TcSMonad.lhs 70.8 KB
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\begin{code}
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{-# OPTIONS -fno-warn-tabs #-}
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-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

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-- Type definitions for the constraint solver
module TcSMonad ( 

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       -- Canonical constraints, definition is now in TcRnTypes
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    WorkList(..), isEmptyWorkList, emptyWorkList,
    workListFromEq, workListFromNonEq, workListFromCt, 
    extendWorkListEq, extendWorkListNonEq, extendWorkListCt, 
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    appendWorkListCt, appendWorkListEqs, unionWorkList, selectWorkItem,
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    getTcSWorkList, updWorkListTcS, updWorkListTcS_return, keepWanted,

    Ct(..), Xi, tyVarsOfCt, tyVarsOfCts, tyVarsOfCDicts, 
    emitFrozenError,
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    isWanted, isGivenOrSolved, isDerived,
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    isGivenOrSolvedCt, isGivenCt, 
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    isWantedCt, isDerivedCt, pprFlavorArising,
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    isFlexiTcsTv,

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    canRewrite, canSolve,
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    mkSolvedLoc, mkGivenLoc,
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    ctWantedLoc,
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    TcS, runTcS, failTcS, panicTcS, traceTcS, -- Basic functionality 
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    traceFireTcS, bumpStepCountTcS, doWithInert,
    tryTcS, nestImplicTcS, recoverTcS,
    wrapErrTcS, wrapWarnTcS,

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    SimplContext(..), isInteractive, simplEqsOnly, performDefaulting,

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    -- Getting and setting the flattening cache
    getFlatCache, updFlatCache, addToSolved, 
    
    
    setEvBind,
    XEvTerm(..),
    MaybeNew (..), isFresh,
    xCtFlavor, -- Transform a CtFlavor during a step 
    rewriteCtFlavor,          -- Specialized version of xCtFlavor for coercions
    newWantedEvVar, newGivenEvVar, instDFunConstraints, newKindConstraint,
    newDerived,
    xCtFlavor_cache, rewriteCtFlavor_cache,
    
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       -- Creation of evidence variables
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{- DELETEME newEvVar, forceNewEvVar, delCachedEvVar, updateFlatCache, flushFlatCache,
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    newGivenEqVar,
    newEqVar, newKindConstraint,
    EvVarCreated (..), isNewEvVar, FlatEqOrigin ( .. ), origin_matches,
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       -- Setting evidence variables 
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    setEqBind,
    setEvBind,
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-}
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    setWantedTyBind,

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    getInstEnvs, getFamInstEnvs,                -- Getting the environments
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    getTopEnv, getGblEnv, getTcEvBinds, getUntouchables,
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    getTcEvBindsMap, getTcSContext, getTcSTyBinds, getTcSTyBindsMap,
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{- DELETEME    
   getTcSEvVarCacheMap, getTcSEvVarFlatCache, setTcSEvVarCacheMap, pprEvVarCache,
-}
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    newFlattenSkolemTy,                         -- Flatten skolems 
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        -- Inerts 
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    InertSet(..), InertCans(..), 
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    getInertEqs, getCtCoercion,
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    emptyInert, getTcSInerts, updInertSet, extractUnsolved,
    extractUnsolvedTcS, modifyInertTcS,
    updInertSetTcS, partitionCCanMap, partitionEqMap,
    getRelevantCts, extractRelevantInerts,
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    CCanMap (..), CtTypeMap, CtFamHeadMap(..), CtPredMap(..),
    pprCtTypeMap, partCtFamHeadMap,
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    instDFunTypes,                              -- Instantiation
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    -- instDFunConstraints,          
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    newFlexiTcSTy, instFlexiTcS,
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    compatKind, mkKindErrorCtxtTcS,
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    TcsUntouchables,
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    isTouchableMetaTyVar,
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    isTouchableMetaTyVar_InRange, 
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    getDefaultInfo, getDynFlags,

    matchClass, matchFam, MatchInstResult (..), 
    checkWellStagedDFun, 
    warnTcS,
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    pprEq                                    -- Smaller utils, re-exported from TcM
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                                             -- TODO (DV): these are only really used in the 
                                             -- instance matcher in TcSimplify. I am wondering
                                             -- if the whole instance matcher simply belongs
                                             -- here 
) where 

#include "HsVersions.h"

import HscTypes
import BasicTypes 

import Inst
import InstEnv 
import FamInst 
import FamInstEnv

import qualified TcRnMonad as TcM
import qualified TcMType as TcM
import qualified TcEnv as TcM 
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       ( checkWellStaged, topIdLvl, tcGetDefaultTys )
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import {-# SOURCE #-} qualified TcUnify as TcM ( mkKindErrorCtxt )
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import Kind
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import TcType
import DynFlags
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import Type
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import TcEvidence
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import Class
import TyCon
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import Name
import Var
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import VarEnv
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import Outputable
import Bag
import MonadUtils
import VarSet
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import FastString
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import Util
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import Id 
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import TcRnTypes
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import Unique 
import UniqFM
import Maybes ( orElse )

import Control.Monad( when )
import StaticFlags( opt_PprStyle_Debug )
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import Data.IORef
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import Data.List ( find )
import Control.Monad ( zipWithM )
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import TrieMap
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\end{code}
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\begin{code}
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compatKind :: Kind -> Kind -> Bool
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compatKind k1 k2 = k1 `tcIsSubKind` k2 || k2 `tcIsSubKind` k1 
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mkKindErrorCtxtTcS :: Type -> Kind 
                   -> Type -> Kind 
                   -> ErrCtxt
mkKindErrorCtxtTcS ty1 ki1 ty2 ki2
  = (False,TcM.mkKindErrorCtxt ty1 ty2 ki1 ki2)

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\end{code}

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%************************************************************************
%*									*
%*                            Worklists                                *
%*  Canonical and non-canonical constraints that the simplifier has to  *
%*  work on. Including their simplification depths.                     *
%*                                                                      *
%*									*
%************************************************************************
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Note [WorkList]
~~~~~~~~~~~~~~~
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A WorkList contains canonical and non-canonical items (of all flavors). 
Notice that each Ct now has a simplification depth. We may 
consider using this depth for prioritization as well in the future. 
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As a simple form of priority queue, our worklist separates out
equalities (wl_eqs) from the rest of the canonical constraints, 
so that it's easier to deal with them first, but the separation 
is not strictly necessary. Notice that non-canonical constraints 
are also parts of the worklist. 
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Note [NonCanonical Semantics]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note that canonical constraints involve a CNonCanonical constructor. In the worklist
we use this constructor for constraints that have not yet been canonicalized such as 
   [Int] ~ [a] 
In other words, all constraints start life as NonCanonicals. 
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On the other hand, in the Inert Set (see below) the presence of a NonCanonical somewhere
means that we have a ``frozen error''. 
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NonCanonical constraints never interact directly with other constraints -- but they can
be rewritten by equalities (for instance if a non canonical exists in the inert, we'd 
better rewrite it as much as possible before reporting it as an error to the user)
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\begin{code}
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-- See Note [WorkList]
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data WorkList = WorkList { wl_eqs  :: [Ct], wl_funeqs :: [Ct], wl_rest :: [Ct] }
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unionWorkList :: WorkList -> WorkList -> WorkList
unionWorkList new_wl orig_wl = 
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   WorkList { wl_eqs    = wl_eqs new_wl ++ wl_eqs orig_wl
            , wl_funeqs = wl_funeqs new_wl ++ wl_funeqs orig_wl
            , wl_rest   = wl_rest new_wl ++ wl_rest orig_wl }
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extendWorkListEq :: Ct -> WorkList -> WorkList
-- Extension by equality
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extendWorkListEq ct wl 
  | Just {} <- isCFunEqCan_Maybe ct
  = wl { wl_funeqs = ct : wl_funeqs wl }
  | otherwise
  = wl { wl_eqs = ct : wl_eqs wl }
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extendWorkListNonEq :: Ct -> WorkList -> WorkList
-- Extension by non equality
extendWorkListNonEq ct wl = wl { wl_rest = ct : wl_rest wl }
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extendWorkListCt :: Ct -> WorkList -> WorkList
-- Agnostic
extendWorkListCt ct wl
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 | isEqPred (ctPred ct) = extendWorkListEq ct wl
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 | otherwise = extendWorkListNonEq ct wl
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appendWorkListCt :: [Ct] -> WorkList -> WorkList
-- Agnostic
appendWorkListCt cts wl = foldr extendWorkListCt wl cts
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appendWorkListEqs :: [Ct] -> WorkList -> WorkList
-- Append a list of equalities
appendWorkListEqs cts wl = foldr extendWorkListEq wl cts
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isEmptyWorkList :: WorkList -> Bool
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isEmptyWorkList wl 
  = null (wl_eqs wl) &&  null (wl_rest wl) && null (wl_funeqs wl)
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emptyWorkList :: WorkList
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emptyWorkList = WorkList { wl_eqs  = [], wl_rest = [], wl_funeqs = []}
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workListFromEq :: Ct -> WorkList
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workListFromEq ct = extendWorkListEq ct emptyWorkList
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workListFromNonEq :: Ct -> WorkList
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workListFromNonEq ct = extendWorkListNonEq ct emptyWorkList
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workListFromCt :: Ct -> WorkList
-- Agnostic 
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workListFromCt ct | isEqPred (ctPred ct) = workListFromEq ct 
                  | otherwise            = workListFromNonEq ct
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selectWorkItem :: WorkList -> (Maybe Ct, WorkList)
selectWorkItem wl@(WorkList { wl_eqs = eqs, wl_funeqs = feqs, wl_rest = rest })
  = case (eqs,feqs,rest) of
      (ct:cts,_,_)     -> (Just ct, wl { wl_eqs    = cts })
      (_,(ct:cts),_)   -> (Just ct, wl { wl_funeqs = cts })
      (_,_,(ct:cts))   -> (Just ct, wl { wl_rest   = cts })
      (_,_,_)          -> (Nothing,wl)

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-- Pretty printing 
instance Outputable WorkList where 
  ppr wl = vcat [ text "WorkList (eqs)   = " <+> ppr (wl_eqs wl)
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                , text "WorkList (funeqs)= " <+> ppr (wl_funeqs wl)
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                , text "WorkList (rest)  = " <+> ppr (wl_rest wl)
                ]
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keepWanted :: Cts -> Cts
keepWanted = filterBag isWantedCt
    -- DV: there used to be a note here that read: 
    -- ``Important: use fold*r*Bag to preserve the order of the evidence variables'' 
    -- DV: Is this still relevant? 
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-- Canonical constraint maps
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data CCanMap a = CCanMap { cts_given   :: UniqFM Cts
                                          -- Invariant: all Given
                         , cts_derived :: UniqFM Cts 
                                          -- Invariant: all Derived
                         , cts_wanted  :: UniqFM Cts } 
                                          -- Invariant: all Wanted

cCanMapToBag :: CCanMap a -> Cts 
cCanMapToBag cmap = foldUFM unionBags rest_wder (cts_given cmap)
  where rest_wder = foldUFM unionBags rest_der  (cts_wanted cmap) 
        rest_der  = foldUFM unionBags emptyCts  (cts_derived cmap)

emptyCCanMap :: CCanMap a 
emptyCCanMap = CCanMap { cts_given = emptyUFM, cts_derived = emptyUFM, cts_wanted = emptyUFM } 

updCCanMap:: Uniquable a => (a,Ct) -> CCanMap a -> CCanMap a 
updCCanMap (a,ct) cmap 
  = case cc_flavor ct of 
      Wanted {}  -> cmap { cts_wanted  = insert_into (cts_wanted cmap)  } 
      Given {}   -> cmap { cts_given   = insert_into (cts_given cmap)   }
      Derived {} -> cmap { cts_derived = insert_into (cts_derived cmap) }
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      Solved {}  -> panic "updCCanMap update with solved!" 
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  where 
    insert_into m = addToUFM_C unionBags m a (singleCt ct)

getRelevantCts :: Uniquable a => a -> CCanMap a -> (Cts, CCanMap a) 
-- Gets the relevant constraints and returns the rest of the CCanMap
getRelevantCts a cmap 
    = let relevant = lookup (cts_wanted cmap) `unionBags`
                     lookup (cts_given cmap)  `unionBags`
                     lookup (cts_derived cmap) 
          residual_map = cmap { cts_wanted  = delFromUFM (cts_wanted cmap) a
                              , cts_given   = delFromUFM (cts_given cmap) a
                              , cts_derived = delFromUFM (cts_derived cmap) a }
      in (relevant, residual_map) 
  where
    lookup map = lookupUFM map a `orElse` emptyCts

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lookupCCanMap :: Uniquable a => a -> (Ct -> Bool) -> CCanMap a -> Maybe Ct
lookupCCanMap a p map
   = let possible_cts = lookupUFM (cts_given map)   a `orElse` 
                        lookupUFM (cts_wanted map)  a `orElse` 
                        lookupUFM (cts_derived map) a `orElse` emptyCts
     in find p (bagToList possible_cts)
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partitionCCanMap :: (Ct -> Bool) -> CCanMap a -> (Cts,CCanMap a) 
-- All constraints that /match/ the predicate go in the bag, the rest remain in the map
partitionCCanMap pred cmap
  = let (ws_map,ws) = foldUFM_Directly aux (emptyUFM,emptyCts) (cts_wanted cmap) 
        (ds_map,ds) = foldUFM_Directly aux (emptyUFM,emptyCts) (cts_derived cmap)
        (gs_map,gs) = foldUFM_Directly aux (emptyUFM,emptyCts) (cts_given cmap) 
    in (ws `andCts` ds `andCts` gs, cmap { cts_wanted  = ws_map
                                         , cts_given   = gs_map
                                         , cts_derived = ds_map }) 
  where aux k this_cts (mp,acc_cts) = (new_mp, new_acc_cts)
                                    where new_mp      = addToUFM mp k cts_keep
                                          new_acc_cts = acc_cts `andCts` cts_out
                                          (cts_out, cts_keep) = partitionBag pred this_cts

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partitionEqMap :: (Ct -> Bool) -> TyVarEnv (Ct,TcCoercion) -> ([Ct], TyVarEnv (Ct,TcCoercion))
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partitionEqMap pred isubst 
  = let eqs_out = foldVarEnv extend_if_pred [] isubst
        eqs_in  = filterVarEnv_Directly (\_ (ct,_) -> not (pred ct)) isubst
    in (eqs_out, eqs_in)
  where extend_if_pred (ct,_) cts = if pred ct then ct : cts else cts


extractUnsolvedCMap :: CCanMap a -> (Cts, CCanMap a)
-- Gets the wanted or derived constraints and returns a residual
-- CCanMap with only givens.
extractUnsolvedCMap cmap =
  let wntd = foldUFM unionBags emptyCts (cts_wanted cmap)
      derd = foldUFM unionBags emptyCts (cts_derived cmap)
  in (wntd `unionBags` derd, 
      cmap { cts_wanted = emptyUFM, cts_derived = emptyUFM })


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-- Maps from PredTypes to Constraints
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type CtTypeMap = TypeMap Ct
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newtype CtPredMap = 
  CtPredMap { unCtPredMap :: CtTypeMap }       -- Indexed by TcPredType
newtype CtFamHeadMap = 
  CtFamHeadMap { unCtFamHeadMap :: CtTypeMap } -- Indexed by family head
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pprCtTypeMap :: TypeMap Ct -> SDoc 
pprCtTypeMap ctmap = ppr (foldTM (:) ctmap [])

ctTypeMapCts :: TypeMap Ct -> Cts
ctTypeMapCts ctmap = foldTM (\ct cts -> extendCts cts ct) ctmap emptyCts

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partCtFamHeadMap :: (Ct -> Bool) 
                 -> CtFamHeadMap 
                 -> (Cts, CtFamHeadMap)
partCtFamHeadMap f ctmap
  = let (cts,tymap_final) = foldTM upd_acc tymap_inside (emptyBag, tymap_inside)
    in (cts, CtFamHeadMap tymap_final)
  where
    tymap_inside = unCtFamHeadMap ctmap 
    upd_acc ct (cts,acc_map)
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         | f ct      = (extendCts cts ct, alterTM ct_key (\_ -> Nothing) acc_map)
         | otherwise = (cts,acc_map)
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         where ct_key | EqPred ty1 _ <- classifyPredType (ctPred ct)
                      = ty1 
                      | otherwise 
                      = panic "partCtFamHeadMap, encountered non equality!"


\end{code}

%************************************************************************
%*									*
%*                            Inert Sets                                *
%*                                                                      *
%*									*
%************************************************************************

\begin{code}


-- All Given (fully known) or Wanted or Derived, never Solved
-- See Note [Detailed InertCans Invariants] for more
data InertCans 
  = IC { inert_eqs :: TyVarEnv Ct
              -- Must all be CTyEqCans! If an entry exists of the form: 
              --   a |-> ct,co
              -- Then ct = CTyEqCan { cc_tyvar = a, cc_rhs = xi } 
              -- And  co : a ~ xi
       , inert_eq_tvs :: InScopeSet
              -- Superset of the type variables of inert_eqs
       , inert_dicts :: CCanMap Class
              -- Dictionaries only, index is the class
              -- NB: index is /not/ the whole type because FD reactions 
              -- need to match the class but not necessarily the whole type.
       , inert_ips :: CCanMap (IPName Name)
              -- Implicit parameters, index is the name
              -- NB: index is /not/ the whole type because IP reactions need 
              -- to match the ip name but not necessarily the whole type.
       , inert_funeqs :: CtFamHeadMap
              -- Family equations, index is the whole family head type.
       , inert_irreds :: Cts       
              -- Irreducible predicates
       }
    
                     
\end{code}

Note [Detailed InertCans Invariants]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The InertCans represents a collection of constraints with the following properties:
  1 All canonical
  2 All Given or Wanted or Derived. No (partially) Solved
  3 No two dictionaries with the same head
  4 No two family equations with the same head
  5 Family equations inert with top-level
  6 Dictionaries have no matching instance at top level
  7 Constraints are fully rewritten with respect to the equality constraints (CTyEqCan)
  8 Equalities form an idempotent substitution (taking flavors into consideration)
  9 Given family or dictionary constraints don't mention touchable unification variables
\begin{code}

{- DV Notes: 23/03/2012
 1) stage: if exact predicate exists then discard immediately else go on
 2) stage: canonicalization (with the newEvVarCache) and the flatCache stuff
 3) stage: whatever we do no but at the interact-top we share previously solved 
           family head equations
-}

-- The Inert Set
data InertSet
  = IS { inert_cans :: InertCans
              -- Canonical Given,Wanted,Solved
       , inert_frozen :: Cts       
              -- Frozen errors (as non-canonicals)
                               
       , inert_solved :: CtPredMap
              -- Solved constraints (for caching): 
              -- (i) key is by predicate type
              -- (ii) all of 'Solved' flavor, may or may not be canonicals
              -- (iii) we use this field for avoiding creating newEvVars
       , inert_flat_cache :: CtFamHeadMap 
              -- All ``flattening equations'' are kept here. 
              -- Always canonical CTyFunEqs (Given or Wanted only!)
              -- Key is by family head. We used this field during flattening only
       , inert_solved_funeqs :: CtFamHeadMap
              -- Memoized *Solved* family equations co :: F xis ~ xi
              -- Stored not necessarily as fully rewritten; we'll do that lazily
              -- when we lookup
       }
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instance Outputable InertCans where 
  ppr ics = vcat [ vcat (map ppr (varEnvElts (inert_eqs ics)))
                 , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_dicts ics)))
                 , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_ips ics))) 
                 , vcat (map ppr (Bag.bagToList $ 
                                  ctTypeMapCts (unCtFamHeadMap $ inert_funeqs ics)))
                 , vcat (map ppr (Bag.bagToList $ inert_irreds ics))
                 ]
            
instance Outputable InertSet where 
  ppr is = vcat [ ppr $ inert_cans is
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                , text "Frozen errors =" <+> -- Clearly print frozen errors
                    braces (vcat (map ppr (Bag.bagToList $ inert_frozen is)))
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                , text "Solved and cached" <+>
                    int (foldTypeMap (\_ x -> x+1) 0 
                             (unCtPredMap $ inert_solved is)) <+> 
                    text "more constraints" ]
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emptyInert :: InertSet
emptyInert
  = IS { inert_cans = IC { inert_eqs    = emptyVarEnv
                         , inert_eq_tvs = emptyInScopeSet
                         , inert_dicts  = emptyCCanMap
                         , inert_ips    = emptyCCanMap
                         , inert_funeqs = CtFamHeadMap emptyTM 
                         , inert_irreds = emptyCts }
       , inert_frozen        = emptyCts
       , inert_flat_cache    = CtFamHeadMap emptyTM
       , inert_solved        = CtPredMap emptyTM 
       , inert_solved_funeqs = CtFamHeadMap emptyTM }
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type AtomicInert = Ct 

updInertSet :: InertSet -> AtomicInert -> InertSet 
-- Add a new inert element to the inert set. 
updInertSet is item 
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  | isSolved (cc_flavor item)
    -- Solved items go in their special place
  = let pty = ctPred item
        upd_solved Nothing = Just item
        upd_solved (Just _existing_solved) = Just item 
               -- .. or Just existing_solved? Is this even possible to happen?
    in is { inert_solved = 
               CtPredMap $ 
               alterTM pty upd_solved (unCtPredMap $ inert_solved is) }

  | isCNonCanonical item 
    -- NB: this may happen if we decide to kick some frozen error 
    -- out to rewrite him. Frozen errors are just NonCanonicals
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  = is { inert_frozen = inert_frozen is `Bag.snocBag` item }
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  | otherwise  
    -- A canonical Given, Wanted, or Derived
  = is { inert_cans = upd_inert_cans (inert_cans is) item }
  
  where upd_inert_cans :: InertCans -> AtomicInert -> InertCans
        -- Precondition: item /is/ canonical
        upd_inert_cans ics item
          | isCTyEqCan item                     
          = let upd_err a b = pprPanic "updInertSet" $
                              vcat [ text "Multiple inert equalities:"
                                   , text "Old (already inert):" <+> ppr a
                                   , text "Trying to insert   :" <+> ppr b ]
        
                eqs'     = extendVarEnv_C upd_err (inert_eqs ics) 
                                                  (cc_tyvar item) item        
                inscope' = extendInScopeSetSet (inert_eq_tvs ics)
                                               (tyVarsOfCt item)
                
            in ics { inert_eqs = eqs', inert_eq_tvs = inscope' }

          | Just x  <- isCIPCan_Maybe item      -- IP 
          = ics { inert_ips   = updCCanMap (x,item) (inert_ips ics) }  
            
          | isCIrredEvCan item                  -- Presently-irreducible evidence
          = ics { inert_irreds = inert_irreds ics `Bag.snocBag` item }

          | Just cls <- isCDictCan_Maybe item   -- Dictionary 
          = ics { inert_dicts = updCCanMap (cls,item) (inert_dicts ics) }

          | Just _tc <- isCFunEqCan_Maybe item  -- Function equality
          = let fam_head = mkTyConApp (cc_fun item) (cc_tyargs item)
                upd_funeqs Nothing = Just item
                upd_funeqs (Just _already_there) 
                  = panic "updInertSet: item already there!"
            in ics { inert_funeqs = CtFamHeadMap 
                                      (alterTM fam_head upd_funeqs $ 
                                         (unCtFamHeadMap $ inert_funeqs ics)) }
          | otherwise
          = pprPanic "upd_inert set: can't happen! Inserting " $ 
            ppr item 
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updInertSetTcS :: AtomicInert -> TcS ()
-- Add a new item in the inerts of the monad
updInertSetTcS item
  = do { traceTcS "updInertSetTcs {" $ 
         text "Trying to insert new inert item:" <+> ppr item

       ; modifyInertTcS (\is -> ((), updInertSet is item)) 
                        
       ; traceTcS "updInertSetTcs }" $ empty }


modifyInertTcS :: (InertSet -> (a,InertSet)) -> TcS a 
-- Modify the inert set with the supplied function
modifyInertTcS upd 
  = do { is_var <- getTcSInertsRef
       ; curr_inert <- wrapTcS (TcM.readTcRef is_var)
       ; let (a, new_inert) = upd curr_inert
       ; wrapTcS (TcM.writeTcRef is_var new_inert)
       ; return a }

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addToSolved :: Ct -> TcS ()
addToSolved ct 
  = ASSERT ( isSolved (cc_flavor ct) )
    updInertSetTcS ct

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extractUnsolvedTcS :: TcS (Cts,Cts) 
-- Extracts frozen errors and remaining unsolved and sets the 
-- inert set to be the remaining! 
extractUnsolvedTcS = 
  modifyInertTcS extractUnsolved 

extractUnsolved :: InertSet -> ((Cts,Cts), InertSet)
-- Postcondition
-- -------------
-- When: 
--   ((frozen,cts),is_solved) <- extractUnsolved inert
-- Then: 
-- -----------------------------------------------------------------------------
--  cts       |  The unsolved (Derived or Wanted only) residual 
--            |  canonical constraints, that is, no CNonCanonicals.
-- -----------|-----------------------------------------------------------------
--  frozen    | The CNonCanonicals of the original inert (frozen errors), 
--            | of all flavors
-- -----------|-----------------------------------------------------------------
--  is_solved | Whatever remains from the inert after removing the previous two. 
-- -----------------------------------------------------------------------------
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extractUnsolved (IS { inert_cans = IC { inert_eqs    = eqs
                                      , inert_eq_tvs = eq_tvs
                                      , inert_irreds = irreds
                                      , inert_ips    = ips
                                      , inert_funeqs = funeqs
                                      , inert_dicts  = dicts
                                      }
                    , inert_frozen = frozen
                    , inert_solved = _solved
                    , inert_flat_cache = _flat_cache })
  
  = let is_solved  = IS { inert_cans = IC { inert_eqs    = solved_eqs
                                          , inert_eq_tvs = eq_tvs
                                          , inert_dicts  = solved_dicts
                                          , inert_ips    = solved_ips
                                          , inert_irreds = solved_irreds
                                          , inert_funeqs = solved_funeqs }
                        , inert_frozen = emptyCts -- All out
                                         
                            -- DV: For solved and the flat cache, I am flushing them here:
                            -- Solved cts may depend on wanteds which we kick out. But later
                            -- we may try to re-solve some kicked-out wanteds and I am worried 
                            -- that there is a danger or evidence loops if we keep the solved 
                            -- in for caching purposes. So I am flushing the solved and the 
                            -- flattening cache, quite conservatively.
                        , inert_solved        = CtPredMap emptyTM
                        , inert_flat_cache    = CtFamHeadMap emptyTM
                        , inert_solved_funeqs = CtFamHeadMap emptyTM
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                        }
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    in ((frozen, unsolved), is_solved)
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  where solved_eqs = filterVarEnv_Directly (\_ ct -> isGivenOrSolvedCt ct) eqs
        unsolved_eqs = foldVarEnv (\ct cts -> cts `extendCts` ct) emptyCts $
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                       eqs `minusVarEnv` solved_eqs

        (unsolved_irreds, solved_irreds) = Bag.partitionBag (not.isGivenOrSolvedCt) irreds
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        (unsolved_ips, solved_ips)       = extractUnsolvedCMap ips
        (unsolved_dicts, solved_dicts)   = extractUnsolvedCMap dicts
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        (unsolved_funeqs, solved_funeqs) = 
          partCtFamHeadMap (not . isGivenOrSolved . cc_flavor) funeqs
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        unsolved = unsolved_eqs `unionBags` unsolved_irreds `unionBags`
                   unsolved_ips `unionBags` unsolved_dicts `unionBags` unsolved_funeqs



extractRelevantInerts :: Ct -> TcS Cts
-- Returns the constraints from the inert set that are 'relevant' to react with 
-- this constraint. The monad is left with the 'thinner' inerts. 
-- NB: This function contains logic specific to the constraint solver, maybe move there?
extractRelevantInerts wi 
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  = modifyInertTcS (extract_relevants wi)
  where extract_relevants wi is 
          = let (cts,ics') = extract_ics_relevants wi (inert_cans is)
            in (cts, is { inert_cans = ics' }) 
            
        extract_ics_relevants (CDictCan {cc_class = cl}) ics = 
            let (cts,dict_map) = getRelevantCts cl (inert_dicts ics) 
            in (cts, ics { inert_dicts = dict_map })
        extract_ics_relevants ct@(CFunEqCan {}) ics = 
            let (cts,feqs_map)  = 
                  let funeq_map = unCtFamHeadMap $ inert_funeqs ics
                      fam_head = mkTyConApp (cc_fun ct) (cc_tyargs ct)
                      lkp = lookupTM fam_head funeq_map
                      new_funeq_map = alterTM fam_head xtm funeq_map
                      xtm Nothing    = Nothing
                      xtm (Just _ct) = Nothing
                  in case lkp of 
                    Nothing -> (emptyCts, funeq_map)
                    Just ct -> (singleCt ct, new_funeq_map)
            in (cts, ics { inert_funeqs = CtFamHeadMap feqs_map })
        extract_ics_relevants (CIPCan { cc_ip_nm = nm } ) ics = 
            let (cts, ips_map) = getRelevantCts nm (inert_ips ics) 
            in (cts, ics { inert_ips = ips_map })
        extract_ics_relevants (CIrredEvCan { }) ics = 
            let cts = inert_irreds ics 
            in (cts, ics { inert_irreds = emptyCts })
        extract_ics_relevants _ ics = (emptyCts,ics)
        

lookupInInerts :: InertSet -> TcPredType -> Maybe Ct
-- Is this exact predicate type cached in the solved or canonicals of the InertSet
lookupInInerts (IS { inert_solved = solved, inert_cans = ics }) pty
  = case lookupInSolved solved pty of
      Just ct -> return ct
      Nothing -> lookupInInertCans ics pty

lookupInSolved :: CtPredMap -> TcPredType -> Maybe Ct
-- Returns just if exactly this predicate type exists in the solved.
lookupInSolved tm pty = lookupTM pty $ unCtPredMap tm

lookupInInertCans :: InertCans -> TcPredType -> Maybe Ct
-- Returns Just if exactly this pred type exists in the inert canonicals
lookupInInertCans ics pty
  = lkp_ics (classifyPredType pty)
  where lkp_ics (ClassPred cls _)
          = lookupCCanMap cls (\ct -> ctPred ct `eqType` pty) (inert_dicts ics)
        lkp_ics (EqPred ty1 _ty2)
          | Just tv <- getTyVar_maybe ty1
          , Just ct <- lookupVarEnv (inert_eqs ics) tv
          , ctPred ct `eqType` pty
          = Just ct
        lkp_ics (EqPred ty1 _ty2) -- Family equation
          | Just _ <- splitTyConApp_maybe ty1
          , Just ct <- lookupTM ty1 (unCtFamHeadMap $ inert_funeqs ics)
          , ctPred ct `eqType` pty
          = Just ct
        lkp_ics (IrredPred {}) 
          = find (\ct -> ctPred ct `eqType` pty) (bagToList (inert_irreds ics))
        lkp_ics _ = Nothing -- NB: No caching for IPs
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\end{code}

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%************************************************************************
%*									*
%*		The TcS solver monad                                    *
%*									*
%************************************************************************

Note [The TcS monad]
~~~~~~~~~~~~~~~~~~~~
The TcS monad is a weak form of the main Tc monad

All you can do is
    * fail
    * allocate new variables
    * fill in evidence variables

Filling in a dictionary evidence variable means to create a binding
for it, so TcS carries a mutable location where the binding can be
added.  This is initialised from the innermost implication constraint.

\begin{code}
data TcSEnv
  = TcSEnv { 
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      tcs_ev_binds    :: EvBindsVar,
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      tcs_ty_binds :: IORef (TyVarEnv (TcTyVar, TcType)),
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          -- Global type bindings

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      tcs_context :: SimplContext,
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      tcs_untch :: TcsUntouchables,

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      tcs_ic_depth   :: Int,       -- Implication nesting depth
      tcs_count      :: IORef Int, -- Global step count

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      tcs_inerts   :: IORef InertSet, -- Current inert set
      tcs_worklist :: IORef WorkList  -- Current worklist
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    }
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{- DELETEME 
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data EvVarCache
  = EvVarCache { evc_cache     :: TypeMap (EvVar,CtFlavor)    
                     -- Map from PredTys to Evidence variables
                     -- used to avoid creating new goals
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               , evc_flat_cache :: TypeMap (TcCoercion,(Xi,CtFlavor,FlatEqOrigin))
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                     -- Map from family-free heads (F xi) to family-free types.
                     -- Useful during flattening to share flatten skolem generation
                     -- The boolean flag:
                     --   True  <-> This equation was generated originally during flattening
                     --   False <-> This equation was generated by having solved a goal
               }

data FlatEqOrigin = WhileFlattening  -- Was it generated during flattening?
                  | WhenSolved       -- Was it generated when a family equation was solved?
                  | Any

origin_matches :: FlatEqOrigin -> FlatEqOrigin -> Bool
origin_matches Any _                           = True
origin_matches WhenSolved WhenSolved           = True
origin_matches WhileFlattening WhileFlattening = True
origin_matches _ _ = False
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-}
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type TcsUntouchables = (Untouchables,TcTyVarSet)
-- Like the TcM Untouchables, 
-- but records extra TcsTv variables generated during simplification
-- See Note [Extra TcsTv untouchables] in TcSimplify
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\end{code}

\begin{code}
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data SimplContext
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  = SimplInfer SDoc	   -- Inferring type of a let-bound thing
  | SimplRuleLhs RuleName  -- Inferring type of a RULE lhs
  | SimplInteractive	   -- Inferring type at GHCi prompt
  | SimplCheck SDoc	   -- Checking a type signature or RULE rhs
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instance Outputable SimplContext where
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  ppr (SimplInfer d)   = ptext (sLit "SimplInfer") <+> d
  ppr (SimplCheck d)   = ptext (sLit "SimplCheck") <+> d
  ppr (SimplRuleLhs n) = ptext (sLit "SimplRuleLhs") <+> doubleQuotes (ftext n)
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  ppr SimplInteractive = ptext (sLit "SimplInteractive")

isInteractive :: SimplContext -> Bool
isInteractive SimplInteractive = True
isInteractive _                = False

simplEqsOnly :: SimplContext -> Bool
-- Simplify equalities only, not dictionaries
-- This is used for the LHS of rules; ee
-- Note [Simplifying RULE lhs constraints] in TcSimplify
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simplEqsOnly (SimplRuleLhs {}) = True
simplEqsOnly _                 = False
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performDefaulting :: SimplContext -> Bool
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performDefaulting (SimplInfer {})   = False
performDefaulting (SimplRuleLhs {}) = False
performDefaulting SimplInteractive  = True
performDefaulting (SimplCheck {})   = True
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---------------
newtype TcS a = TcS { unTcS :: TcSEnv -> TcM a } 

instance Functor TcS where
  fmap f m = TcS $ fmap f . unTcS m

instance Monad TcS where 
  return x  = TcS (\_ -> return x) 
  fail err  = TcS (\_ -> fail err) 
  m >>= k   = TcS (\ebs -> unTcS m ebs >>= \r -> unTcS (k r) ebs)

-- Basic functionality 
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
wrapTcS :: TcM a -> TcS a 
-- Do not export wrapTcS, because it promotes an arbitrary TcM to TcS,
-- and TcS is supposed to have limited functionality
wrapTcS = TcS . const -- a TcM action will not use the TcEvBinds

wrapErrTcS :: TcM a -> TcS a 
-- The thing wrapped should just fail
-- There's no static check; it's up to the user
-- Having a variant for each error message is too painful
wrapErrTcS = wrapTcS

wrapWarnTcS :: TcM a -> TcS a 
-- The thing wrapped should just add a warning, or no-op
-- There's no static check; it's up to the user
wrapWarnTcS = wrapTcS

failTcS, panicTcS :: SDoc -> TcS a
failTcS      = wrapTcS . TcM.failWith
panicTcS doc = pprPanic "TcCanonical" doc

traceTcS :: String -> SDoc -> TcS ()
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traceTcS herald doc = wrapTcS (TcM.traceTc herald doc)
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bumpStepCountTcS :: TcS ()
bumpStepCountTcS = TcS $ \env -> do { let ref = tcs_count env
                                    ; n <- TcM.readTcRef ref
                                    ; TcM.writeTcRef ref (n+1) }

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traceFireTcS :: SubGoalDepth -> SDoc -> TcS ()
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-- Dump a rule-firing trace
traceFireTcS depth doc 
  = TcS $ \env -> 
    TcM.ifDOptM Opt_D_dump_cs_trace $ 
    do { n <- TcM.readTcRef (tcs_count env)
       ; let msg = int n 
                <> text (replicate (tcs_ic_depth env) '>')
                <> brackets (int depth) <+> doc
       ; TcM.dumpTcRn msg }
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runTcS :: SimplContext
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       -> Untouchables 	       -- Untouchables
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       -> InertSet             -- Initial inert set
       -> WorkList             -- Initial work list
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       -> TcS a		       -- What to run
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       -> TcM (a, Bag EvBind)
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runTcS context untouch is wl tcs 
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  = do { ty_binds_var <- TcM.newTcRef emptyVarEnv
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       ; ev_binds_var <- TcM.newTcEvBinds
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       ; step_count <- TcM.newTcRef 0
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       ; inert_var <- TcM.newTcRef is 
       ; wl_var <- TcM.newTcRef wl

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       ; let env = TcSEnv { tcs_ev_binds = ev_binds_var
                          , tcs_ty_binds = ty_binds_var
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                          , tcs_context  = context
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                          , tcs_untch    = (untouch, emptyVarSet) -- No Tcs untouchables yet
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			  , tcs_count    = step_count
			  , tcs_ic_depth = 0
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                          , tcs_inerts   = inert_var
                          , tcs_worklist = wl_var }
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	     -- Run the computation
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       ; res <- unTcS tcs env
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	     -- Perform the type unifications required
       ; ty_binds <- TcM.readTcRef ty_binds_var
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       ; mapM_ do_unification (varEnvElts ty_binds)
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       ; when debugIsOn $ do {
             count <- TcM.readTcRef step_count
           ; when (opt_PprStyle_Debug && count > 0) $
             TcM.debugDumpTcRn (ptext (sLit "Constraint solver steps =") 
                                <+> int count <+> ppr context)
         }
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             -- And return
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       ; ev_binds <- TcM.getTcEvBinds ev_binds_var
       ; return (res, ev_binds) }
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  where
    do_unification (tv,ty) = TcM.writeMetaTyVar tv ty
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doWithInert :: InertSet -> TcS a -> TcS a 
doWithInert inert (TcS action)
  = TcS $ \env -> do { new_inert_var <- TcM.newTcRef inert
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                     ; action (env { tcs_inerts = new_inert_var }) }
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nestImplicTcS :: EvBindsVar -> TcsUntouchables -> TcS a -> TcS a 
nestImplicTcS ref (inner_range, inner_tcs) (TcS thing_inside) 
  = TcS $ \ TcSEnv { tcs_ty_binds = ty_binds
                   , tcs_untch = (_outer_range, outer_tcs)
                   , tcs_count = count
                   , tcs_ic_depth = idepth
                   , tcs_context = ctxt
                   , tcs_inerts = inert_var
                   , tcs_worklist = wl_var } -> 
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    do { let inner_untch = (inner_range, outer_tcs `unionVarSet` inner_tcs)
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       		   -- The inner_range should be narrower than the outer one
		   -- (thus increasing the set of untouchables) but 
		   -- the inner Tcs-untouchables must be unioned with the
		   -- outer ones!
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         -- Inherit the inerts from the outer scope
       ; orig_inerts <- TcM.readTcRef inert_var
       ; new_inert_var <- TcM.newTcRef orig_inerts
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       ; let nest_env = TcSEnv { tcs_ev_binds    = ref
                               , tcs_ty_binds    = ty_binds
                               , tcs_untch       = inner_untch
                               , tcs_count       = count
                               , tcs_ic_depth    = idepth+1
                               , tcs_context     = ctxtUnderImplic ctxt 
                               , tcs_inerts      = new_inert_var
                               , tcs_worklist    = wl_var 
                               -- NB: worklist is going to be empty anyway, 
                               -- so reuse the same ref cell
                               }
       ; thing_inside nest_env } 
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recoverTcS :: TcS a -> TcS a -> TcS a
recoverTcS (TcS recovery_code) (TcS thing_inside)
  = TcS $ \ env ->
    TcM.recoverM (recovery_code env) (thing_inside env)

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ctxtUnderImplic :: SimplContext -> SimplContext
-- See Note [Simplifying RULE lhs constraints] in TcSimplify
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ctxtUnderImplic (SimplRuleLhs n) = SimplCheck (ptext (sLit "lhs of rule") 
                                               <+> doubleQuotes (ftext n))
ctxtUnderImplic ctxt              = ctxt
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tryTcS :: TcS a -> TcS a
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-- Like runTcS, but from within the TcS monad 
-- Completely afresh inerts and worklist, be careful! 
-- Moreover, we will simply throw away all the evidence generated. 
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tryTcS tcs
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  = TcS (\env -> 
             do { wl_var <- TcM.newTcRef emptyWorkList
                ; is_var <- TcM.newTcRef emptyInert

                ; ty_binds_var <- TcM.newTcRef emptyVarEnv
                ; ev_binds_var <- TcM.newTcEvBinds

                ; let env1 = env { tcs_ev_binds = ev_binds_var
                                 , tcs_ty_binds = ty_binds_var
                                 , tcs_inerts   = is_var
                                 , tcs_worklist = wl_var } 
                ; unTcS tcs env1 })

-- Getters and setters of TcEnv fields
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-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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-- Getter of inerts and worklist
getTcSInertsRef :: TcS (IORef InertSet)
getTcSInertsRef = TcS (return . tcs_inerts)

getTcSWorkListRef :: TcS (IORef WorkList) 
getTcSWorkListRef = TcS (return . tcs_worklist) 

getTcSInerts :: TcS InertSet 
getTcSInerts = getTcSInertsRef >>= wrapTcS . (TcM.readTcRef) 

getTcSWorkList :: TcS WorkList
getTcSWorkList = getTcSWorkListRef >>= wrapTcS . (TcM.readTcRef) 

updWorkListTcS :: (WorkList -> WorkList) -> TcS () 
updWorkListTcS f 
  = updWorkListTcS_return (\w -> ((),f w))

updWorkListTcS_return :: (WorkList -> (a,WorkList)) -> TcS a
updWorkListTcS_return f
  = do { wl_var <- getTcSWorkListRef
       ; wl_curr <- wrapTcS (TcM.readTcRef wl_var)
       ; let (res,new_work) = f wl_curr
       ; wrapTcS (TcM.writeTcRef wl_var new_work)
       ; return res }

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emitFrozenError :: CtFlavor -> SubGoalDepth -> TcS ()
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-- Emits a non-canonical constraint that will stand for a frozen error in the inerts. 
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emitFrozenError fl depth 
  = do { traceTcS "Emit frozen error" (ppr (ctFlavPred fl))
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       ; inert_ref <- getTcSInertsRef 
       ; inerts <- wrapTcS (TcM.readTcRef inert_ref)
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       ; let ct = CNonCanonical { cc_flavor = fl
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                                , cc_depth = depth } 
             inerts_new = inerts { inert_frozen = extendCts (inert_frozen inerts) ct } 
       ; wrapTcS (TcM.writeTcRef inert_ref inerts_new) }

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instance HasDynFlags TcS where
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    getDynFlags = wrapTcS getDynFlags
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getTcSContext :: TcS SimplContext
getTcSContext = TcS (return . tcs_context)

getTcEvBinds :: TcS EvBindsVar
getTcEvBinds = TcS (return . tcs_ev_binds) 

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getFlatCache :: TcS CtTypeMap 
getFlatCache = getTcSInerts >>= (return . unCtFamHeadMap . inert_flat_cache)

updFlatCache :: Ct -> TcS ()
-- Pre: constraint is a flat family equation (equal to a flatten skolem)
updFlatCache flat_eq@(CFunEqCan { cc_flavor = fl, cc_fun = tc, cc_tyargs = xis })
  = modifyInertTcS upd_inert_cache
  where upd_inert_cache is = ((), is { inert_flat_cache = CtFamHeadMap new_fc })
                           where new_fc = alterTM pred_key upd_cache fc
                                 fc = unCtFamHeadMap $ inert_flat_cache is
        pred_key = mkTyConApp tc xis
        upd_cache (Just ct) | cc_flavor ct `canSolve` fl = Just ct 
        upd_cache (Just _ct) = Just flat_eq 
        upd_cache Nothing    = Just flat_eq
updFlatCache other_ct = pprPanic "updFlatCache: non-family constraint" $
                        ppr other_ct
                        
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{- DELETEME 
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flushFlatCache :: TcS ()
flushFlatCache
  = do { cache_var <- getTcSEvVarCache
       ; the_cache <- wrapTcS $ TcM.readTcRef cache_var
       ; wrapTcS $ TcM.writeTcRef cache_var (the_cache { evc_flat_cache = emptyTM }) }


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getTcSEvVarCacheMap :: TcS (TypeMap (EvVar,CtFlavor))
getTcSEvVarCacheMap = do { cache_var <- getTcSEvVarCache 
                         ; the_cache <- wrapTcS $ TcM.readTcRef cache_var 
                         ; return (evc_cache the_cache) }

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getTcSEvVarFlatCache :: TcS (TypeMap (TcCoercion,(Type,CtFlavor,FlatEqOrigin)))
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getTcSEvVarFlatCache = do { cache_var <- getTcSEvVarCache 
                          ; the_cache <- wrapTcS $ TcM.readTcRef cache_var 
                          ; return (evc_flat_cache the_cache) }

setTcSEvVarCacheMap :: TypeMap (EvVar,CtFlavor) -> TcS () 
setTcSEvVarCacheMap cache = do { cache_var <- getTcSEvVarCache 
                               ; orig_cache <- wrapTcS $ TcM.readTcRef cache_var
                               ; let new_cache = orig_cache { evc_cache = cache } 
                               ; wrapTcS $ TcM.writeTcRef cache_var new_cache }
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-}

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getUntouchables :: TcS TcsUntouchables
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getUntouchables = TcS (return . tcs_untch)

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getTcSTyBinds :: TcS (IORef (TyVarEnv (TcTyVar, TcType)))
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getTcSTyBinds = TcS (return . tcs_ty_binds)

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getTcSTyBindsMap :: TcS (TyVarEnv (TcTyVar, TcType))
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getTcSTyBindsMap = getTcSTyBinds >>= wrapTcS . (TcM.readTcRef) 
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getTcEvBindsMap :: TcS EvBindMap
getTcEvBindsMap
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  = do { EvBindsVar ev_ref _ <- getTcEvBinds 
       ; wrapTcS $ TcM.readTcRef ev_ref }

setWantedTyBind :: TcTyVar -> TcType -> TcS () 
-- Add a type binding
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-- We never do this twice!
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setWantedTyBind tv ty 
  = do { ref <- getTcSTyBinds
       ; wrapTcS $ 
         do { ty_binds <- TcM.readTcRef ref
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            ; when debugIsOn $
                  TcM.checkErr (not (tv `elemVarEnv` ty_binds)) $
                  vcat [ text "TERRIBLE ERROR: double set of meta type variable"
                       , ppr tv <+> text ":=" <+> ppr ty
                       , text "Old value =" <+> ppr (lookupVarEnv_NF ty_binds tv)]
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            ; TcM.writeTcRef ref (extendVarEnv ty_binds tv (tv,ty)) } }
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\end{code}
Note [Optimizing Spontaneously Solved Coercions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

Spontaneously solved coercions such as alpha := tau used to be bound as everything else
in the evidence binds. Subsequently they were used for rewriting other wanted or solved
goals. For instance: 

WorkItem = [S] g1 : a ~ tau
Inerts   = [S] g2 : b ~ [a]
           [S] g3 : c ~ [(a,a)]

Would result, eventually, after the workitem rewrites the inerts, in the
following evidence bindings:

        g1 = ReflCo tau
        g2 = ReflCo [a]
        g3 = ReflCo [(a,a)]
        g2' = g2 ; [g1] 
        g3' = g3 ; [(g1,g1)]

This ia annoying because it puts way too much stress to the zonker and
desugarer, since we /know/ at the generation time (spontaneously
solving) that the evidence for a particular evidence variable is the
identity.

For this reason, our solution is to cache inside the GivenSolved
flavor of a constraint the term which is actually solving this
constraint. Whenever we perform a setEvBind, a new flavor is returned
so that if it was a GivenSolved to start with, it remains a
GivenSolved with a new evidence term inside. Then, when we use solved
goals to rewrite other constraints we simply use whatever is in the
GivenSolved flavor and not the constraint cc_id.

In our particular case we'd get the following evidence bindings, eventually: 

       g1 = ReflCo tau
       g2 = ReflCo [a]
       g3 = ReflCo [(a,a)]
       g2'= ReflCo [a]
       g3'= ReflCo [(a,a)]

Since we use smart constructors to get rid of g;ReflCo t ~~> g etc.

\begin{code}
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warnTcS :: CtLoc orig -> Bool -> SDoc -> TcS ()
warnTcS loc warn_if doc 
  | warn_if   = wrapTcS $ TcM.setCtLoc loc $ TcM.addWarnTc doc
  | otherwise = return ()

getDefaultInfo ::  TcS (SimplContext, [Type], (Bool, Bool))
getDefaultInfo 
  = do { ctxt <- getTcSContext
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       ; (tys, flags) <- wrapTcS TcM.tcGetDefaultTys
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       ; return (ctxt, tys, flags) }

-- Just get some environments needed for instance looking up and matching
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

getInstEnvs :: TcS (InstEnv, InstEnv) 
getInstEnvs = wrapTcS $ Inst.tcGetInstEnvs 

getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv) 
getFamInstEnvs = wrapTcS $ FamInst.tcGetFamInstEnvs

getTopEnv :: TcS HscEnv 
getTopEnv = wrapTcS $ TcM.getTopEnv 

getGblEnv :: TcS TcGblEnv 
getGblEnv = wrapTcS $ TcM.getGblEnv 

-- Various smaller utilities [TODO, maybe will be absorbed in the instance matcher]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

checkWellStagedDFun :: PredType -> DFunId -> WantedLoc -> TcS () 
checkWellStagedDFun pred dfun_id loc 
  = wrapTcS $ TcM.setCtLoc loc $ 
    do { use_stage <- TcM.getStage
       ; TcM.checkWellStaged pp_thing bind_lvl (thLevel use_stage) }
  where
    pp_thing = ptext (sLit "instance for") <+> quotes (ppr pred)
    bind_lvl = TcM.topIdLvl dfun_id

pprEq :: TcType -> TcType -> SDoc
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pprEq ty1 ty2 = pprType $ mkEqPred ty1 ty2
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isTouchableMetaTyVar :: TcTyVar -> TcS Bool
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  = do { untch <- getUntouchables
       ; return $ isTouchableMetaTyVar_InRange untch tv } 

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isTouchableMetaTyVar_InRange :: TcsUntouchables -> TcTyVar -> Bool 
isTouchableMetaTyVar_InRange (untch,untch_tcs) tv 
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  = ASSERT2 ( isTcTyVar tv, ppr tv )
    case tcTyVarDetails tv of 
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      MetaTv TcsTv _ -> not (tv `elemVarSet` untch_tcs)
                        -- See Note [Touchable meta type variables] 
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      MetaTv {}      -> inTouchableRange untch tv 
      _              -> False 


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\end{code}

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Note [Touchable meta type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Meta type variables allocated *by the constraint solver itself* are always
touchable.  Example: 
   instance C a b => D [a] where...
if we use this instance declaration we "make up" a fresh meta type
variable for 'b', which we must later guess.  (Perhaps C has a
functional dependency.)  But since we aren't in the constraint *generator*
we can't allocate a Unique in the touchable range for this implication
constraint.  Instead, we mark it as a "TcsTv", which makes it always-touchable.
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\begin{code}
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-- Flatten skolems
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

newFlattenSkolemTy :: TcType -> TcS TcType
newFlattenSkolemTy ty = mkTyVarTy <$> newFlattenSkolemTyVar ty
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newFlattenSkolemTyVar :: TcType -> TcS TcTyVar
newFlattenSkolemTyVar ty
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  = do { tv <- wrapTcS $ 
               do { uniq <- TcM.newUnique
                  ; let name = TcM.mkTcTyVarName uniq (fsLit "f")
                  ; return $ mkTcTyVar name (typeKind ty) (FlatSkol ty) } 
       ; traceTcS "New Flatten Skolem Born" $
         ppr tv <+> text "[:= " <+> ppr ty <+> text "]"
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       ; return tv }
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-- Instantiations 
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

instDFunTypes :: [Either TyVar TcType] -> TcS [TcType] 
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instDFunTypes mb_inst_tys 
  = mapM inst_tv mb_inst_tys
  where
    inst_tv :: Either TyVar TcType -> TcS Type
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    inst_tv (Right ty) = return ty 
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instFlexiTcS :: TyVar -> TcS TcTyVar 
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-- Like TcM.instMetaTyVar but the variable that is created is 
-- always touchable; we are supposed to guess its instantiation.
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-- See Note [Touchable meta type variables] 
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instFlexiTcS tv = instFlexiTcSHelper (tyVarName tv) (tyVarKind tv) 
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newFlexiTcSTy :: Kind -> TcS TcType  
newFlexiTcSTy knd 
  = wrapTcS $
    do { uniq <- TcM.newUnique 
       ; ref  <- TcM.newMutVar  Flexi 
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       ; let name = TcM.mkTcTyVarName uniq (fsLit "uf")
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       ; return $ mkTyVarTy (mkTcTyVar name knd (MetaTv TcsTv ref)) }

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isFlexiTcsTv :: TyVar -> Bool
isFlexiTcsTv tv
  | not (isTcTyVar tv)                  = False
  | MetaTv TcsTv _ <- tcTyVarDetails tv = True
  | otherwise                           = False

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instFlexiTcSHelper :: Name -> Kind -> TcS TcTyVar
instFlexiTcSHelper tvname tvkind
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  = wrapTcS $ 
    do { uniq <- TcM.newUnique 
       ; ref  <- TcM.newMutVar  Flexi 
       ; let name = setNameUnique tvname uniq 
             kind = tvkind 
       ; return (mkTcTyVar name kind (MetaTv TcsTv ref)) }
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-- Creating and setting evidence variables and CtFlavors
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

data XEvTerm = 
  XEvTerm { ev_comp   :: [EvVar] -> EvTerm
                         -- How to compose evidence 
          , ev_decomp :: EvVar -> [EvTerm]
                         -- How to decompose evidence 
          }

data MaybeNew a = Fresh  { mn_thing :: a } 
                | Cached { mn_thing :: a }

isFresh :: MaybeNew a -> Bool
isFresh (Fresh {}) = True
isFresh _ = False

setEvBind :: EvVar -> EvTerm -> TcS ()
setEvBind ev t
  = do { tc_evbinds <- getTcEvBinds
       ; wrapTcS $ TcM.addTcEvBind tc_evbinds ev t

#ifdef DEBUG
       ; binds <- getTcEvBindsMap
       ; let cycle = any (reaches binds) (evVarsOfTerm t)
       ; when cycle (fail_if_co_loop binds)
#endif
       ; return () }

#ifdef DEBUG
  where fail_if_co_loop binds
          = pprTrace "setEvBind" (vcat [ text "Cycle in evidence binds, evvar =" <+> ppr ev
                                       , ppr (evBindMapBinds binds) ]) $
            when (isEqVar ev) (pprPanic "setEvBind" (text "BUG: Coercion loop!"))

        reaches :: EvBindMap -> Var -> Bool 
        -- Does this evvar reach ev? 
        reaches ebm ev0 = go ev0
          where go ev0
                  | ev0 == ev = True
                  | Just (EvBind _ evtrm) <- lookupEvBind ebm ev0
                  = any go (evVarsOfTerm evtrm)
                  | otherwise = False
#endif

newGivenEvVar  :: TcPredType -> EvTerm -> TcS (MaybeNew EvVar)
newGivenEvVar pty evterm
  = do { is <- getTcSInerts
       ; case lookupInInerts is pty of
            Just ct | isGivenOrSolvedCt ct 
                    -> return (Cached (ctId "newGivenEvVar" ct))
            _ -> do { new_ev <- wrapTcS $ TcM.newEvVar pty
                    ; setEvBind new_ev evterm
                    ; return (Fresh new_ev) } }

newWantedEvVar :: TcPredType -> TcS (MaybeNew EvVar)
newWantedEvVar pty
  = do { is <- getTcSInerts
       ; case lookupInInerts is pty of
            Just ct | not (isDerivedCt ct) 
                    -> return (Cached (ctId "newWantedEvVar" ct))
            _ -> do { new_ev <- wrapTcS $ TcM.newEvVar pty
                    ; return (Fresh new_ev) } }

newDerived :: TcPredType -> TcS (MaybeNew TcPredType)
newDerived pty
  = do { is <- getTcSInerts
       ; case lookupInInerts is pty of
            Just {} -> return (Cached pty)
            _       -> return (Fresh pty) }
    
newKindConstraint :: TcTyVar -> Kind -> TcS (MaybeNew EvVar)
-- Create new wanted CoVar that constrains the type to have the specified kind. 
newKindConstraint tv knd
  = do { tv_k <- instFlexiTcSHelper (tyVarName tv) knd 
       ; let ty_k = mkTyVarTy tv_k
       ; newWantedEvVar (mkTcEqPred (mkTyVarTy tv) ty_k) }

instDFunConstraints :: TcThetaType -> TcS [MaybeNew EvVar]
instDFunConstraints = mapM newWantedEvVar

                
xCtFlavor :: CtFlavor              -- Original flavor   
          -> [TcPredType]          -- New predicate types
          -> XEvTerm               -- Instructions about how to manipulate evidence
          -> ([CtFlavor] -> TcS a) -- What to do with any remaining /fresh/ goals!
          -> TcS a
xCtFlavor = xCtFlavor_cache True          


xCtFlavor_cache :: Bool -- True = if wanted add to the solved bag!    
          -> CtFlavor              -- Original flavor   
          -> [TcPredType]          -- New predicate types
          -> XEvTerm               -- Instructions about how to manipulate evidence
          -> ([CtFlavor] -> TcS a) -- What to do with any remaining /fresh/ goals!
          -> TcS a
xCtFlavor_cache _ (Given { flav_gloc = gl, flav_evar = evar }) ptys xev cont_with
  = do { let ev_trms = ev_decomp xev evar
       ; new_evars <- zipWithM newGivenEvVar ptys ev_trms
       ; cont_with $
         map (\x -> Given gl (mn_thing x)) (filter isFresh new_evars) }
  
xCtFlavor_cache cache (Wanted { flav_wloc = wl, flav_evar = evar }) ptys xev cont_with
  = do { new_evars <- mapM newWantedEvVar ptys
       ; let evars  = map mn_thing new_evars
             evterm = ev_comp xev evars
       ; setEvBind evar evterm
       ; let solved_flav = Solved { flav_gloc = mkSolvedLoc wl UnkSkol
                                  , flav_evar = evar }
       ; when cache $ addToSolved (mkNonCanonical solved_flav)
       ; cont_with $
         map (\x -> Wanted wl (mn_thing x)) (filter isFresh new_evars) }
    
xCtFlavor_cache _ (Derived { flav_wloc = wl }) ptys _xev cont_with
  = do { ders <- mapM newDerived ptys
       ; cont_with $ 
         map (\x -> Derived wl (mn_thing x)) (filter isFresh ders) }
    
    -- I am not sure I actually want to do this (e.g. from recanonicalizing a solved?)
    -- but if we plan to use xCtFlavor for rewriting as well then I might as well add a case
xCtFlavor_cache _ (Solved { flav_gloc = gl, flav_evar = evar }) ptys xev cont_with
  = do { let ev_trms = ev_decomp xev evar
       ; new_evars <- zipWithM newGivenEvVar ptys ev_trms
       ; cont_with $
         map (\x -> Solved gl (mn_thing x)) (filter isFresh new_evars) }

rewriteCtFlavor :: CtFlavor
                -> TcPredType   -- new predicate
                -> TcCoercion   -- new ~ old     
                -> TcS (Maybe CtFlavor)
rewriteCtFlavor = rewriteCtFlavor_cache True
-- Returns Nothing only if rewriting has happened and the rewritten constraint is cached
-- Returns Just if either (i) we rewrite by reflexivity or 
--                        (ii) we rewrite and original not cached

rewriteCtFlavor_cache :: Bool 
                -> CtFlavor
                -> TcPredType   -- new predicate
                -> TcCoercion   -- new ~ old     
                -> TcS (Maybe CtFlavor)
-- If derived, don't even look at the coercion
-- NB: this allows us to sneak away with ``error'' thunks for 
-- coercions that come from derived ids (which don't exist!) 
rewriteCtFlavor_cache _cache (Derived wl _pty_orig) pty_new _co
  = newDerived pty_new >>= from_mn
  where from_mn (Cached {}) = return Nothing
        from_mn (Fresh {})  = return $ Just (Derived wl pty_new)
        
rewriteCtFlavor_cache cache fl pty co
  | isTcReflCo co
  -- If just reflexivity then you may re-use the same variable as optimization
  = return (Just $ case fl of
               Derived wl _pty_orig -> Derived wl pty
               Given gl ev  -> Given  gl (setVarType ev pty)
               Wanted wl ev -> Wanted wl (setVarType ev pty)
               Solved gl ev -> Solved gl (setVarType ev pty))
  | otherwise 
  = xCtFlavor_cache cache fl [pty] (XEvTerm ev_comp ev_decomp) cont
  where ev_comp [x] = EvCast x co
        ev_comp _   = panic "Coercion can only have one subgoal"
        ev_decomp x = [EvCast x (mkTcSymCo co)]
        cont []     = return Nothing
        cont [fl]   = return $ Just fl
        cont _      = panic "At most one constraint can be subgoal of coercion!"

{- REFACTOR -- HERE HERE HERE 
rewriteCtFlavor :: Ct                   -- Original ct  
                -> PredType             -- New predicate
                -> TcCoercion           -- ctPred ct ~ new_predicate
                -> TcS (Maybe CtFlavor) -- Nothing if we already have a constraint 
                                        -- like this in the solved or in the inert set

rewriteCtFlavor orig_ct pty co
  | isReflCo co
    -- If the coercion is just reflexivity then you may re-use the same variable
  = return (Just $ case cc_flavor orig_ct of
               Derived wl pty_orig -> Derived wl pty
               Given gl ev  -> Given  gl (setVarType ev pty)
               Wanted wl ev -> Wanted wl (setVarType ev pty)
               Solved gl ev -> Solved gl (setVarType ev pty))
  | otherwise 
  = new_ct_flav orig_ct (cc_flavor orig_ct) pty co
  where 
        -- Given
        new_ct_flav _orig_ct (Given { flav_gloc = gl, flav_evar = ev }) pty co          
          = do { new_ev <- wrapTcS $ TcM.newEvVar pty -- 1) Create new variable
               ; setEvBind new_ev (mkEvCast ev co)    -- 2) Set evidence
               ; return $ Just (Given { flav_gloc = gl, flav_evar = new_ev }) }
                                                      -- 3) Return new given
        -- Wanted
        new_ct_flav orig_ct (Wanted { flav_wloc = wl, flav_evar = ev }) pty co
          = do { is <- getTcSInerts
               ; case lookupInInerts is pty of
                   Just ct | not (isDerived ct) -> -- Cached (and has a ctId)
                     do { setEvBind ev (mkCast (ctId ct) (mkSymCo co))
                        ; return Nothing }
                   _ ->                            -- Not Cached!
                     do { new_ev <- wrapTcS $ TcM.newEvVar pty      -- 1) Create new variable
                        ; setEvBind ev (mkCast new_ev (mkSymCo co)) -- 2) Solve old from new
                        ; let gl = mkSolvedLoc wl UnkSkol
                        ; addToSolved (orig_ct { cc_flavor = Solved gl ev }) 
                                                                    -- 3) Add old as solved
                        ; return (Just (Wanted { flav_wloc = wl     -- 4) Return new wanted
                                               , flav_evar = new_ev })) } }
        -- Derived
        new_ct_flav _orig_ct (Derived { flav_wloc = wl, flav_der_pty = _pty }) pty _co
          = do { is <- getTcSInerts
               ; case lookupInInerts is pty of
                 Just ct -> return Nothing -- Some other constraint already there
                 _ -> return $ 
                      Just (Derived {flav_wloc = wl, flav_der_pty = pty }) }
            
        -- Solved
        new_ct_flav _orig_ct (Solved { flav_gloc = gl, flav_evar = ev }) pty co
          = do { new_ev <- wrapTcS $ TcM.newEvVar pty      -- 1) Create new variable
               ; setEvBind new_ev (mkEvCast ev co)         -- 2) Set new evidence         
               ; let new_fl = Solved gl new_ev
               ; return $ Just new_fl } -- 3) Return. NB: no need to addToSolved here




newGivenFlavor :: GivenLoc -> TcPredType -> TcS CtFlavor
newGivenFlavor gl pty 
  = do { new_ev <- wrapTcS $ TcM.newEvVar pty
       ; return (Given gl pty) }

newWantedFlavor :: WantedLoc 
               -> TcPredType 
               -> TcS (Either CtFlavor Ct)  
-- Returns either a new flavor or a cached constraint
newWantedFlavor wl pty
  = do { is <- getTcSInerts
       ; case lookupInInerts is pty of 
            Just ct | not (isDerived ct) -> return (Right ct)
            _ -> do { new_ev <- wrapTcS $ TcM.newEvVar pty
                    ; return $ Left (Wanted wl new_ev) } }
    
newDerivedFlavor :: WantedLoc
                 -> TcPredType -> TcS CtFlavor
newDerivedFlavor wl pty = return (Derived wl pty)


newCtFlavor :: CtFlavor -> TcPredType -> TcS CtFlavor
-- CtFlavor might not necessarily be fresh if constraint is cached
newCtFlavor (Wanted { flav_wloc = wl }) pty 
  = do { lr <- newWantedFlavor wl pty 
       ; case lr of Left fl  -> return fl
                    Right ct -> return (cc_flavor ct) }
newCtFlavor (Given { flav_gloc = gl }) pty
  = newGivenFlavor gl pty
newCtFlavor (Derived { flav_wloc = wl }) pty
  = newDerivedFlavor wl pty

-} 

{- DELETEME 
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data EvVarCreated 
  = EvVarCreated { evc_is_new    :: Bool    -- True iff the variable was just created
                 , evc_the_evvar :: EvVar } -- The actual evidence variable could be cached or new

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instance Outputable EvVarCreated where
  ppr (EvVarCreated { evc_is_new = is_new, evc_the_evvar = ev })
    = ppr ev <> parens (if is_new then ptext (sLit "new") else ptext (sLit "old"))
  
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newKindConstraint :: TcTyVar -> Kind -> CtFlavor -> TcS EvVarCreated
-- Create new wanted CoVar that constrains the type to have the specified kind. 
newKindConstraint tv knd fl
  = do { tv_k <- instFlexiTcSHelper (tyVarName tv) knd 
       ; let ty_k = mkTyVarTy tv_k
       ; eqv <- newEqVar fl (mkTyVarTy tv) ty_k
       ; return eqv }


setEqBind :: EqVar -> TcCoercion -> CtFlavor -> TcS CtFlavor
setEqBind eqv co fl = setEvBind eqv (EvCoercion co) fl

setEvBind :: EvVar -> EvTerm -> CtFlavor -> TcS CtFlavor
-- If the flavor is Solved, we cache the new evidence term inside the returned flavor
-- see Note [Optimizing Spontaneously Solved Coercions]
setEvBind ev t fl
  = do { tc_evbinds <- getTcEvBinds
       ; wrapTcS $ TcM.addTcEvBind tc_evbinds ev t

#ifdef DEBUG
       ; binds <- getTcEvBindsMap
       ; let cycle = any (reaches binds) (evVarsOfTerm t)
       ; when cycle (fail_if_co_loop binds)
#endif
       ; return $ 
         case fl of 
           Given gl (GivenSolved _) 
               -> Given gl (GivenSolved (Just t))
           _   -> fl
       }

#ifdef DEBUG
  where fail_if_co_loop binds
          = pprTrace "setEvBind" (vcat [ text "Cycle in evidence binds, evvar =" <+> ppr ev
                                       , ppr (evBindMapBinds binds) ]) $
            when (isEqVar ev) (pprPanic "setEvBind" (text "BUG: Coercion loop!"))

        reaches :: EvBindMap -> Var -> Bool 
        -- Does this evvar reach ev? 
        reaches ebm ev0 = go ev0
          where go ev0
                  | ev0 == ev = True
                  | Just (EvBind _ evtrm) <- lookupEvBind ebm ev0
                  = any go (evVarsOfTerm evtrm)
                  | otherwise = False
#endif


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isNewEvVar :: EvVarCreated -> Bool
isNewEvVar = evc_is_new

newEvVar :: CtFlavor -> TcPredType -> TcS EvVarCreated
-- Post: If Given then evc_is_new is True
-- Hence it is safe to do a setEvBind right after a newEvVar with a Given flavor
-- NB: newEvVar may temporarily break the TcSEnv invariant but it is expected in 
--     the call sites for this invariant to be quickly restored.
newEvVar fl pty
  | isGivenOrSolved fl    -- Create new variable and update the cache
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  = do { 
{- We lose a lot of time if we enable this check:
         eref <- getTcSEvVarCache
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       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; case lookupTM pty (evc_cache ecache) of
           Just (_,cached_fl) 
               | cached_fl `canSolve` fl 
               -> pprTrace "Interesting: given newEvVar, missed caching opportunity!" empty $
                  return ()
           _ -> return ()
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-}
         new <- forceNewEvVar fl pty
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       ; return (EvVarCreated True new) }

  | otherwise             -- Otherwise lookup first
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  = {-# SCC "newEvVarWanted" #-}
    do { eref <- getTcSEvVarCache
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       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; case lookupTM pty (evc_cache ecache) of
           Just (cached_evvar, cached_flavor)
             | cached_flavor `canSolve` fl -- NB: 
                                           -- We want to use the cache /only/ if he can solve
                                           -- the workitem. If cached_flavor is Derived
                                           -- but we have a real Wanted, we want to create
                                           -- new evidence, otherwise we are in danger to
                                           -- have unsolved goals in the end. 
                                           -- (Remember: Derived's are just unification hints
                                           --            but they don't come with guarantees
                                           --            that they can be solved and we don't 
                                           --            quantify over them.
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             -> do { traceTcS "newEvVar: already cached, doing nothing" 
                              (ppr (evc_cache ecache))
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                   ; return (EvVarCreated False cached_evvar) }
           _   -- Not cached or cached with worse flavor
             -> do { new <- force_new_ev_var eref ecache fl pty
                   ; return (EvVarCreated True new) } }

forceNewEvVar :: CtFlavor -> TcPredType -> TcS EvVar
-- Create a new EvVar, regardless of whether or not the
-- cache already contains one like it, and update the cache
forceNewEvVar fl pty 
  = do { eref   <- getTcSEvVarCache
       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; force_new_ev_var eref ecache fl pty }

force_new_ev_var :: IORef EvVarCache -> EvVarCache -> CtFlavor -> TcPredType -> TcS EvVar
-- Create a new EvVar, and update the cache with it
force_new_ev_var eref ecache fl pty
  = wrapTcS $
    do { TcM.traceTc "newEvVar" $ text "updating cache"

       ; new_evvar <-TcM.newEvVar pty
            -- This is THE PLACE where we finally call TcM.newEvVar

       ; let new_cache = updateCache ecache (new_evvar,fl,pty)
       ; TcM.writeTcRef eref new_cache 
       ; return new_evvar }

updateCache :: EvVarCache -> (EvVar,CtFlavor,Type) -> EvVarCache
updateCache ecache (ev,fl,pty)
  | IPPred {} <- classifier
  = ecache
  | otherwise
  = ecache { evc_cache = ecache' }
  where classifier = classifyPredType pty
        ecache'    = alterTM pty (\_ -> Just (ev,fl)) $
                     evc_cache ecache

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delCachedEvVar :: EvVar -> CtFlavor -> TcS ()
delCachedEvVar ev _fl
  = {-# SCC "delCachedEvVarOther" #-}
    do { eref   <- getTcSEvVarCache
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       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; wrapTcS $ TcM.writeTcRef eref (delFromCache ecache ev) }

delFromCache :: EvVarCache -> EvVar -> EvVarCache 
delFromCache (EvVarCache { evc_cache      = ecache
                         , evc_flat_cache = flat_cache }) ev
  = EvVarCache { evc_cache = ecache', evc_flat_cache = flat_cache }
  where ecache' = alterTM pty x_del ecache
        x_del Nothing = Nothing
        x_del r@(Just (ev0,_))
           | ev0 == ev = Nothing
           | otherwise = r
        pty = evVarPred ev



updateFlatCache :: EvVar -> CtFlavor 
                -> TyCon -> [Xi] -> TcType 
                -> FlatEqOrigin
                -> TcS () 
updateFlatCache ev fl fn xis rhs_ty feq_origin
  = do { eref <- getTcSEvVarCache
       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; let flat_cache     = evc_flat_cache ecache
             new_flat_cache = alterTM fun_ty x_flat_cache flat_cache
             new_evc = ecache { evc_flat_cache = new_flat_cache }
       ; wrapTcS $ TcM.writeTcRef eref new_evc }
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  where x_flat_cache _ = Just (mkTcCoVarCo ev,(rhs_ty,fl,feq_origin))
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        fun_ty = mkTyConApp fn xis


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pprEvVarCache :: TypeMap (TcCoercion,a) -> SDoc
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pprEvVarCache tm = ppr (foldTM mk_pair tm [])
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 where mk_pair (co,_) cos = (co, tcCoercionKind co) : cos
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newGivenEqVar :: CtFlavor -> TcType -> TcType -> TcCoercion -> TcS (CtFlavor,EvVar)
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-- Pre: fl is Given
newGivenEqVar fl ty1 ty2 co 
  = do { ecv <- newEqVar fl ty1 ty2
       ; let v = evc_the_evvar ecv -- Will be a new EvVar by post of newEvVar
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       ; fl' <- setEvBind v (EvCoercion co) fl
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       ; return (fl',v) }
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newEqVar :: CtFlavor -> TcType -> TcType -> TcS EvVarCreated
newEqVar fl ty1 ty2