TcPat.hs 51.3 KB
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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

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TcPat: Typechecking patterns
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-}
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{-# LANGUAGE CPP, RankNTypes #-}
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module TcPat ( tcLetPat, TcSigFun
             , TcPragEnv, lookupPragEnv, emptyPragEnv
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             , TcSigInfo(..), TcPatSynInfo(..)
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             , findScopedTyVars, isPartialSig
             , completeSigPolyId, completeSigPolyId_maybe
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             , LetBndrSpec(..), addInlinePrags
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             , tcPat, tcPats, newNoSigLetBndr
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             , addDataConStupidTheta, badFieldCon, polyPatSig ) where
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#include "HsVersions.h"
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import {-# SOURCE #-}   TcExpr( tcSyntaxOp, tcInferRho)
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import HsSyn
import TcHsSyn
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import TcRnMonad
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import Inst
import Id
import Var
import Name
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import NameSet
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import NameEnv
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import TcEnv
import TcMType
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import TcValidity( arityErr )
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import TcType
import TcUnify
import TcHsType
import TysWiredIn
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import TcEvidence
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import TyCon
import DataCon
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import PatSyn
import ConLike
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import PrelNames
import BasicTypes hiding (SuccessFlag(..))
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import DynFlags
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import SrcLoc
import Util
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import Outputable
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import FastString
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import Maybes( orElse )
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import Control.Monad
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{-
************************************************************************
*                                                                      *
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                External interface
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*                                                                      *
************************************************************************
-}
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tcLetPat :: TcSigFun -> LetBndrSpec
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         -> LPat Name -> TcSigmaType
         -> TcM a
         -> TcM (LPat TcId, a)
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tcLetPat sig_fn no_gen pat pat_ty thing_inside
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  = tc_lpat pat pat_ty penv thing_inside
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  where
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    penv = PE { pe_lazy = True
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              , pe_ctxt = LetPat sig_fn no_gen }
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-----------------
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tcPats :: HsMatchContext Name
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       -> [LPat Name]            -- Patterns,
       -> [TcSigmaType]          --   and their types
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       -> TcM a                  --   and the checker for the body
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       -> TcM ([LPat TcId], a)
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-- This is the externally-callable wrapper function
-- Typecheck the patterns, extend the environment to bind the variables,
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-- do the thing inside, use any existentially-bound dictionaries to
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-- discharge parts of the returning LIE, and deal with pattern type
-- signatures

--   1. Initialise the PatState
--   2. Check the patterns
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--   3. Check the body
--   4. Check that no existentials escape
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tcPats ctxt pats pat_tys thing_inside
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  = tc_lpats penv pats pat_tys thing_inside
  where
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    penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }
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tcPat :: HsMatchContext Name
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      -> LPat Name -> TcSigmaType
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      -> TcM a                 -- Checker for body, given
                               -- its result type
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      -> TcM (LPat TcId, a)
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tcPat ctxt pat pat_ty thing_inside
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  = tc_lpat pat pat_ty penv thing_inside
  where
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    penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt }
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-----------------
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data PatEnv
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  = PE { pe_lazy :: Bool        -- True <=> lazy context, so no existentials allowed
       , pe_ctxt :: PatCtxt     -- Context in which the whole pattern appears
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       }
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data PatCtxt
  = LamPat   -- Used for lambdas, case etc
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       (HsMatchContext Name)
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  | LetPat   -- Used only for let(rec) pattern bindings
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             -- See Note [Typing patterns in pattern bindings]
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       TcSigFun        -- Tells type sig if any
       LetBndrSpec     -- True <=> no generalisation of this let

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data LetBndrSpec
  = LetLclBndr            -- The binder is just a local one;
                          -- an AbsBinds will provide the global version
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  | LetGblBndr TcPragEnv  -- Generalisation plan is NoGen, so there isn't going
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                          -- to be an AbsBinds; So we must bind the global version
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                          -- of the binder right away.
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                          -- Oh, and here is the inline-pragma information
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makeLazy :: PatEnv -> PatEnv
makeLazy penv = penv { pe_lazy = True }

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inPatBind :: PatEnv -> Bool
inPatBind (PE { pe_ctxt = LetPat {} }) = True
inPatBind (PE { pe_ctxt = LamPat {} }) = False
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---------------
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type TcPragEnv = NameEnv [LSig Name]
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type TcSigFun  = Name -> Maybe TcSigInfo
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emptyPragEnv :: TcPragEnv
emptyPragEnv = emptyNameEnv

lookupPragEnv :: TcPragEnv -> Name -> [LSig Name]
lookupPragEnv prag_fn n = lookupNameEnv prag_fn n `orElse` []

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data TcSigInfo
  = TcSigInfo {
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        sig_name    :: Name,  -- The binder name of the type signature. When
                              -- sig_id = Just id, then sig_name = idName id.

        sig_poly_id :: Maybe TcId,
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             -- Just f <=> the type signature had no wildcards, so the precise,
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             --            complete polymorphic type is known.  In that case,
             --            f is the polymorphic Id, with that type
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             -- Nothing <=> the type signature is partial (i.e. includes one or more
             --             wildcards). In this case it doesn't make sense to give
             --             the polymorphic Id, because we are going to /infer/ its
             --             type, so we can't make the polymorphic Id ab-initio
             --
             -- See Note [Complete and partial type signatures]
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        sig_tvs    :: [(Maybe Name, TcTyVar)],
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                           -- Instantiated type and kind variables
                           -- Just n <=> this skolem is lexically in scope with name n
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                           -- See Note [Binding scoped type variables]
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        sig_nwcs   :: [(Name, TcTyVar)],
                           -- Instantiated wildcard variables
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                           -- If sig_poly_id = Just f, then sig_nwcs must be empty
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        sig_extra_cts :: Maybe SrcSpan,
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                           -- Just loc <=> An extra-constraints wildcard was present
                           --              at location loc
                           --   e.g.   f :: (Eq a, _) => a -> a
                           -- Any extra constraints inferred during
                           -- type-checking will be added to the sig_theta.
                           -- If sig_poly_id = Just f, sig_extra_cts must be Nothing
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        sig_theta  :: TcThetaType,  -- Instantiated theta
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        sig_tau    :: TcSigmaType,  -- Instantiated tau
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                                    -- See Note [sig_tau may be polymorphic]
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        sig_loc    :: SrcSpan,      -- The location of the signature

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        sig_warn_redundant :: Bool  -- True <=> report redundant constraints
                                    --          when typechecking the value binding
                                    --          for this type signature
           -- This is usually True, but False for
           --   * Record selectors (not important here)
           --   * Class and instance methods.  Here the code may legitimately
           --     be more polymorphic than the signature generated from the
           --     class declaration
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    }
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  | TcPatSynInfo TcPatSynInfo

data TcPatSynInfo
  = TPSI {
        patsig_name  :: Name,
        patsig_tau   :: TcSigmaType,
        patsig_ex    :: [TcTyVar],
        patsig_prov  :: TcThetaType,
        patsig_univ  :: [TcTyVar],
        patsig_req   :: TcThetaType
    }
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findScopedTyVars  -- See Note [Binding scoped type variables]
  :: LHsType Name             -- The HsType
  -> TcType                   -- The corresponding Type:
                              --   uses same Names as the HsType
  -> [TcTyVar]                -- The instantiated forall variables of the Type
  -> [(Maybe Name, TcTyVar)]  -- In 1-1 correspondence with the instantiated vars
findScopedTyVars hs_ty sig_ty inst_tvs
  = zipWith find sig_tvs inst_tvs
  where
    find sig_tv inst_tv
      | tv_name `elemNameSet` scoped_names = (Just tv_name, inst_tv)
      | otherwise                          = (Nothing,      inst_tv)
      where
        tv_name = tyVarName sig_tv

    scoped_names = mkNameSet (hsExplicitTvs hs_ty)
    (sig_tvs,_)  = tcSplitForAllTys sig_ty

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instance NamedThing TcSigInfo where
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    getName TcSigInfo{ sig_name = name } = name
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    getName (TcPatSynInfo tpsi) = patsig_name tpsi

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instance Outputable TcSigInfo where
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    ppr (TcSigInfo { sig_name = name, sig_poly_id = mb_poly_id, sig_tvs = tyvars
                   , sig_theta = theta, sig_tau = tau })
        = maybe (ppr name) ppr mb_poly_id <+> dcolon <+>
          vcat [ pprSigmaType (mkSigmaTy (map snd tyvars) theta tau)
               , ppr (map fst tyvars) ]
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    ppr (TcPatSynInfo tpsi) = text "TcPatSynInfo" <+> ppr tpsi

instance Outputable TcPatSynInfo where
    ppr (TPSI{ patsig_name = name}) = ppr name

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isPartialSig :: TcSigInfo -> Bool
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isPartialSig (TcSigInfo { sig_poly_id = Nothing }) = True
isPartialSig _ = False

-- Helper for cases when we know for sure we have a complete type
-- signature, e.g. class methods.
completeSigPolyId :: TcSigInfo -> TcId
completeSigPolyId (TcSigInfo { sig_poly_id = Just id }) = id
completeSigPolyId _ = panic "completeSigPolyId"
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completeSigPolyId_maybe :: TcSigInfo -> Maybe TcId
completeSigPolyId_maybe (TcSigInfo { sig_poly_id = mb_id }) = mb_id
completeSigPolyId_maybe (TcPatSynInfo {})                   = Nothing

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{-
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Note [Binding scoped type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The type variables *brought into lexical scope* by a type signature may
be a subset of the *quantified type variables* of the signatures, for two reasons:

* With kind polymorphism a signature like
    f :: forall f a. f a -> f a
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  may actually give rise to
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    f :: forall k. forall (f::k -> *) (a:k). f a -> f a
  So the sig_tvs will be [k,f,a], but only f,a are scoped.
  NB: the scoped ones are not necessarily the *inital* ones!

* Even aside from kind polymorphism, tere may be more instantiated
  type variables than lexically-scoped ones.  For example:
        type T a = forall b. b -> (a,b)
        f :: forall c. T c
  Here, the signature for f will have one scoped type variable, c,
  but two instantiated type variables, c' and b'.

The function findScopedTyVars takes
  * hs_ty:    the original HsForAllTy
  * sig_ty:   the corresponding Type (which is guaranteed to use the same Names
              as the HsForAllTy)
  * inst_tvs: the skolems instantiated from the forall's in sig_ty
It returns a [(Maybe Name, TcTyVar)], in 1-1 correspondence with inst_tvs
but with a (Just n) for the lexically scoped name of each in-scope tyvar.
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Note [sig_tau may be polymorphic]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note that "sig_tau" might actually be a polymorphic type,
if the original function had a signature like
   forall a. Eq a => forall b. Ord b => ....
But that's ok: tcMatchesFun (called by tcRhs) can deal with that
It happens, too!  See Note [Polymorphic methods] in TcClassDcl.
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Note [Existential check]
~~~~~~~~~~~~~~~~~~~~~~~~
Lazy patterns can't bind existentials.  They arise in two ways:
  * Let bindings      let { C a b = e } in b
  * Twiddle patterns  f ~(C a b) = e
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The pe_lazy field of PatEnv says whether we are inside a lazy
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pattern (perhaps deeply)
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If we aren't inside a lazy pattern then we can bind existentials,
but we need to be careful about "extra" tyvars. Consider
    (\C x -> d) : pat_ty -> res_ty
When looking for existential escape we must check that the existential
bound by C don't unify with the free variables of pat_ty, OR res_ty
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(or of course the environment).   Hence we need to keep track of the
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res_ty free vars.
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Note [Complete and partial type signatures]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A type signature is partial when it contains one or more wildcards
(= type holes).  The wildcard can either be:
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* A (type) wildcard occurring in sig_theta or sig_tau. These are
  stored in sig_nwcs.
      f :: Bool -> _
      g :: Eq _a => _a -> _a -> Bool
* Or an extra-constraints wildcard, stored in sig_extra_cts:
      h :: (Num a, _) => a -> a

A type signature is a complete type signature when there are no
wildcards in the type signature, i.e. iff sig_nwcs is empty and
sig_extra_cts is Nothing.
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************************************************************************
*                                                                      *
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                Binders
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*                                                                      *
************************************************************************
-}
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tcPatBndr :: PatEnv -> Name -> TcSigmaType -> TcM (TcCoercion, TcId)
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-- (coi, xp) = tcPatBndr penv x pat_ty
-- Then coi : pat_ty ~ typeof(xp)
--
tcPatBndr (PE { pe_ctxt = LetPat lookup_sig no_gen}) bndr_name pat_ty
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          -- See Note [Typing patterns in pattern bindings]
  | LetGblBndr prags <- no_gen
  , Just sig <- lookup_sig bndr_name
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  , Just poly_id <- sig_poly_id sig
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  = do { bndr_id <- addInlinePrags poly_id (lookupPragEnv prags bndr_name)
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       ; traceTc "tcPatBndr(gbl,sig)" (ppr bndr_id $$ ppr (idType bndr_id))
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       ; co <- unifyPatType (idType bndr_id) pat_ty
       ; return (co, bndr_id) }
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  | otherwise
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  = do { bndr_id <- newNoSigLetBndr no_gen bndr_name pat_ty
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       ; traceTc "tcPatBndr(no-sig)" (ppr bndr_id $$ ppr (idType bndr_id))
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       ; return (mkTcNomReflCo pat_ty, bndr_id) }
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tcPatBndr (PE { pe_ctxt = _lam_or_proc }) bndr_name pat_ty
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  = return (mkTcNomReflCo pat_ty, mkLocalId bndr_name pat_ty)
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------------
newNoSigLetBndr :: LetBndrSpec -> Name -> TcType -> TcM TcId
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-- In the polymorphic case (no_gen = LetLclBndr), generate a "monomorphic version"
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--    of the Id; the original name will be bound to the polymorphic version
--    by the AbsBinds
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-- In the monomorphic case (no_gen = LetBglBndr) there is no AbsBinds, and we
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--    use the original name directly
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newNoSigLetBndr LetLclBndr name ty
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  =do  { mono_name <- newLocalName name
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       ; return (mkLocalId mono_name ty) }
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newNoSigLetBndr (LetGblBndr prags) name ty
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  = addInlinePrags (mkLocalId name ty) (lookupPragEnv prags name)
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----------
addInlinePrags :: TcId -> [LSig Name] -> TcM TcId
addInlinePrags poly_id prags
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  | inl@(L _ prag) : inls <- inl_prags
  = do { traceTc "addInlinePrag" (ppr poly_id $$ ppr prag)
       ; unless (null inls) (warn_multiple_inlines inl inls)
       ; return (poly_id `setInlinePragma` prag) }
  | otherwise
  = return poly_id
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  where
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    inl_prags = [L loc prag | L loc (InlineSig _ prag) <- prags]

    warn_multiple_inlines _ [] = return ()

    warn_multiple_inlines inl1@(L loc prag1) (inl2@(L _ prag2) : inls)
       | inlinePragmaActivation prag1 == inlinePragmaActivation prag2
       , isEmptyInlineSpec (inlinePragmaSpec prag1)
       =    -- Tiresome: inl1 is put there by virtue of being in a hs-boot loop
            -- and inl2 is a user NOINLINE pragma; we don't want to complain
         warn_multiple_inlines inl2 inls
       | otherwise
       = setSrcSpan loc $
         addWarnTc (hang (ptext (sLit "Multiple INLINE pragmas for") <+> ppr poly_id)
                       2 (vcat (ptext (sLit "Ignoring all but the first")
                                : map pp_inl (inl1:inl2:inls))))

    pp_inl (L loc prag) = ppr prag <+> parens (ppr loc)
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{-
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Note [Typing patterns in pattern bindings]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we are typing a pattern binding
    pat = rhs
Then the PatCtxt will be (LetPat sig_fn let_bndr_spec).

There can still be signatures for the binders:
     data T = MkT (forall a. a->a) Int
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     x :: forall a. a->a
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     y :: Int
     MkT x y = <rhs>

Two cases, dealt with by the LetPat case of tcPatBndr

 * If we are generalising (generalisation plan is InferGen or
   CheckGen), then the let_bndr_spec will be LetLclBndr.  In that case
   we want to bind a cloned, local version of the variable, with the
   type given by the pattern context, *not* by the signature (even if
   there is one; see Trac #7268). The mkExport part of the
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   generalisation step will do the checking and impedance matching
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   against the signature.

 * If for some some reason we are not generalising (plan = NoGen), the
   LetBndrSpec will be LetGblBndr.  In that case we must bind the
   global version of the Id, and do so with precisely the type given
   in the signature.  (Then we unify with the type from the pattern
   context type.

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************************************************************************
*                                                                      *
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                The main worker functions
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*                                                                      *
************************************************************************
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Note [Nesting]
~~~~~~~~~~~~~~
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tcPat takes a "thing inside" over which the pattern scopes.  This is partly
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so that tcPat can extend the environment for the thing_inside, but also
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so that constraints arising in the thing_inside can be discharged by the
pattern.

This does not work so well for the ErrCtxt carried by the monad: we don't
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want the error-context for the pattern to scope over the RHS.
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Hence the getErrCtxt/setErrCtxt stuff in tcMultiple
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-}
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--------------------
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type Checker inp out =  forall r.
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                          inp
                       -> PatEnv
                       -> TcM r
                       -> TcM (out, r)
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tcMultiple :: Checker inp out -> Checker [inp] [out]
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tcMultiple tc_pat args penv thing_inside
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  = do  { err_ctxt <- getErrCtxt
        ; let loop _ []
                = do { res <- thing_inside
                     ; return ([], res) }

              loop penv (arg:args)
                = do { (p', (ps', res))
                                <- tc_pat arg penv $
                                   setErrCtxt err_ctxt $
                                   loop penv args
                -- setErrCtxt: restore context before doing the next pattern
                -- See note [Nesting] above

                     ; return (p':ps', res) }

        ; loop penv args }
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--------------------
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tc_lpat :: LPat Name
        -> TcSigmaType
        -> PatEnv
        -> TcM a
        -> TcM (LPat TcId, a)
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tc_lpat (L span pat) pat_ty penv thing_inside
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  = setSrcSpan span $
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    do  { (pat', res) <- maybeWrapPatCtxt pat (tc_pat penv pat pat_ty)
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                                          thing_inside
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        ; return (L span pat', res) }
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tc_lpats :: PatEnv
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         -> [LPat Name] -> [TcSigmaType]
         -> TcM a
         -> TcM ([LPat TcId], a)
tc_lpats penv pats tys thing_inside
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  = ASSERT2( equalLength pats tys, ppr pats $$ ppr tys )
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    tcMultiple (\(p,t) -> tc_lpat p t)
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                (zipEqual "tc_lpats" pats tys)
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                penv thing_inside
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--------------------
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tc_pat  :: PatEnv
        -> Pat Name
        -> TcSigmaType  -- Fully refined result type
        -> TcM a                -- Thing inside
        -> TcM (Pat TcId,       -- Translated pattern
                a)              -- Result of thing inside
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tc_pat penv (VarPat name) pat_ty thing_inside
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  = do  { (co, id) <- tcPatBndr penv name pat_ty
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        ; res <- tcExtendIdEnv1 name id thing_inside
        ; return (mkHsWrapPatCo co (VarPat id) pat_ty, res) }
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tc_pat penv (ParPat pat) pat_ty thing_inside
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  = do  { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
        ; return (ParPat pat', res) }
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tc_pat penv (BangPat pat) pat_ty thing_inside
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  = do  { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
        ; return (BangPat pat', res) }
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tc_pat penv lpat@(LazyPat pat) pat_ty thing_inside
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  = do  { (pat', (res, pat_ct))
                <- tc_lpat pat pat_ty (makeLazy penv) $
                   captureConstraints thing_inside
                -- Ignore refined penv', revert to penv
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        ; emitConstraints pat_ct
        -- captureConstraints/extendConstraints:
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        --   see Note [Hopping the LIE in lazy patterns]
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        -- Check there are no unlifted types under the lazy pattern
        ; when (any (isUnLiftedType . idType) $ collectPatBinders pat') $
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               lazyUnliftedPatErr lpat

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        -- Check that the expected pattern type is itself lifted
        ; pat_ty' <- newFlexiTyVarTy liftedTypeKind
        ; _ <- unifyType pat_ty pat_ty'
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        ; return (LazyPat pat', res) }
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tc_pat _ (WildPat _) pat_ty thing_inside
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  = do  { res <- thing_inside
        ; return (WildPat pat_ty, res) }
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tc_pat penv (AsPat (L nm_loc name) pat) pat_ty thing_inside
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  = do  { (co, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
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        ; (pat', res) <- tcExtendIdEnv1 name bndr_id $
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                         tc_lpat pat (idType bndr_id) penv thing_inside
            -- NB: if we do inference on:
            --          \ (y@(x::forall a. a->a)) = e
            -- we'll fail.  The as-pattern infers a monotype for 'y', which then
            -- fails to unify with the polymorphic type for 'x'.  This could
            -- perhaps be fixed, but only with a bit more work.
            --
            -- If you fix it, don't forget the bindInstsOfPatIds!
        ; return (mkHsWrapPatCo co (AsPat (L nm_loc bndr_id) pat') pat_ty, res) }

tc_pat penv (ViewPat expr pat _) overall_pat_ty thing_inside
  = do  {
         -- Morally, expr must have type `forall a1...aN. OPT' -> B`
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         -- where overall_pat_ty is an instance of OPT'.
         -- Here, we infer a rho type for it,
         -- which replaces the leading foralls and constraints
         -- with fresh unification variables.
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        ; (expr',expr'_inferred) <- tcInferRho expr

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         -- next, we check that expr is coercible to `overall_pat_ty -> pat_ty`
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         -- NOTE: this forces pat_ty to be a monotype (because we use a unification
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         -- variable to find it).  this means that in an example like
         -- (view -> f)    where view :: _ -> forall b. b
         -- we will only be able to use view at one instantation in the
         -- rest of the view
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        ; (expr_co, pat_ty) <- tcInfer $ \ pat_ty ->
                unifyType expr'_inferred (mkFunTy overall_pat_ty pat_ty)

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         -- pattern must have pat_ty
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        ; (pat', res) <- tc_lpat pat pat_ty penv thing_inside

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        ; return (ViewPat (mkLHsWrapCo expr_co expr') pat' overall_pat_ty, res) }
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-- Type signatures in patterns
-- See Note [Pattern coercions] below
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tc_pat penv (SigPatIn pat sig_ty) pat_ty thing_inside
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  = do  { (inner_ty, tv_binds, nwc_binds, wrap) <- tcPatSig (inPatBind penv)
                                                            sig_ty pat_ty
        ; (pat', res) <- tcExtendTyVarEnv2 (tv_binds ++ nwc_binds) $
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                         tc_lpat pat inner_ty penv thing_inside
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        ; return (mkHsWrapPat wrap (SigPatOut pat' inner_ty) pat_ty, res) }
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------------------------
-- Lists, tuples, arrays
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tc_pat penv (ListPat pats _ Nothing) pat_ty thing_inside
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  = do  { (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTyR pat_ty
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        ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
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                                     pats penv thing_inside
        ; return (mkHsWrapPat coi (ListPat pats' elt_ty Nothing) pat_ty, res)
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        }

tc_pat penv (ListPat pats _ (Just (_,e))) pat_ty thing_inside
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  = do  { list_pat_ty <- newFlexiTyVarTy liftedTypeKind
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        ; e' <- tcSyntaxOp ListOrigin e (mkFunTy pat_ty list_pat_ty)
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        ; (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTyR list_pat_ty
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        ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
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                                     pats penv thing_inside
        ; return (mkHsWrapPat coi (ListPat pats' elt_ty (Just (pat_ty,e'))) list_pat_ty, res)
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        }
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tc_pat penv (PArrPat pats _) pat_ty thing_inside
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  = do  { (coi, elt_ty) <- matchExpectedPatTy matchExpectedPArrTyR pat_ty
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        ; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
                                     pats penv thing_inside
        ; return (mkHsWrapPat coi (PArrPat pats' elt_ty) pat_ty, res)
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        }
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tc_pat penv (TuplePat pats boxity _) pat_ty thing_inside
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  = do  { let tc = tupleTyCon boxity (length pats)
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        ; (coi, arg_tys) <- matchExpectedPatTy (matchExpectedTyConAppR tc) pat_ty
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        ; (pats', res) <- tc_lpats penv pats arg_tys thing_inside
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        ; dflags <- getDynFlags
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        -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
        -- so that we can experiment with lazy tuple-matching.
        -- This is a pretty odd place to make the switch, but
        -- it was easy to do.
        ; let
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              unmangled_result = TuplePat pats' boxity arg_tys
                                 -- pat_ty /= pat_ty iff coi /= IdCo
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              possibly_mangled_result
                | gopt Opt_IrrefutableTuples dflags &&
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                  isBoxed boxity            = LazyPat (noLoc unmangled_result)
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                | otherwise                 = unmangled_result
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        ; ASSERT( length arg_tys == length pats )      -- Syntactically enforced
          return (mkHsWrapPat coi possibly_mangled_result pat_ty, res)
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        }
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------------------------
-- Data constructors
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tc_pat penv (ConPatIn con arg_pats) pat_ty thing_inside
  = tcConPat penv con pat_ty arg_pats thing_inside
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------------------------
-- Literal patterns
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tc_pat _ (LitPat simple_lit) pat_ty thing_inside
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  = do  { let lit_ty = hsLitType simple_lit
        ; co <- unifyPatType lit_ty pat_ty
                -- coi is of kind: pat_ty ~ lit_ty
        ; res <- thing_inside
        ; return ( mkHsWrapPatCo co (LitPat simple_lit) pat_ty
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                 , res) }
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------------------------
-- Overloaded patterns: n, and n+k
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tc_pat _ (NPat (L l over_lit) mb_neg eq) pat_ty thing_inside
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  = do  { let orig = LiteralOrigin over_lit
        ; lit'    <- newOverloadedLit orig over_lit pat_ty
        ; eq'     <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
        ; mb_neg' <- case mb_neg of
                        Nothing  -> return Nothing      -- Positive literal
                        Just neg ->     -- Negative literal
                                        -- The 'negate' is re-mappable syntax
                            do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
                               ; return (Just neg') }
        ; res <- thing_inside
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        ; return (NPat (L l lit') mb_neg' eq', res) }
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tc_pat penv (NPlusKPat (L nm_loc name) (L loc lit) ge minus) pat_ty thing_inside
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  = do  { (co, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
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        ; let pat_ty' = idType bndr_id
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              orig    = LiteralOrigin lit
        ; lit' <- newOverloadedLit orig lit pat_ty'
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        -- The '>=' and '-' parts are re-mappable syntax
        ; ge'    <- tcSyntaxOp orig ge    (mkFunTys [pat_ty', pat_ty'] boolTy)
        ; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
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        ; let pat' = NPlusKPat (L nm_loc bndr_id) (L loc lit') ge' minus'
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        -- The Report says that n+k patterns must be in Integral
        -- We may not want this when using re-mappable syntax, though (ToDo?)
        ; icls <- tcLookupClass integralClassName
        ; instStupidTheta orig [mkClassPred icls [pat_ty']]
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        ; res <- tcExtendIdEnv1 name bndr_id thing_inside
        ; return (mkHsWrapPatCo co pat' pat_ty, res) }

tc_pat _ _other_pat _ _ = panic "tc_pat"        -- ConPatOut, SigPatOut
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----------------
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unifyPatType :: TcType -> TcType -> TcM TcCoercion
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-- In patterns we want a coercion from the
-- context type (expected) to the actual pattern type
-- But we don't want to reverse the args to unifyType because
-- that controls the actual/expected stuff in error messages
unifyPatType actual_ty expected_ty
  = do { coi <- unifyType actual_ty expected_ty
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       ; return (mkTcSymCo coi) }
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{-
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Note [Hopping the LIE in lazy patterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a lazy pattern, we must *not* discharge constraints from the RHS
from dictionaries bound in the pattern.  E.g.
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        f ~(C x) = 3
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We can't discharge the Num constraint from dictionaries bound by
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the pattern C!
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So we have to make the constraints from thing_inside "hop around"
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the pattern.  Hence the captureConstraints and emitConstraints.
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The same thing ensures that equality constraints in a lazy match
are not made available in the RHS of the match. For example
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        data T a where { T1 :: Int -> T Int; ... }
        f :: T a -> Int -> a
        f ~(T1 i) y = y
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It's obviously not sound to refine a to Int in the right
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hand side, because the argument might not match T1 at all!
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Finally, a lazy pattern should not bind any existential type variables
because they won't be in scope when we do the desugaring

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************************************************************************
*                                                                      *
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        Most of the work for constructors is here
        (the rest is in the ConPatIn case of tc_pat)
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*                                                                      *
************************************************************************
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[Pattern matching indexed data types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following declarations:

  data family Map k :: * -> *
  data instance Map (a, b) v = MapPair (Map a (Pair b v))

and a case expression

  case x :: Map (Int, c) w of MapPair m -> ...

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As explained by [Wrappers for data instance tycons] in MkIds.hs, the
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worker/wrapper types for MapPair are

  $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
  $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v

So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
:R123Map, which means the straight use of boxySplitTyConApp would give a type
error.  Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
boxySplitTyConApp with the family tycon Map instead, which gives us the family
type list {(Int, c), w}.  To get the correct split for :R123Map, we need to
unify the family type list {(Int, c), w} with the instance types {(a, b), v}
(provided by tyConFamInst_maybe together with the family tycon).  This
unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
the split arguments for the representation tycon :R123Map as {Int, c, w}

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In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
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  Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
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moving between representation and family type into account.  To produce type
correct Core, this coercion needs to be used to case the type of the scrutinee
from the family to the representation type.  This is achieved by
unwrapFamInstScrutinee using a CoPat around the result pattern.

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Now it might appear seem as if we could have used the previous GADT type
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refinement infrastructure of refineAlt and friends instead of the explicit
unification and CoPat generation.  However, that would be wrong.  Why?  The
whole point of GADT refinement is that the refinement is local to the case
alternative.  In contrast, the substitution generated by the unification of
the family type list and instance types needs to be propagated to the outside.
Imagine that in the above example, the type of the scrutinee would have been
(Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
substitution [x -> (a, b), v -> w].  In contrast to GADT matching, the
instantiation of x with (a, b) must be global; ie, it must be valid in *all*
alternatives of the case expression, whereas in the GADT case it might vary
between alternatives.

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RIP GADT refinement: refinements have been replaced by the use of explicit
equality constraints that are used in conjunction with implication constraints
to express the local scope of GADT refinements.
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-}
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--      Running example:
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-- MkT :: forall a b c. (a~[b]) => b -> c -> T a
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--       with scrutinee of type (T ty)
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tcConPat :: PatEnv -> Located Name
         -> TcRhoType           -- Type of the pattern
         -> HsConPatDetails Name -> TcM a
         -> TcM (Pat TcId, a)
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tcConPat penv con_lname@(L _ con_name) pat_ty arg_pats thing_inside
  = do  { con_like <- tcLookupConLike con_name
        ; case con_like of
            RealDataCon data_con -> tcDataConPat penv con_lname data_con
                                                 pat_ty arg_pats thing_inside
            PatSynCon pat_syn -> tcPatSynPat penv con_lname pat_syn
                                             pat_ty arg_pats thing_inside
        }

tcDataConPat :: PatEnv -> Located Name -> DataCon
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             -> TcRhoType               -- Type of the pattern
             -> HsConPatDetails Name -> TcM a
             -> TcM (Pat TcId, a)
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tcDataConPat penv (L con_span con_name) data_con pat_ty arg_pats thing_inside
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  = do  { let tycon = dataConTyCon data_con
                  -- For data families this is the representation tycon
              (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _)
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                = dataConFullSig data_con
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              header = L con_span (RealDataCon data_con)
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          -- Instantiate the constructor type variables [a->ty]
          -- This may involve doing a family-instance coercion,
          -- and building a wrapper
        ; (wrap, ctxt_res_tys) <- matchExpectedPatTy (matchExpectedConTy tycon) pat_ty
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          -- Add the stupid theta
        ; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys
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        ; checkExistentials ex_tvs penv
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        ; (tenv, ex_tvs') <- tcInstSuperSkolTyVarsX
                               (zipTopTvSubst univ_tvs ctxt_res_tys) ex_tvs
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                     -- Get location from monad, not from ex_tvs

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        ; let -- pat_ty' = mkTyConApp tycon ctxt_res_tys
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              -- pat_ty' is type of the actual constructor application
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              -- pat_ty' /= pat_ty iff coi /= IdCo
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              arg_tys' = substTys tenv arg_tys
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        ; traceTc "tcConPat" (vcat [ ppr con_name, ppr univ_tvs, ppr ex_tvs, ppr eq_spec
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                                   , ppr ex_tvs', ppr ctxt_res_tys, ppr arg_tys' ])
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        ; if null ex_tvs && null eq_spec && null theta
          then do { -- The common case; no class bindings etc
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                    -- (see Note [Arrows and patterns])
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                    (arg_pats', res) <- tcConArgs (RealDataCon data_con) arg_tys'
                                                  arg_pats penv thing_inside
                  ; let res_pat = ConPatOut { pat_con = header,
                                              pat_tvs = [], pat_dicts = [],
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                                              pat_binds = emptyTcEvBinds,
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                                              pat_args = arg_pats',
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                                              pat_arg_tys = ctxt_res_tys,
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                                              pat_wrap = idHsWrapper }
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                  ; return (mkHsWrapPat wrap res_pat pat_ty, res) }
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          else do   -- The general case, with existential,
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                    -- and local equality constraints
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        { let theta'   = substTheta tenv (eqSpecPreds eq_spec ++ theta)
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                           -- order is *important* as we generate the list of
                           -- dictionary binders from theta'
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              no_equalities = not (any isEqPred theta')
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              skol_info = case pe_ctxt penv of
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                            LamPat mc -> PatSkol (RealDataCon data_con) mc
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                            LetPat {} -> UnkSkol -- Doesn't matter
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        ; gadts_on    <- xoptM Opt_GADTs
        ; families_on <- xoptM Opt_TypeFamilies
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        ; checkTc (no_equalities || gadts_on || families_on)
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                  (text "A pattern match on a GADT requires the" <+>
                   text "GADTs or TypeFamilies language extension")
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                  -- Trac #2905 decided that a *pattern-match* of a GADT
                  -- should require the GADT language flag.
                  -- Re TypeFamilies see also #7156
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        ; given <- newEvVars theta'
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        ; (ev_binds, (arg_pats', res))
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             <- checkConstraints skol_info ex_tvs' given $
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                tcConArgs (RealDataCon data_con) arg_tys' arg_pats penv thing_inside