TcInteract.lhs 97.1 KB
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\begin{code}
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{-# OPTIONS -fno-warn-tabs #-}
-- 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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module TcInteract ( 
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     solveInteract, solveInteractGiven, solveInteractWanted,
     AtomicInert, tyVarsOfInert, 
     InertSet, emptyInert, updInertSet, extractUnsolved, solveOne,
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  ) where  

#include "HsVersions.h"

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import BasicTypes 
import TcCanonical
import VarSet
import Type
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import Unify
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import Id 
import Var

import TcType
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import HsBinds
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import Inst( tyVarsOfEvVar )
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import Class
import TyCon
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import Name

import FunDeps

import Coercion
import Outputable

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import TcRnTypes
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import TcErrors
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import TcSMonad
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import Maybes( orElse )
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import Bag
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import Control.Monad( when )
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import Unique
import UniqFM
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import FastString ( sLit ) 
import DynFlags
\end{code}

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Note [InertSet invariants]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
An InertSet is a bag of canonical constraints, with the following invariants:

  1 No two constraints react with each other. 
    
    A tricky case is when there exists a given (solved) dictionary 
    constraint and a wanted identical constraint in the inert set, but do 
    not react because reaction would create loopy dictionary evidence for 
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    the wanted. See note [Recursive instances and superclases]
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  2 Given equalities form an idempotent substitution [none of the
    given LHS's occur in any of the given RHS's or reactant parts]

  3 Wanted equalities also form an idempotent substitution
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  4 The entire set of equalities is acyclic.

  5 Wanted dictionaries are inert with the top-level axiom set 

  6 Equalities of the form tv1 ~ tv2 always have a touchable variable
    on the left (if possible).
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  7 No wanted constraints tv1 ~ tv2 with tv1 touchable. Such constraints
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    will be marked as solved right before being pushed into the inert set. 
    See note [Touchables and givens].
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  8 No Given constraint mentions a touchable unification variable, but 
    Given/Solved may do so. 

  9 Given constraints will also have their superclasses in the inert set, 
    but Given/Solved will not. 
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Note that 6 and 7 are /not/ enforced by canonicalization but rather by 
insertion in the inert list, ie by TcInteract. 

During the process of solving, the inert set will contain some
previously given constraints, some wanted constraints, and some given
constraints which have arisen from solving wanted constraints. For
now we do not distinguish between given and solved constraints.

Note that we must switch wanted inert items to given when going under an
implication constraint (when in top-level inference mode).

\begin{code}

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data CCanMap a = CCanMap { cts_given   :: UniqFM CanonicalCts
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                                          -- Invariant: all Given
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                         , cts_derived :: UniqFM CanonicalCts 
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                                          -- Invariant: all Derived
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                         , cts_wanted  :: UniqFM CanonicalCts } 
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                                          -- Invariant: all Wanted
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cCanMapToBag :: CCanMap a -> CanonicalCts 
cCanMapToBag cmap = foldUFM unionBags rest_wder (cts_given cmap)
  where rest_wder = foldUFM unionBags rest_der  (cts_wanted cmap) 
        rest_der  = foldUFM unionBags emptyCCan (cts_derived cmap)
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emptyCCanMap :: CCanMap a 
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emptyCCanMap = CCanMap { cts_given = emptyUFM, cts_derived = emptyUFM, cts_wanted = emptyUFM } 
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updCCanMap:: Uniquable a => (a,CanonicalCt) -> CCanMap a -> CCanMap a 
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updCCanMap (a,ct) cmap 
  = case cc_flavor ct of 
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      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) }
  where 
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    insert_into m = addToUFM_C unionBags m a (singleCCan ct)
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getRelevantCts :: Uniquable a => a -> CCanMap a -> (CanonicalCts, CCanMap a) 
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-- Gets the relevant constraints and returns the rest of the CCanMap
getRelevantCts a cmap 
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    = 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 }
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      in (relevant, residual_map) 
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  where
    lookup map = lookupUFM map a `orElse` emptyCCan
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extractUnsolvedCMap :: CCanMap a -> (CanonicalCts, CCanMap a)
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-- Gets the wanted or derived constraints and returns a residual
-- CCanMap with only givens.
extractUnsolvedCMap cmap =
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  let wntd = foldUFM unionBags emptyCCan (cts_wanted cmap)
      derd = foldUFM unionBags emptyCCan (cts_derived cmap)
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  in (wntd `unionBags` derd, 
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      cmap { cts_wanted = emptyUFM, cts_derived = emptyUFM })
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-- See Note [InertSet invariants]
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data InertSet 
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  = IS { inert_eqs          :: CanonicalCts               -- Equalities only (CTyEqCan)
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       , inert_dicts        :: CCanMap Class              -- Dictionaries only
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       , inert_ips          :: CCanMap (IPName Name)      -- Implicit parameters
       , inert_irreds       :: CanonicalCts               -- Irreducible predicates
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       , inert_frozen       :: CanonicalCts
       , inert_funeqs       :: CCanMap TyCon              -- Type family equalities only
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               -- This representation allows us to quickly get to the relevant 
               -- inert constraints when interacting a work item with the inert set.
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       }
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tyVarsOfInert :: InertSet -> TcTyVarSet 
tyVarsOfInert (IS { inert_eqs    = eqs
                  , inert_dicts  = dictmap
                  , inert_ips    = ipmap
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                  , inert_irreds = irreds
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                  , inert_frozen = frozen
                  , inert_funeqs = funeqmap }) = tyVarsOfCanonicals cts
  where
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    cts = eqs `andCCan` frozen `andCCan` irreds `andCCan` cCanMapToBag dictmap
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              `andCCan` cCanMapToBag ipmap `andCCan` cCanMapToBag funeqmap
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instance Outputable InertSet where
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  ppr is = vcat [ vcat (map ppr (Bag.bagToList $ inert_eqs is))
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                , vcat (map ppr (Bag.bagToList $ inert_irreds is)) 
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                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_dicts is)))
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                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_ips is))) 
                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_funeqs is)))
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                , text "Frozen errors =" <+> -- Clearly print frozen errors
                    vcat (map ppr (Bag.bagToList $ inert_frozen is))
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                ]
                       
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emptyInert :: InertSet
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emptyInert = IS { inert_eqs    = Bag.emptyBag
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                , inert_frozen = Bag.emptyBag
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                , inert_irreds = Bag.emptyBag
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                , inert_dicts  = emptyCCanMap
                , inert_ips    = emptyCCanMap
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                , inert_funeqs = emptyCCanMap }
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updInertSet :: InertSet -> AtomicInert -> InertSet 
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updInertSet is item 
  | isCTyEqCan item                     -- Other equality 
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  = let eqs' = inert_eqs is `Bag.snocBag` item
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    in is { inert_eqs = eqs' } 
  | Just cls <- isCDictCan_Maybe item   -- Dictionary 
  = is { inert_dicts = updCCanMap (cls,item) (inert_dicts is) } 
  | Just x  <- isCIPCan_Maybe item      -- IP 
  = is { inert_ips   = updCCanMap (x,item) (inert_ips is) }  
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  | isCIrredEvCan item                     -- Presently-irreducible evidence
  = is { inert_irreds = inert_irreds is `Bag.snocBag` item }
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  | Just tc <- isCFunEqCan_Maybe item   -- Function equality 
  = is { inert_funeqs = updCCanMap (tc,item) (inert_funeqs is) }
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  | otherwise 
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  = is { inert_frozen = inert_frozen is `Bag.snocBag` item }
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extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
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-- Postcondition: the returned canonical cts are either Derived, or Wanted.
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extractUnsolved is@(IS {inert_eqs = eqs, inert_irreds = irreds}) 
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  = let is_solved  = is { inert_eqs    = solved_eqs
                        , inert_dicts  = solved_dicts
                        , inert_ips    = solved_ips
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                        , inert_irreds = solved_irreds
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                        , inert_frozen = emptyCCan
                        , inert_funeqs = solved_funeqs }
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    in (is_solved, unsolved)
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  where (unsolved_eqs, solved_eqs)       = Bag.partitionBag (not.isGivenOrSolvedCt) eqs
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        (unsolved_irreds, solved_irreds) = Bag.partitionBag (not.isGivenOrSolvedCt) irreds
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        (unsolved_ips, solved_ips)       = extractUnsolvedCMap (inert_ips is) 
        (unsolved_dicts, solved_dicts)   = extractUnsolvedCMap (inert_dicts is) 
        (unsolved_funeqs, solved_funeqs) = extractUnsolvedCMap (inert_funeqs is) 
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        unsolved = unsolved_eqs `unionBags` inert_frozen is `unionBags` unsolved_irreds `unionBags`
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                   unsolved_ips `unionBags` unsolved_dicts `unionBags` unsolved_funeqs
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\end{code}

%*********************************************************************
%*                                                                   * 
*                      Main Interaction Solver                       *
*                                                                    *
**********************************************************************

Note [Basic plan] 
~~~~~~~~~~~~~~~~~
1. Canonicalise (unary)
2. Pairwise interaction (binary)
    * Take one from work list 
    * Try all pair-wise interactions with each constraint in inert
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   As an optimisation, we prioritize the equalities both in the 
   worklist and in the inerts. 

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3. Try to solve spontaneously for equalities involving touchables 
4. Top-level interaction (binary wrt top-level)
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   Superclass decomposition belongs in (1), see note [Adding superclasses]
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\begin{code}
type AtomicInert = CanonicalCt     -- constraint pulled from InertSet
type WorkItem    = CanonicalCt     -- constraint pulled from WorkList

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------------------------
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data StopOrContinue 
  = Stop			-- Work item is consumed
  | ContinueWith WorkItem	-- Not consumed

instance Outputable StopOrContinue where
  ppr Stop             = ptext (sLit "Stop")
  ppr (ContinueWith w) = ptext (sLit "ContinueWith") <+> ppr w

-- Results after interacting a WorkItem as far as possible with an InertSet
data StageResult
  = SR { sr_inerts     :: InertSet
           -- The new InertSet to use (REPLACES the old InertSet)
       , sr_new_work   :: WorkList
           -- Any new work items generated (should be ADDED to the old WorkList)
           -- Invariant: 
           --    sr_stop = Just workitem => workitem is *not* in sr_inerts and
           --                               workitem is inert wrt to sr_inerts
       , sr_stop       :: StopOrContinue
       }

instance Outputable StageResult where
  ppr (SR { sr_inerts = inerts, sr_new_work = work, sr_stop = stop })
    = ptext (sLit "SR") <+> 
      braces (sep [ ptext (sLit "inerts =") <+> ppr inerts <> comma
             	  , ptext (sLit "new work =") <+> ppr work <> comma
             	  , ptext (sLit "stop =") <+> ppr stop])

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type SubGoalDepth = Int	  -- Starts at zero; used to limit infinite
     		    	  -- recursion of sub-goals
type SimplifierStage = SubGoalDepth -> WorkItem -> InertSet -> TcS StageResult 
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-- Combine a sequence of simplifier 'stages' to create a pipeline 
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runSolverPipeline :: SubGoalDepth
                  -> [(String, SimplifierStage)]
		  -> InertSet -> WorkItem 
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                  -> TcS (InertSet, WorkList)
-- Precondition: non-empty list of stages 
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runSolverPipeline depth pipeline inerts workItem
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  = do { traceTcS "Start solver pipeline" $ 
            vcat [ ptext (sLit "work item =") <+> ppr workItem
                 , ptext (sLit "inerts    =") <+> ppr inerts]

       ; let itr_in = SR { sr_inerts = inerts
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                         , sr_new_work = emptyWorkList
                         , sr_stop = ContinueWith workItem }
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       ; itr_out <- run_pipeline pipeline itr_in
       ; let new_inert 
              = case sr_stop itr_out of 
       	          Stop              -> sr_inerts itr_out
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                  ContinueWith item -> sr_inerts itr_out `updInertSet` item
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       ; return (new_inert, sr_new_work itr_out) }
  where 
    run_pipeline :: [(String, SimplifierStage)]
                 -> StageResult -> TcS StageResult
    run_pipeline [] itr                         = return itr
    run_pipeline _  itr@(SR { sr_stop = Stop }) = return itr

    run_pipeline ((name,stage):stages) 
                 (SR { sr_new_work = accum_work
                     , sr_inerts   = inerts
                     , sr_stop     = ContinueWith work_item })
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      = do { itr <- stage depth work_item inerts 
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           ; traceTcS ("Stage result (" ++ name ++ ")") (ppr itr)
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           ; let itr' = itr { sr_new_work = accum_work `unionWorkList` sr_new_work itr }
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           ; run_pipeline stages itr' }
\end{code}

Example 1:
  Inert:   {c ~ d, F a ~ t, b ~ Int, a ~ ty} (all given)
  Reagent: a ~ [b] (given)

React with (c~d)     ==> IR (ContinueWith (a~[b]))  True    []
React with (F a ~ t) ==> IR (ContinueWith (a~[b]))  False   [F [b] ~ t]
React with (b ~ Int) ==> IR (ContinueWith (a~[Int]) True    []

Example 2:
  Inert:  {c ~w d, F a ~g t, b ~w Int, a ~w ty}
  Reagent: a ~w [b]

React with (c ~w d)   ==> IR (ContinueWith (a~[b]))  True    []
React with (F a ~g t) ==> IR (ContinueWith (a~[b]))  True    []    (can't rewrite given with wanted!)
etc.

Example 3:
  Inert:  {a ~ Int, F Int ~ b} (given)
  Reagent: F a ~ b (wanted)

React with (a ~ Int)   ==> IR (ContinueWith (F Int ~ b)) True []
React with (F Int ~ b) ==> IR Stop True []    -- after substituting we re-canonicalize and get nothing

\begin{code}
-- Main interaction solver: we fully solve the worklist 'in one go', 
-- returning an extended inert set.
--
-- See Note [Touchables and givens].
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solveInteractGiven :: InertSet -> GivenLoc -> [EvVar] -> TcS InertSet
solveInteractGiven inert gloc evs
  = do { (_, inert_ret) <- solveInteract inert $ listToBag $
                           map mk_given evs
       ; return inert_ret }
  where
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    flav = Given gloc GivenOrig
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    mk_given ev = mkEvVarX ev flav

solveInteractWanted :: InertSet -> [WantedEvVar] -> TcS InertSet
solveInteractWanted inert wvs
  = do { (_,inert_ret) <- solveInteract inert $ listToBag $
                          map wantedToFlavored wvs
       ; return inert_ret }

solveInteract :: InertSet -> Bag FlavoredEvVar -> TcS (Bool, InertSet)
-- Post: (True,  inert_set) means we managed to discharge all constraints
--                          without actually doing any interactions!
--       (False, inert_set) means some interactions occurred
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solveInteract inert ws 
  = do { dyn_flags <- getDynFlags
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       ; sctx <- getTcSContext

       ; traceTcS "solveInteract, before clever canonicalization:" $
         vcat [ text "ws = " <+>  ppr (mapBag (\(EvVarX ev ct)
                                                   -> (ct,evVarPred ev)) ws)
              , text "inert = " <+> ppr inert ]

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       ; can_ws <- mkCanonicalFEVs ws

       ; (flag, inert_ret)
           <- foldrWorkListM (tryPreSolveAndInteract sctx dyn_flags) (True,inert) can_ws
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       ; traceTcS "solveInteract, after clever canonicalization (and interaction):" $
         vcat [ text "No interaction happened = " <+> ppr flag
              , text "inert_ret = " <+> ppr inert_ret ]

       ; return (flag, inert_ret) }

tryPreSolveAndInteract :: SimplContext
                       -> DynFlags
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                       -> CanonicalCt
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                       -> (Bool, InertSet)
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                       -> TcS (Bool, InertSet)
-- Returns: True if it was able to discharge this constraint AND all previous ones
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tryPreSolveAndInteract sctx dyn_flags ct (all_previous_discharged, inert)
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  = do { let inert_cts = get_inert_cts (predTypePredTree (evVarPred ev_var))
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       ; this_one_discharged <- 
           if isCFrozenErr ct then 
               return False
           else
               dischargeFromCCans inert_cts ev_var fl
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       ; if this_one_discharged
         then return (all_previous_discharged, inert)
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         else do
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       { inert_ret <- solveOneWithDepth (ctxtStkDepth dyn_flags,0,[]) ct inert
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       ; return (False, inert_ret) } }

  where
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    ev_var = cc_id ct
    fl = cc_flavor ct 

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    get_inert_cts (ClassPred clas _)
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      | simplEqsOnly sctx = emptyCCan
      | otherwise         = fst (getRelevantCts clas (inert_dicts inert))
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    get_inert_cts (IPPred {})
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      = emptyCCan -- We must not do the same thing for IParams, because (contrary
                  -- to dictionaries), work items /must/ override inert items.
                 -- See Note [Overriding implicit parameters] in TcInteract.
    get_inert_cts (EqPred {})
      = inert_eqs inert `unionBags` cCanMapToBag (inert_funeqs inert)
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    get_inert_cts (TuplePred ts)
      = andCCans $ map get_inert_cts ts
    get_inert_cts (IrredPred {})
      = inert_irreds inert
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dischargeFromCCans :: CanonicalCts -> EvVar -> CtFlavor -> TcS Bool
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-- See if this (pre-canonicalised) work-item is identical to a 
-- one already in the inert set. Reasons:
--    a) Avoid creating superclass constraints for millions of incoming (Num a) constraints
--    b) Termination for improve_eqs in TcSimplify.simpl_loop
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dischargeFromCCans cans ev fl
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  = Bag.foldrBag discharge_ct (return False) cans
  where 
    the_pred = evVarPred ev

    discharge_ct :: CanonicalCt -> TcS Bool -> TcS Bool
    discharge_ct ct _rest
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      | evVarPred (cc_id ct) `eqPred` the_pred
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      , cc_flavor ct `canSolve` fl
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      = do { when (isWanted fl) $ setEvBind ev (EvId (cc_id ct))
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           	 -- Deriveds need no evidence
    	         -- For Givens, we already have evidence, and we don't need it twice 
           ; return True }

    discharge_ct _ct rest = rest
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\end{code}

Note [Avoiding the superclass explosion] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
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This note now is not as significant as it used to be because we no
longer add the superclasses of Wanted as Derived, except only if they
have equality superclasses or superclasses with functional
dependencies. The fear was that hundreds of identical wanteds would
give rise each to the same superclass or equality Derived's which
would lead to a blo-up in the number of interactions.

Instead, what we do with tryPreSolveAndCanon, is when we encounter a
new constraint, we very quickly see if it can be immediately
discharged by a class constraint in our inert set or the previous
canonicals. If so, we add nothing to the returned canonical
constraints.
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\begin{code}
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solveOne :: WorkItem -> InertSet -> TcS InertSet 
solveOne workItem inerts 
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  = do { dyn_flags <- getDynFlags
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       ; solveOneWithDepth (ctxtStkDepth dyn_flags,0,[]) workItem inerts
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       }

-----------------
solveInteractWithDepth :: (Int, Int, [WorkItem])
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                       -> WorkList -> InertSet -> TcS InertSet
solveInteractWithDepth ctxt@(max_depth,n,stack) ws inert
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  | isEmptyWorkList ws
  = return inert

  | n > max_depth 
  = solverDepthErrorTcS n stack

  | otherwise 
  = do { traceTcS "solveInteractWithDepth" $ 
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              vcat [ text "Current depth =" <+> ppr n
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                   , text "Max depth =" <+> ppr max_depth
                   , text "ws =" <+> ppr ws ]
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       ; foldrWorkListM (solveOneWithDepth ctxt) inert ws }
              -- use foldr to preserve the order
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------------------
-- Fully interact the given work item with an inert set, and return a
-- new inert set which has assimilated the new information.
solveOneWithDepth :: (Int, Int, [WorkItem])
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                  -> WorkItem -> InertSet -> TcS InertSet
solveOneWithDepth (max_depth, depth, stack) work inert
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  = do { traceFireTcS depth (text "Solving {" <+> ppr work)
       ; (new_inert, new_work) <- runSolverPipeline depth thePipeline inert work
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	 -- Recursively solve the new work generated 
         -- from workItem, with a greater depth
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       ; res_inert <- solveInteractWithDepth (max_depth, depth+1, work:stack) new_work new_inert 
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       ; traceFireTcS depth (text "Done }" <+> ppr work) 

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       ; return res_inert }

thePipeline :: [(String,SimplifierStage)]
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thePipeline = [ ("interact with inert eqs", interactWithInertEqsStage)
              , ("interact with inerts",    interactWithInertsStage)
              , ("spontaneous solve",       spontaneousSolveStage)
              , ("top-level reactions",     topReactionsStage) ]
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\end{code}

*********************************************************************************
*                                                                               * 
                       The spontaneous-solve Stage
*                                                                               *
*********************************************************************************

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Note [Efficient Orientation] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are two cases where we have to be careful about 
orienting equalities to get better efficiency. 

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Case 1: In Rewriting Equalities (function rewriteEqLHS) 
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    When rewriting two equalities with the same LHS:
          (a)  (tv ~ xi1) 
          (b)  (tv ~ xi2) 
    We have a choice of producing work (xi1 ~ xi2) (up-to the
    canonicalization invariants) However, to prevent the inert items
    from getting kicked out of the inerts first, we prefer to
    canonicalize (xi1 ~ xi2) if (b) comes from the inert set, or (xi2
    ~ xi1) if (a) comes from the inert set.
    
    This choice is implemented using the WhichComesFromInert flag. 
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Case 2: Functional Dependencies 
    Again, we should prefer, if possible, the inert variables on the RHS

Case 3: IP improvement work
    We must always rewrite so that the inert type is on the right. 
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\begin{code}
spontaneousSolveStage :: SimplifierStage 
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spontaneousSolveStage depth workItem inerts 
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  = do { mSolve <- trySpontaneousSolve workItem

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       ; case mSolve of 
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           SPCantSolve -> -- No spontaneous solution for him, keep going
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               return $ SR { sr_new_work   = emptyWorkList
                           , sr_inerts     = inerts
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                           , sr_stop       = ContinueWith workItem }

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           SPSolved workItem'
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               | not (isGivenOrSolvedCt workItem) 
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	       	 -- Original was wanted or derived but we have now made him 
                 -- given so we have to interact him with the inerts due to
                 -- its status change. This in turn may produce more work.
		 -- We do this *right now* (rather than just putting workItem'
		 -- back into the work-list) because we've solved 
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               -> do { bumpStepCountTcS
	       	     ; traceFireTcS depth (ptext (sLit "Spontaneous (w/d)") <+> ppr workItem)
                     ; (new_inert, new_work) <- runSolverPipeline depth
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                             [ ("recursive interact with inert eqs", interactWithInertEqsStage)
                             , ("recursive interact with inerts", interactWithInertsStage)
                             ] inerts workItem'
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                     ; return $ SR { sr_new_work = new_work 
                                   , sr_inerts   = new_inert -- will include workItem' 
                                   , sr_stop     = Stop }
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                     }
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               | otherwise 
                   -> -- Original was given; he must then be inert all right, and
                      -- workList' are all givens from flattening
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                      do { bumpStepCountTcS
	       	         ; traceFireTcS depth (ptext (sLit "Spontaneous (g)") <+> ppr workItem)
                         ; return $ SR { sr_new_work = emptyWorkList
                                       , sr_inerts   = inerts `updInertSet` workItem' 
                                       , sr_stop     = Stop } }
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           SPError -> -- Return with no new work
               return $ SR { sr_new_work = emptyWorkList
                           , sr_inerts   = inerts
                           , sr_stop     = Stop }
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       }
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data SPSolveResult = SPCantSolve | SPSolved WorkItem | SPError
-- SPCantSolve means that we can't do the unification because e.g. the variable is untouchable
-- SPSolved workItem' gives us a new *given* to go on 
-- SPError means that it's completely impossible to solve this equality, eg due to a kind error


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-- @trySpontaneousSolve wi@ solves equalities where one side is a
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-- touchable unification variable.
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--     	    See Note [Touchables and givens] 
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trySpontaneousSolve :: WorkItem -> TcS SPSolveResult
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trySpontaneousSolve workItem@(CTyEqCan { cc_id = eqv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi })
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  | isGivenOrSolved gw
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  = return SPCantSolve
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  | Just tv2 <- tcGetTyVar_maybe xi
  = do { tch1 <- isTouchableMetaTyVar tv1
       ; tch2 <- isTouchableMetaTyVar tv2
       ; case (tch1, tch2) of
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           (True,  True)  -> trySpontaneousEqTwoWay eqv gw tv1 tv2
           (True,  False) -> trySpontaneousEqOneWay eqv gw tv1 xi
           (False, True)  -> trySpontaneousEqOneWay eqv gw tv2 (mkTyVarTy tv1)
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	   _ -> return SPCantSolve }
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  | otherwise
  = do { tch1 <- isTouchableMetaTyVar tv1
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       ; if tch1 then trySpontaneousEqOneWay eqv gw tv1 xi
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                 else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:" 
                                    (ppr workItem) 
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                         ; return SPCantSolve }
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       }
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  -- No need for 
  --      trySpontaneousSolve (CFunEqCan ...) = ...
  -- See Note [No touchables as FunEq RHS] in TcSMonad
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trySpontaneousSolve _ = return SPCantSolve
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----------------
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trySpontaneousEqOneWay :: EqVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
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-- tv is a MetaTyVar, not untouchable
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trySpontaneousEqOneWay eqv gw tv xi	
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  | not (isSigTyVar tv) || isTyVarTy xi 
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  = do { let kxi = typeKind xi -- NB: 'xi' is fully rewritten according to the inerts 
                               -- so we have its more specific kind in our hands
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       ; if kxi `isSubKind` tyVarKind tv then
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             solveWithIdentity eqv gw tv xi
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         else return SPCantSolve
{-
         else if tyVarKind tv `isSubKind` kxi then
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             return SPCantSolve -- kinds are compatible but we can't solveWithIdentity this way
                                -- This case covers the  a_touchable :: * ~ b_untouchable :: ?? 
                                -- which has to be deferred or floated out for someone else to solve 
                                -- it in a scope where 'b' is no longer untouchable.
         else do { addErrorTcS KindError gw (mkTyVarTy tv) xi -- See Note [Kind errors]
                 ; return SPError }
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-}
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       }
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  | otherwise -- Still can't solve, sig tyvar and non-variable rhs
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  = return SPCantSolve
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----------------
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trySpontaneousEqTwoWay :: EqVar -> CtFlavor -> TcTyVar -> TcTyVar -> TcS SPSolveResult
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-- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here
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trySpontaneousEqTwoWay eqv gw tv1 tv2
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  | k1 `isSubKind` k2
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  , nicer_to_update_tv2 = solveWithIdentity eqv gw tv2 (mkTyVarTy tv1)
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  | k2 `isSubKind` k1 
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  = solveWithIdentity eqv gw tv1 (mkTyVarTy tv2)
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  | otherwise -- None is a subkind of the other, but they are both touchable! 
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  = return SPCantSolve
    -- do { addErrorTcS KindError gw (mkTyVarTy tv1) (mkTyVarTy tv2)
    --   ; return SPError }
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  where
    k1 = tyVarKind tv1
    k2 = tyVarKind tv2
    nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}

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Note [Kind errors] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the wanted problem: 
      alpha ~ (# Int, Int #) 
where alpha :: ?? and (# Int, Int #) :: (#). We can't spontaneously solve this constraint, 
but we should rather reject the program that give rise to it. If 'trySpontaneousEqTwoWay' 
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simply returns @CantSolve@ then that wanted constraint is going to propagate all the way and 
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get quantified over in inference mode. That's bad because we do know at this point that the 
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constraint is insoluble. Instead, we call 'recKindErrorTcS' here, which will fail later on.
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The same applies in canonicalization code in case of kind errors in the givens. 
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However, when we canonicalize givens we only check for compatibility (@compatKind@). 
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If there were a kind error in the givens, this means some form of inconsistency or dead code.
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You may think that when we spontaneously solve wanteds we may have to look through the 
bindings to determine the right kind of the RHS type. E.g one may be worried that xi is 
@alpha@ where alpha :: ? and a previous spontaneous solving has set (alpha := f) with (f :: *).
But we orient our constraints so that spontaneously solved ones can rewrite all other constraint
so this situation can't happen. 
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Note [Spontaneous solving and kind compatibility] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Note that our canonical constraints insist that *all* equalities (tv ~
xi) or (F xis ~ rhs) require the LHS and the RHS to have *compatible*
the same kinds.  ("compatible" means one is a subKind of the other.)
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  - It can't be *equal* kinds, because
     b) wanted constraints don't necessarily have identical kinds
               eg   alpha::? ~ Int
     b) a solved wanted constraint becomes a given

  - SPJ thinks that *given* constraints (tv ~ tau) always have that
    tau has a sub-kind of tv; and when solving wanted constraints
    in trySpontaneousEqTwoWay we re-orient to achieve this.

  - Note that the kind invariant is maintained by rewriting.
    Eg wanted1 rewrites wanted2; if both were compatible kinds before,
       wanted2 will be afterwards.  Similarly givens.

Caveat:
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  - Givens from higher-rank, such as: 
          type family T b :: * -> * -> * 
          type instance T Bool = (->) 

          f :: forall a. ((T a ~ (->)) => ...) -> a -> ... 
          flop = f (...) True 
     Whereas we would be able to apply the type instance, we would not be able to 
     use the given (T Bool ~ (->)) in the body of 'flop' 

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Note [Avoid double unifications] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The spontaneous solver has to return a given which mentions the unified unification
variable *on the left* of the equality. Here is what happens if not: 
  Original wanted:  (a ~ alpha),  (alpha ~ Int) 
We spontaneously solve the first wanted, without changing the order! 
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      given : a ~ alpha      [having unified alpha := a] 
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Now the second wanted comes along, but he cannot rewrite the given, so we simply continue.
At the end we spontaneously solve that guy, *reunifying*  [alpha := Int] 

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We avoid this problem by orienting the resulting given so that the unification
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variable is on the left.  [Note that alternatively we could attempt to
enforce this at canonicalization]
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See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding
double unifications is the main reason we disallow touchable
unification variables as RHS of type family equations: F xis ~ alpha.
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\begin{code}
----------------
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solveWithIdentity :: EqVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
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-- Solve with the identity coercion 
-- Precondition: kind(xi) is a sub-kind of kind(tv)
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-- Precondition: CtFlavor is Wanted or Derived
-- See [New Wanted Superclass Work] to see why solveWithIdentity 
--     must work for Derived as well as Wanted
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-- Returns: workItem where 
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--        workItem = the new Given constraint
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solveWithIdentity eqv wd tv xi 
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  = do { traceTcS "Sneaky unification:" $ 
                       vcat [text "Coercion variable:  " <+> ppr wd, 
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                             text "Coercion:           " <+> pprEq (mkTyVarTy tv) xi,
                             text "Left  Kind is     : " <+> ppr (typeKind (mkTyVarTy tv)),
                             text "Right Kind is     : " <+> ppr (typeKind xi)
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                  ]
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       ; setWantedTyBind tv xi
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       ; let refl_xi = mkReflCo xi
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       ; eqv_given <- newGivenEqVar (mkTyVarTy tv) xi refl_xi
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       ; when (isWanted wd) (setEqBind eqv refl_xi)
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           -- We don't want to do this for Derived, that's why we use 'when (isWanted wd)'
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       ; return $ SPSolved (CTyEqCan { cc_id = eqv_given
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                                     , cc_flavor = mkSolvedFlavor wd UnkSkol
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                                     , cc_tyvar  = tv, cc_rhs = xi }) }
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\end{code}


*********************************************************************************
*                                                                               * 
                       The interact-with-inert Stage
*                                                                               *
*********************************************************************************

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Note [The Solver Invariant]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We always add Givens first.  So you might think that the solver has
the invariant

   If the work-item is Given, 
   then the inert item must Given

But this isn't quite true.  Suppose we have, 
    c1: [W] beta ~ [alpha], c2 : [W] blah, c3 :[W] alpha ~ Int
After processing the first two, we get
     c1: [G] beta ~ [alpha], c2 : [W] blah
Now, c3 does not interact with the the given c1, so when we spontaneously
solve c3, we must re-react it with the inert set.  So we can attempt a 
reaction between inert c2 [W] and work-item c3 [G].

It *is* true that [Solver Invariant]
   If the work-item is Given, 
   AND there is a reaction
   then the inert item must Given
or, equivalently,
   If the work-item is Given, 
   and the inert item is Wanted/Derived
   then there is no reaction

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\begin{code}
-- Interaction result of  WorkItem <~> AtomicInert
data InteractResult
   = IR { ir_stop         :: StopOrContinue
            -- Stop
            --   => Reagent (work item) consumed.
            -- ContinueWith new_reagent
            --   => Reagent transformed but keep gathering interactions. 
            --      The transformed item remains inert with respect 
            --      to any previously encountered inerts.

        , ir_inert_action :: InertAction
            -- Whether the inert item should remain in the InertSet.

        , ir_new_work     :: WorkList
            -- new work items to add to the WorkList
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        , ir_fire :: Maybe String    -- Tells whether a rule fired, and if so what
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        }

-- What to do with the inert reactant.
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data InertAction = KeepInert | DropInert 
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mkIRContinue :: String -> WorkItem -> InertAction -> WorkList -> TcS InteractResult
mkIRContinue rule wi keep newWork 
  = return $ IR { ir_stop = ContinueWith wi, ir_inert_action = keep
                , ir_new_work = newWork, ir_fire = Just rule }
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mkIRStopK :: String -> WorkList -> TcS InteractResult
mkIRStopK rule newWork
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  = return $ IR { ir_stop = Stop, ir_inert_action = KeepInert
                , ir_new_work = newWork, ir_fire = Just rule }
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mkIRStopD :: String -> WorkList -> TcS InteractResult
mkIRStopD rule newWork
  = return $ IR { ir_stop = Stop, ir_inert_action = DropInert
                , ir_new_work = newWork, ir_fire = Just rule }

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noInteraction :: Monad m => WorkItem -> m InteractResult
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noInteraction wi
  = return $ IR { ir_stop = ContinueWith wi, ir_inert_action = KeepInert
                , ir_new_work = emptyWorkList, ir_fire = Nothing }
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data WhichComesFromInert = LeftComesFromInert | RightComesFromInert 
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     -- See Note [Efficient Orientation] 
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---------------------------------------------------
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-- Interact a single WorkItem with the equalities of an inert set as
-- far as possible, i.e. until we get a Stop result from an individual
-- reaction (i.e. when the WorkItem is consumed), or until we've
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-- interact the WorkItem with the entire equalities of the InertSet

interactWithInertEqsStage :: SimplifierStage 
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interactWithInertEqsStage depth workItem inert
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  = Bag.foldrBagM (interactNext depth) initITR (inert_eqs inert)
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                        -- use foldr to preserve the order          
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  where
    initITR = SR { sr_inerts   = inert { inert_eqs = emptyCCan }
                 , sr_new_work = emptyWorkList
                 , sr_stop     = ContinueWith workItem }
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---------------------------------------------------
-- Interact a single WorkItem with *non-equality* constraints in the inert set. 
-- Precondition: equality interactions must have already happened, hence we have 
-- to pick up some information from the incoming inert, before folding over the 
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-- "Other" constraints it contains!

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interactWithInertsStage :: SimplifierStage
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interactWithInertsStage depth workItem inert
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  = let (relevant, inert_residual) = getISRelevant workItem inert 
        initITR = SR { sr_inerts   = inert_residual
                     , sr_new_work = emptyWorkList
                     , sr_stop     = ContinueWith workItem } 
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    in Bag.foldrBagM (interactNext depth) initITR relevant 
                        -- use foldr to preserve the order
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  where 
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    getISRelevant :: CanonicalCt -> InertSet -> (CanonicalCts, InertSet) 
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    getISRelevant (CFrozenErr {}) is = (emptyCCan, is)
                  -- Nothing s relevant; we have alread interacted
                  -- it with the equalities in the inert set

    getISRelevant (CDictCan { cc_class = cls } ) is
      = let (relevant, residual_map) = getRelevantCts cls (inert_dicts is)
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        in (relevant, is { inert_dicts = residual_map }) 
    getISRelevant (CFunEqCan { cc_fun = tc } ) is 
      = let (relevant, residual_map) = getRelevantCts tc (inert_funeqs is) 
        in (relevant, is { inert_funeqs = residual_map })
    getISRelevant (CIPCan { cc_ip_nm = nm }) is 
      = let (relevant, residual_map) = getRelevantCts nm (inert_ips is)
        in (relevant, is { inert_ips = residual_map }) 
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    getISRelevant (CIrredEvCan {}) is
      = (inert_irreds is, is { inert_irreds = emptyCCan })
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    -- An equality, finally, may kick everything except equalities out 
    -- because we have already interacted the equalities in interactWithInertEqsStage
    getISRelevant _eq_ct is  -- Equality, everything is relevant for this one 
                             -- TODO: if we were caching variables, we'd know that only 
                             --       some are relevant. Experiment with this for now. 
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      = let cts = cCanMapToBag (inert_ips is) `unionBags`
                    cCanMapToBag (inert_dicts is) `unionBags`
                    cCanMapToBag (inert_funeqs is) `unionBags`
                    inert_irreds is
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        in (cts, is { inert_dicts  = emptyCCanMap
                    , inert_ips    = emptyCCanMap
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                    , inert_funeqs = emptyCCanMap
                    , inert_irreds = emptyBag })
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interactNext :: SubGoalDepth -> AtomicInert -> StageResult -> TcS StageResult 
interactNext depth inert it
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  | ContinueWith work_item <- sr_stop it
  = do { let inerts = sr_inerts it 

       ; IR { ir_new_work = new_work, ir_inert_action = inert_action
            , ir_fire = fire_info, ir_stop = stop } 
            <- interactWithInert inert work_item

       ; let mk_msg rule 
      	       = text rule <+> keep_doc
      	         <+> vcat [ ptext (sLit "Inert =") <+> ppr inert
      	                  , ptext (sLit "Work =")  <+> ppr work_item
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      	                  , ppUnless (isEmptyWorkList new_work) $
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                            ptext (sLit "New =") <+> ppr new_work ]
             keep_doc = case inert_action of
                 	  KeepInert -> ptext (sLit "[keep]")
                 	  DropInert -> ptext (sLit "[drop]")
       ; case fire_info of
           Just rule -> do { bumpStepCountTcS
                           ; traceFireTcS depth (mk_msg rule) }
           Nothing  -> return ()

       -- New inerts depend on whether we KeepInert or not 
       ; let inerts_new = case inert_action of
                            KeepInert -> inerts `updInertSet` inert
                            DropInert -> inerts
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       ; return $ SR { sr_inerts   = inerts_new
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                     , sr_new_work = sr_new_work it `unionWorkList` new_work
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                     , sr_stop     = stop } }
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  | otherwise 
  = return $ it { sr_inerts = (sr_inerts it) `updInertSet` inert }
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-- Do a single interaction of two constraints.
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interactWithInert :: AtomicInert -> WorkItem -> TcS InteractResult
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interactWithInert inert workItem 
  = do { ctxt <- getTcSContext
       ; let is_allowed  = allowedInteraction (simplEqsOnly ctxt) inert workItem 
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       ; if is_allowed then 
              doInteractWithInert inert workItem 
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          else 
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              noInteraction workItem 
       }
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allowedInteraction :: Bool -> AtomicInert -> WorkItem -> Bool 
-- Allowed interactions 
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allowedInteraction eqs_only (CDictCan {})    (CDictCan {})    = not eqs_only
allowedInteraction eqs_only (CIPCan {})      (CIPCan {})      = not eqs_only
allowedInteraction eqs_only (CIrredEvCan {}) (CIrredEvCan {}) = not eqs_only
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allowedInteraction _ _ _ = True 

--------------------------------------------
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doInteractWithInert :: CanonicalCt -> CanonicalCt -> TcS InteractResult
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-- Identical class constraints.

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doInteractWithInert
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  inertItem@(CDictCan { cc_id = d1, cc_flavor = fl1, cc_class = cls1, cc_tyargs = tys1 }) 
   workItem@(CDictCan { cc_id = d2, cc_flavor = fl2, cc_class = cls2, cc_tyargs = tys2 })
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  | cls1 == cls2  
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  = do { let pty1 = mkClassPred cls1 tys1
             pty2 = mkClassPred cls2 tys2
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             inert_pred_loc     = (pty1, pprFlavorArising fl1)
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             work_item_pred_loc = (pty2, pprFlavorArising fl2)
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       ; any_fundeps 
           <- if isGivenOrSolved fl1 && isGivenOrSolved fl2 then return Nothing
              -- NB: We don't create fds for given (and even solved), have not seen a useful
              -- situation for these and even if we did we'd have to be very careful to only
              -- create Derived's and not Wanteds. 

              else let fd_eqns = improveFromAnother inert_pred_loc work_item_pred_loc
                       wloc    = get_workitem_wloc fl2 
                   in rewriteWithFunDeps fd_eqns tys2 wloc
                      -- See Note [Efficient Orientation], [When improvement happens]

       ; case any_fundeps of
           -- No Functional Dependencies 
           Nothing             
               | eqTypes tys1 tys2 -> solveOneFromTheOther "Cls/Cls" (EvId d1,fl1) workItem
               | otherwise         -> noInteraction workItem

           -- Actual Functional Dependencies
           Just (rewritten_tys2,cos2,fd_work) 
               | not (eqTypes tys1 rewritten_tys2) 
               -- Standard thing: create derived fds and keep on going. Importantly we don't
               -- throw workitem back in the worklist because this can cause loops. See #5236.
               -> do { fd_cans <- mkCanonicalFDAsDerived fd_work
                     ; mkIRContinue "Cls/Cls fundep (not solved)" workItem KeepInert fd_cans }

               -- This WHOLE otherwise branch is an optimization where the fd made the things match
               | otherwise  
               , let dict_co = mkTyConAppCo (classTyCon cls1) cos2
               -> case fl2 of
                    Given {} 
                        -> pprPanic "Unexpected given!" (ppr inertItem $$ ppr workItem)
                           -- The only way to have created a fundep is if the inert was
                           -- wanted or derived, in which case the workitem can't be given!
                    Derived {}
                        -- The types were made to exactly match so we don't need 
                        -- the workitem any longer.
                        -> do { fd_cans <- mkCanonicalFDAsDerived fd_work
                               -- No rewriting really, so let's create deriveds fds
                              ; mkIRStopK "Cls/Cls fundep (solved)" fd_cans }
		    Wanted  {} 
		        | isDerived fl1 
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                            -> do { setEvBind d2 (EvCast d1 dict_co)
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			          ; let inert_w = inertItem { cc_flavor = fl2 }
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			   -- A bit naughty: we take the inert Derived, 
			   -- turn it into a Wanted, use it to solve the work-item
			   -- and put it back into the work-list
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			   -- Maybe rather than starting again, we could keep going 
                           -- with the rewritten workitem, having dropped the inert, but its
                           -- safe to restart.
                          
                           -- Also: we have rewriting so lets create wanted fds
                                  ; fd_cans <- mkCanonicalFDAsWanted fd_work
                                  ; mkIRStopD "Cls/Cls fundep (solved)" $ 
                                    workListFromNonEq inert_w `unionWorkList` fd_cans }
		        | otherwise
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                        -> do { setEvBind d2 (EvCast d1 dict_co)
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                          -- Rewriting is happening, so we have to create wanted fds
                              ; fd_cans <- mkCanonicalFDAsWanted fd_work
                              ; mkIRStopK "Cls/Cls fundep (solved)" fd_cans }
       }
  where get_workitem_wloc (Wanted wl)  = wl 
        get_workitem_wloc (Derived wl) = wl 
        get_workitem_wloc (Given {})   = panic "Unexpected given!"

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-- Class constraint and given equality: use the equality to rewrite
-- the class constraint. 
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doInteractWithInert (CTyEqCan { cc_id = eqv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi }) 
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                    (CDictCan { cc_id = dv, cc_flavor = wfl, cc_class = cl, cc_tyargs = xis }) 
  | ifl `canRewrite` wfl 
  , tv `elemVarSet` tyVarsOfTypes xis
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  = do { rewritten_dict <- rewriteDict (eqv,tv,xi) (dv,wfl,cl,xis)
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            -- Continue with rewritten Dictionary because we can only be in the 
            -- interactWithEqsStage, so the dictionary is inert. 
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       ; mkIRContinue "Eq/Cls" rewritten_dict KeepInert emptyWorkList }
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doInteractWithInert (CDictCan { cc_id = dv, cc_flavor = ifl, cc_class = cl, cc_tyargs = xis }) 
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           workItem@(CTyEqCan { cc_id = eqv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
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  | wfl `canRewrite` ifl
  , tv `elemVarSet` tyVarsOfTypes xis
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  = do { rewritten_dict <- rewriteDict (eqv,tv,xi) (dv,ifl,cl,xis)
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       ; mkIRContinue "Cls/Eq" workItem DropInert (workListFromNonEq rewritten_dict) }
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-- Irreducible evidence and given equality: use the equality to rewrite
-- the irreducible evidence.
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doInteractWithInert (CTyEqCan { cc_id = eqv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi }) 
                    (CIrredEvCan { cc_id = id, cc_flavor = wfl, cc_ty = ty })
  | ifl `canRewrite` wfl
  , tv `elemVarSet` tyVarsOfType ty 
  = do { rewritten_irred <- rewriteIrred (eqv,tv,xi) (id,wfl,ty) 
       ; mkIRStopK "Eq/Irred" rewritten_irred } 

doInteractWithInert (CIrredEvCan { cc_id = id, cc_flavor = ifl, cc_ty = ty }) 
           workItem@(CTyEqCan { cc_id = eqv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
  | wfl `canRewrite` ifl
  , tv `elemVarSet` tyVarsOfType ty
  = do { rewritten_irred <- rewriteIrred (eqv,tv,xi) (id,ifl,ty) 
       ; mkIRContinue "Irred/Eq" workItem DropInert rewritten_irred }

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-- Two pieces of irreducible evidence: if their types are *exactly identical* we can
-- rewrite them. We can never improve using this: if we want ty1 :: Constraint and have
-- ty2 :: Constraint it clearly does not mean that (ty1 ~ ty2)
doInteractWithInert (CIrredEvCan { cc_id = id1, cc_flavor = ifl, cc_ty = ty1 })
           workItem@(CIrredEvCan { cc_ty = ty2 })
  | ty1 `eqType` ty2
  = solveOneFromTheOther "Irred/Irred" (EvId id1,ifl) workItem

-- Implicit param and given equality: use the equality to rewrite
-- the implicit param.
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doInteractWithInert (CTyEqCan { cc_id = eqv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi }) 
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                    (CIPCan { cc_id = ipid, cc_flavor = wfl, cc_ip_nm = nm, cc_ip_ty = ty }) 
  | ifl `canRewrite` wfl
  , tv `elemVarSet` tyVarsOfType ty 
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  = do { rewritten_ip <- rewriteIP (eqv,tv,xi) (ipid,wfl,nm,ty) 
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       ; mkIRContinue "Eq/IP" rewritten_ip KeepInert emptyWorkList } 
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doInteractWithInert (CIPCan { cc_id = ipid, cc_flavor = ifl, cc_ip_nm = nm, cc_ip_ty = ty }) 
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           workItem@(CTyEqCan { cc_id = eqv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
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  | wfl `canRewrite` ifl
  , tv `elemVarSet` tyVarsOfType ty
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  = do { rewritten_ip <- rewriteIP (eqv,tv,xi) (ipid,ifl,nm,ty) 
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       ; mkIRContinue "IP/Eq" workItem DropInert (workListFromNonEq rewritten_ip) }
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-- Two implicit parameter constraints.  If the names are the same,
-- but their types are not, we generate a wanted type equality 
-- that equates the type (this is "improvement").  
-- However, we don't actually need the coercion evidence,
-- so we just generate a fresh coercion variable that isn't used anywhere.
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doInteractWithInert (CIPCan { cc_id = id1, cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 }) 
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           workItem@(CIPCan { cc_flavor = wfl, cc_ip_nm = nm2, cc_ip_ty = ty2 })
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  | nm1 == nm2 && isGivenOrSolved wfl && isGivenOrSolved ifl
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  = 	-- See Note [Overriding implicit parameters]
        -- Dump the inert item, override totally with the new one
	-- Do not require type equality
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	-- For example, given let ?x::Int = 3 in let ?x::Bool = True in ...
	--              we must *override* the outer one with the inner one
    mkIRContinue "IP/IP override" workItem DropInert emptyWorkList
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