TcPatSyn.hs 35.3 KB
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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

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\section[TcPatSyn]{Typechecking pattern synonym declarations}
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-}
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{-# LANGUAGE CPP #-}
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{-# LANGUAGE FlexibleContexts #-}
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module TcPatSyn ( tcInferPatSynDecl, tcCheckPatSynDecl
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                , tcPatSynBuilderBind, tcPatSynBuilderOcc, nonBidirectionalErr
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  ) where
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import HsSyn
import TcPat
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import Type( mkTyVarBinders, mkEmptyTCvSubst
           , tidyTyVarBinders, tidyTypes, tidyType )
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import TcRnMonad
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import TcSigs( emptyPragEnv, completeSigFromId )
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import TcEnv
import TcMType
import TysPrim
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import TysWiredIn  ( runtimeRepTy )
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import Name
import SrcLoc
import PatSyn
import NameSet
import Panic
import Outputable
import FastString
import Var
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import VarEnv( emptyTidyEnv, mkInScopeSet )
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import Id
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import IdInfo( RecSelParent(..))
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import TcBinds
import BasicTypes
import TcSimplify
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import TcUnify
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import TcType
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import TcEvidence
import BuildTyCl
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import VarSet
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import MkId
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import TcTyDecls
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import ConLike
import FieldLabel
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import Bag
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import Util
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import ErrUtils
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import FV
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import Control.Monad ( zipWithM )
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import Data.List( partition )
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#include "HsVersions.h"

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{-
************************************************************************
*                                                                      *
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                    Type checking a pattern synonym
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*                                                                      *
************************************************************************
-}
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tcInferPatSynDecl :: PatSynBind Name Name
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                  -> TcM (LHsBinds Id, TcGblEnv)
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tcInferPatSynDecl PSB{ psb_id = lname@(L _ name), psb_args = details,
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                       psb_def = lpat, psb_dir = dir }
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  = addPatSynCtxt lname $
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    do { traceTc "tcInferPatSynDecl {" $ ppr name
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       ; tcCheckPatSynPat lpat
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       ; let (arg_names, rec_fields, is_infix) = collectPatSynArgInfo details
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       ; (tclvl, wanted, ((lpat', args), pat_ty))
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            <- pushLevelAndCaptureConstraints  $
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               do { pat_ty <- newOpenInferExpType
                  ; stuff <- tcPat PatSyn lpat pat_ty $
                             mapM tcLookupId arg_names
                  ; pat_ty <- readExpType pat_ty
                  ; return (stuff, pat_ty) }
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       ; let named_taus = (name, pat_ty) : map (\arg -> (getName arg, varType arg)) args
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       ; (qtvs, req_dicts, ev_binds) <- simplifyInfer tclvl NoRestrictions []
                                                      named_taus wanted
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       ; let ((ex_tvs, ex_vars), prov_dicts) = tcCollectEx lpat'
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             univ_tvs   = filter (not . (`elemVarSet` ex_vars)) qtvs
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             prov_theta = map evVarPred prov_dicts
             req_theta  = map evVarPred req_dicts
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       ; traceTc "tcInferPatSynDecl }" $ ppr name
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       ; tc_patsyn_finish lname dir is_infix lpat'
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                          (mkTyVarBinders Inferred univ_tvs
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                            , req_theta,  ev_binds, req_dicts)
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                          (mkTyVarBinders Inferred ex_tvs
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                            , mkTyVarTys ex_tvs, prov_theta, map EvId prov_dicts)
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                          (map nlHsVar args, map idType args)
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                          pat_ty rec_fields }

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tcCheckPatSynDecl :: PatSynBind Name Name
                  -> TcPatSynInfo
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                  -> TcM (LHsBinds Id, TcGblEnv)
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tcCheckPatSynDecl psb@PSB{ psb_id = lname@(L _ name), psb_args = details
                         , psb_def = lpat, psb_dir = dir }
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                  TPSI{ patsig_implicit_bndrs = implicit_tvs
                      , patsig_univ_bndrs = explicit_univ_tvs, patsig_prov = prov_theta
                      , patsig_ex_bndrs   = explicit_ex_tvs,   patsig_req  = req_theta
                      , patsig_body_ty    = sig_body_ty }
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  = addPatSynCtxt lname $
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    do { let decl_arity = length arg_names
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             (arg_names, rec_fields, is_infix) = collectPatSynArgInfo details

       ; traceTc "tcCheckPatSynDecl" $
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         vcat [ ppr implicit_tvs, ppr explicit_univ_tvs, ppr req_theta
              , ppr explicit_ex_tvs, ppr prov_theta, ppr sig_body_ty ]
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       ; tcCheckPatSynPat lpat

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       ; (arg_tys, pat_ty) <- case tcSplitFunTysN decl_arity sig_body_ty of
                                 Right stuff  -> return stuff
                                 Left missing -> wrongNumberOfParmsErr name decl_arity missing

       -- Complain about:  pattern P :: () => forall x. x -> P x
       -- The existential 'x' should not appear in the result type
       -- Can't check this until we know P's arity
       ; let bad_tvs = filter (`elemVarSet` tyCoVarsOfType pat_ty) explicit_ex_tvs
       ; checkTc (null bad_tvs) $
         hang (sep [ text "The result type of the signature for" <+> quotes (ppr name) <> comma
                   , text "namely" <+> quotes (ppr pat_ty) ])
            2 (text "mentions existential type variable" <> plural bad_tvs
               <+> pprQuotedList bad_tvs)

         -- See Note [The pattern-synonym signature splitting rule]
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       ; let univ_fvs = closeOverKinds $
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                        (tyCoVarsOfTypes (pat_ty : req_theta) `extendVarSetList` explicit_univ_tvs)
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             (extra_univ, extra_ex) = partition ((`elemVarSet` univ_fvs) . binderVar) implicit_tvs
             univ_bndrs = extra_univ ++ mkTyVarBinders Specified explicit_univ_tvs
             ex_bndrs   = extra_ex   ++ mkTyVarBinders Specified explicit_ex_tvs
             univ_tvs   = binderVars univ_bndrs
             ex_tvs     = binderVars ex_bndrs
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       -- Right!  Let's check the pattern against the signature
       -- See Note [Checking against a pattern signature]
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       ; req_dicts <- newEvVars req_theta
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       ; (tclvl, wanted, (lpat', (ex_tvs', prov_dicts, args'))) <-
           ASSERT2( equalLength arg_names arg_tys, ppr name $$ ppr arg_names $$ ppr arg_tys )
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           pushLevelAndCaptureConstraints            $
           tcExtendTyVarEnv univ_tvs                 $
           tcPat PatSyn lpat (mkCheckExpType pat_ty) $
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           do { let new_tv | isUnidirectional dir = newMetaTyVarX
                           | otherwise            = newMetaSigTyVarX
                    in_scope    = mkInScopeSet (mkVarSet univ_tvs)
                    empty_subst = mkEmptyTCvSubst in_scope
              ; (subst, ex_tvs') <- mapAccumLM new_tv empty_subst ex_tvs
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                    -- See the "Existential type variables" part of
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                    -- Note [Checking against a pattern signature]
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              ; traceTc "tcpatsyn1" (vcat [ ppr v <+> dcolon <+> ppr (tyVarKind v) | v <- ex_tvs])
              ; traceTc "tcpatsyn2" (vcat [ ppr v <+> dcolon <+> ppr (tyVarKind v) | v <- ex_tvs'])
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              ; let prov_theta' = substTheta subst prov_theta
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                  -- Add univ_tvs to the in_scope set to
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                  -- satisfy the substition invariant. There's no need to
                  -- add 'ex_tvs' as they are already in the domain of the
                  -- substitution.
                  -- See also Note [The substitution invariant] in TyCoRep.
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              ; prov_dicts <- mapM (emitWanted (ProvCtxtOrigin psb)) prov_theta'
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              ; args'      <- zipWithM (tc_arg subst) arg_names arg_tys
              ; return (ex_tvs', prov_dicts, args') }

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       ; let skol_info = SigSkol (PatSynCtxt name) (mkPhiTy req_theta pat_ty)
                         -- The type here is a bit bogus, but we do not print
                         -- the type for PatSynCtxt, so it doesn't matter
                         -- See TcRnTypes Note [Skolem info for pattern synonyms]
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       ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info univ_tvs req_dicts wanted
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       -- Solve the constraints now, because we are about to make a PatSyn,
       -- which should not contain unification variables and the like (Trac #10997)
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       ; empty_binds <- simplifyTop (mkImplicWC implics)

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       -- Since all the inputs are implications the returned bindings will be empty
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       ; MASSERT2( isEmptyBag empty_binds, ppr empty_binds )
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       -- ToDo: in the bidirectional case, check that the ex_tvs' are all distinct
       -- Otherwise we may get a type error when typechecking the builder,
       -- when that should be impossible
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       ; traceTc "tcCheckPatSynDecl }" $ ppr name
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       ; tc_patsyn_finish lname dir is_infix lpat'
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                          (univ_bndrs, req_theta, ev_binds, req_dicts)
                          (ex_bndrs, mkTyVarTys ex_tvs', prov_theta, prov_dicts)
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                          (args', arg_tys)
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                          pat_ty rec_fields }
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  where
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    tc_arg :: TCvSubst -> Name -> Type -> TcM (LHsExpr TcId)
    tc_arg subst arg_name arg_ty
      = do {   -- Look up the variable actually bound by lpat
               -- and check that it has the expected type
             arg_id <- tcLookupId arg_name
           ; coi <- unifyType (Just arg_id)
                              (idType arg_id)
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                              (substTyUnchecked subst arg_ty)
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           ; return (mkLHsWrapCo coi $ nlHsVar arg_id) }

{- Note [Checking against a pattern signature]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When checking the actual supplied pattern against the pattern synonym
signature, we need to be quite careful.

----- Provided constraints
Example

    data T a where
      MkT :: Ord a => a -> T a

    pattern P :: () => Eq a => a -> [T a]
    pattern P x = [MkT x]

We must check that the (Eq a) that P claims to bind (and to
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make available to matches against P), is derivable from the
actual pattern.  For example:
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    f (P (x::a)) = ...here (Eq a) should be available...
And yes, (Eq a) is derivable from the (Ord a) bound by P's rhs.

----- Existential type variables
Unusually, we instantiate the existential tyvars of the pattern with
*meta* type variables.  For example

    data S where
      MkS :: Eq a => [a] -> S

    pattern P :: () => Eq x => x -> S
    pattern P x <- MkS x

The pattern synonym conceals from its client the fact that MkS has a
list inside it.  The client just thinks it's a type 'x'.  So we must
unify x := [a] during type checking, and then use the instantiating type
[a] (called ex_tys) when building the matcher.  In this case we'll get

   $mP :: S -> (forall x. Ex x => x -> r) -> r -> r
   $mP x k = case x of
               MkS a (d:Eq a) (ys:[a]) -> let dl :: Eq [a]
                                              dl = $dfunEqList d
                                          in k [a] dl ys

This "concealing" story works for /uni-directional/ pattern synonmys,
but obviously not for bidirectional ones.  So in the bidirectional case
we use SigTv, rather than a generic TauTv, meta-tyvar so that.  And
we should really check that those SigTvs don't get unified with each
other.

-}
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collectPatSynArgInfo :: HsPatSynDetails (Located Name) -> ([Name], [Name], Bool)
collectPatSynArgInfo details =
  case details of
    PrefixPatSyn names      -> (map unLoc names, [], False)
    InfixPatSyn name1 name2 -> (map unLoc [name1, name2], [], True)
    RecordPatSyn names ->
      let (vars, sels) = unzip (map splitRecordPatSyn names)
      in (vars, sels, False)

  where
    splitRecordPatSyn :: RecordPatSynField (Located Name) -> (Name, Name)
    splitRecordPatSyn (RecordPatSynField { recordPatSynPatVar = L _ patVar
                                         , recordPatSynSelectorId = L _ selId })
      = (patVar, selId)

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addPatSynCtxt :: Located Name -> TcM a -> TcM a
addPatSynCtxt (L loc name) thing_inside
  = setSrcSpan loc $
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    addErrCtxt (text "In the declaration for pattern synonym"
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                <+> quotes (ppr name)) $
    thing_inside

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wrongNumberOfParmsErr :: Name -> Arity -> Arity -> TcM a
wrongNumberOfParmsErr name decl_arity missing
  = failWithTc $
    hang (text "Pattern synonym" <+> quotes (ppr name) <+> ptext (sLit "has")
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          <+> speakNOf decl_arity (text "argument"))
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       2 (text "but its type signature has" <+> int missing <+> text "fewer arrows")
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-------------------------
-- Shared by both tcInferPatSyn and tcCheckPatSyn
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tc_patsyn_finish :: Located Name  -- ^ PatSyn Name
                 -> HsPatSynDir Name  -- ^ PatSyn type (Uni/Bidir/ExplicitBidir)
                 -> Bool              -- ^ Whether infix
                 -> LPat Id           -- ^ Pattern of the PatSyn
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                 -> ([TcTyVarBinder], [PredType], TcEvBinds, [EvVar])
                 -> ([TcTyVarBinder], [TcType], [PredType], [EvTerm])
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                 -> ([LHsExpr TcId], [TcType])   -- ^ Pattern arguments and types
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                 -> TcType              -- ^ Pattern type
                 -> [Name]              -- ^ Selector names
                 -- ^ Whether fields, empty if not record PatSyn
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                 -> TcM (LHsBinds Id, TcGblEnv)
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tc_patsyn_finish lname dir is_infix lpat'
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                 (univ_bndrs, req_theta, req_ev_binds, req_dicts)
                 (ex_bndrs, ex_tys, prov_theta, prov_dicts)
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                 (args, arg_tys)
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                 pat_ty field_labels
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  = do { -- Zonk everything.  We are about to build a final PatSyn
         -- so there had better be no unification variables in there
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         univ_tvs'    <- mapMaybeM zonk_qtv univ_bndrs
       ; ex_tvs'      <- mapMaybeM zonk_qtv ex_bndrs
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       ; prov_theta'  <- zonkTcTypes prov_theta
       ; req_theta'   <- zonkTcTypes req_theta
       ; pat_ty'      <- zonkTcType pat_ty
       ; arg_tys'     <- zonkTcTypes arg_tys

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       ; let (env1, univ_tvs) = tidyTyVarBinders emptyTidyEnv univ_tvs'
             (env2, ex_tvs)   = tidyTyVarBinders env1 ex_tvs'
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             req_theta  = tidyTypes env2 req_theta'
             prov_theta = tidyTypes env2 prov_theta'
             arg_tys    = tidyTypes env2 arg_tys'
             pat_ty     = tidyType  env2 pat_ty'
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       ; traceTc "tc_patsyn_finish {" $
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           ppr (unLoc lname) $$ ppr (unLoc lpat') $$
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           ppr (univ_tvs, req_theta, req_ev_binds, req_dicts) $$
           ppr (ex_tvs, prov_theta, prov_dicts) $$
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           ppr args $$
           ppr arg_tys $$
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           ppr pat_ty
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       -- Make the 'matcher'
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       ; (matcher_id, matcher_bind) <- tcPatSynMatcher lname lpat'
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                                         (binderVars univ_tvs, req_theta, req_ev_binds, req_dicts)
                                         (binderVars ex_tvs, ex_tys, prov_theta, prov_dicts)
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                                         (args, arg_tys)
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                                         pat_ty
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       -- Make the 'builder'
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       ; builder_id <- mkPatSynBuilderId dir lname
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                                         univ_tvs req_theta
                                         ex_tvs   prov_theta
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                                         arg_tys pat_ty
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         -- TODO: Make this have the proper information
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       ; let mkFieldLabel name = FieldLabel { flLabel = occNameFS (nameOccName name)
                                            , flIsOverloaded = False
                                            , flSelector = name }
             field_labels' = map mkFieldLabel field_labels
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       -- Make the PatSyn itself
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       ; let patSyn = mkPatSyn (unLoc lname) is_infix
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                        (univ_tvs, req_theta)
                        (ex_tvs, prov_theta)
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                        arg_tys
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                        pat_ty
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                        matcher_id builder_id
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                        field_labels'

       -- Selectors
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       ; let rn_rec_sel_binds = mkPatSynRecSelBinds patSyn (patSynFieldLabels patSyn)
             tything = AConLike (PatSynCon patSyn)
       ; tcg_env <- tcExtendGlobalEnv [tything] $
                    tcRecSelBinds rn_rec_sel_binds
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       ; traceTc "tc_patsyn_finish }" empty
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       ; return (matcher_bind, tcg_env) }
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  where
    -- This is a bit of an odd functions; why does it not occur elsewhere
    zonk_qtv :: TcTyVarBinder -> TcM (Maybe TcTyVarBinder)
    zonk_qtv (TvBndr tv vis)
      = do { mb_tv' <- zonkQuantifiedTyVar False tv
                    -- ToDo: The False means that we behave here as if
                    -- -XPolyKinds was always on, which isn't right.
           ; return (fmap (\tv' -> TvBndr tv' vis) mb_tv') }
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{-
************************************************************************
*                                                                      *
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         Constructing the "matcher" Id and its binding
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*                                                                      *
************************************************************************
-}
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tcPatSynMatcher :: Located Name
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                -> LPat Id
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                -> ([TcTyVar], ThetaType, TcEvBinds, [EvVar])
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                -> ([TcTyVar], [TcType], ThetaType, [EvTerm])
                -> ([LHsExpr TcId], [TcType])
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                -> TcType
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                -> TcM ((Id, Bool), LHsBinds Id)
-- See Note [Matchers and builders for pattern synonyms] in PatSyn
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tcPatSynMatcher (L loc name) lpat
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                (univ_tvs, req_theta, req_ev_binds, req_dicts)
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                (ex_tvs, ex_tys, prov_theta, prov_dicts)
                (args, arg_tys) pat_ty
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  = do { rr_name <- newNameAt (mkTyVarOcc "rep") loc
       ; tv_name <- newNameAt (mkTyVarOcc "r")   loc
       ; let rr_tv    = mkTcTyVar rr_name runtimeRepTy (SkolemTv False)
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             rr       = mkTyVarTy rr_tv
             res_tv   = mkTcTyVar tv_name  (tYPE rr) (SkolemTv False)
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             is_unlifted = null args && null prov_dicts
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             res_ty = mkTyVarTy res_tv
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             (cont_args, cont_arg_tys)
               | is_unlifted = ([nlHsVar voidPrimId], [voidPrimTy])
               | otherwise   = (args,                 arg_tys)
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             cont_ty = mkInfSigmaTy ex_tvs prov_theta $
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                       mkFunTys cont_arg_tys res_ty

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             fail_ty  = mkFunTy voidPrimTy res_ty
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       ; matcher_name <- newImplicitBinder name mkMatcherOcc
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       ; scrutinee    <- newSysLocalId (fsLit "scrut") pat_ty
       ; cont         <- newSysLocalId (fsLit "cont")  cont_ty
       ; fail         <- newSysLocalId (fsLit "fail")  fail_ty

       ; let matcher_tau   = mkFunTys [pat_ty, cont_ty, fail_ty] res_ty
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             matcher_sigma = mkInfSigmaTy (rr_tv:res_tv:univ_tvs) req_theta matcher_tau
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             matcher_id    = mkExportedVanillaId matcher_name matcher_sigma
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                             -- See Note [Exported LocalIds] in Id
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             inst_wrap = mkWpEvApps prov_dicts <.> mkWpTyApps ex_tys
             cont' = foldl nlHsApp (mkLHsWrap inst_wrap (nlHsVar cont)) cont_args
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             fail' = nlHsApps fail [nlHsVar voidPrimId]
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             args = map nlVarPat [scrutinee, cont, fail]
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             lwpat = noLoc $ WildPat pat_ty
             cases = if isIrrefutableHsPat lpat
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                     then [mkHsCaseAlt lpat  cont']
                     else [mkHsCaseAlt lpat  cont',
                           mkHsCaseAlt lwpat fail']
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             body = mkLHsWrap (mkWpLet req_ev_binds) $
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                    L (getLoc lpat) $
                    HsCase (nlHsVar scrutinee) $
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                    MG{ mg_alts = L (getLoc lpat) cases
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                      , mg_arg_tys = [pat_ty]
                      , mg_res_ty = res_ty
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                      , mg_origin = Generated
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                      }
             body' = noLoc $
                     HsLam $
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                     MG{ mg_alts = noLoc [mkSimpleMatch LambdaExpr
                                                        args body]
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                       , mg_arg_tys = [pat_ty, cont_ty, res_ty]
                       , mg_res_ty = res_ty
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                       , mg_origin = Generated
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                       }
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             match = mkMatch (FunRhs (L loc name) Prefix) []
                             (mkHsLams (rr_tv:res_tv:univ_tvs)
                             req_dicts body')
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                             (noLoc EmptyLocalBinds)
             mg = MG{ mg_alts = L (getLoc match) [match]
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                    , mg_arg_tys = []
                    , mg_res_ty = res_ty
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                    , mg_origin = Generated
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                    }

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       ; let bind = FunBind{ fun_id = L loc matcher_id
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                           , fun_matches = mg
                           , fun_co_fn = idHsWrapper
                           , bind_fvs = emptyNameSet
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                           , fun_tick = [] }
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             matcher_bind = unitBag (noLoc bind)
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       ; traceTc "tcPatSynMatcher" (ppr name $$ ppr (idType matcher_id))
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       ; traceTc "tcPatSynMatcher" (ppr matcher_bind)

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       ; return ((matcher_id, is_unlifted), matcher_bind) }

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mkPatSynRecSelBinds :: PatSyn
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                    -> [FieldLabel]  -- ^ Visible field labels
                    -> HsValBinds Name
mkPatSynRecSelBinds ps fields
  = ValBindsOut selector_binds sigs
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  where
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    (sigs, selector_binds) = unzip (map mkRecSel fields)
    mkRecSel fld_lbl = mkOneRecordSelector [PatSynCon ps] (RecSelPatSyn ps) fld_lbl
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isUnidirectional :: HsPatSynDir a -> Bool
isUnidirectional Unidirectional          = True
isUnidirectional ImplicitBidirectional   = False
isUnidirectional ExplicitBidirectional{} = False

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{-
************************************************************************
*                                                                      *
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         Constructing the "builder" Id
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*                                                                      *
************************************************************************
-}
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mkPatSynBuilderId :: HsPatSynDir a -> Located Name
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                  -> [TyVarBinder] -> ThetaType
                  -> [TyVarBinder] -> ThetaType
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                  -> [Type] -> Type
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                  -> TcM (Maybe (Id, Bool))
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mkPatSynBuilderId dir (L _ name)
                  univ_bndrs req_theta ex_bndrs prov_theta
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                  arg_tys pat_ty
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  | isUnidirectional dir
  = return Nothing
  | otherwise
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  = do { builder_name <- newImplicitBinder name mkBuilderOcc
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       ; let theta          = req_theta ++ prov_theta
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             need_dummy_arg = isUnliftedType pat_ty && null arg_tys && null theta
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             builder_sigma  = add_void need_dummy_arg $
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                              mkForAllTys univ_bndrs $
                              mkForAllTys ex_bndrs $
                              mkFunTys theta $
                              mkFunTys arg_tys $
                              pat_ty
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             builder_id     = mkExportedVanillaId builder_name builder_sigma
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              -- See Note [Exported LocalIds] in Id
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       ; return (Just (builder_id, need_dummy_arg)) }
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  where
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tcPatSynBuilderBind :: PatSynBind Name Name
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                    -> TcM (LHsBinds Id)
-- See Note [Matchers and builders for pattern synonyms] in PatSyn
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tcPatSynBuilderBind (PSB { psb_id = L loc name, psb_def = lpat
                         , psb_dir = dir, psb_args = details })
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  | isUnidirectional dir
  = return emptyBag

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  | Left why <- mb_match_group       -- Can't invert the pattern
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  = setSrcSpan (getLoc lpat) $ failWithTc $
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    vcat [ hang (text "Invalid right-hand side of bidirectional pattern synonym"
                 <+> quotes (ppr name) <> colon)
              2 why
         , text "RHS pattern:" <+> ppr lpat ]
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  | Right match_group <- mb_match_group  -- Bidirectional
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  = do { patsyn <- tcLookupPatSyn name
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       ; traceTc "tcPatSynBuilderBind {" $ ppr patsyn
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       ; let Just (builder_id, need_dummy_arg) = patSynBuilder patsyn
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                   -- Bidirectional, so patSynBuilder returns Just

             match_group' | need_dummy_arg = add_dummy_arg match_group
                          | otherwise      = match_group

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             bind = FunBind { fun_id      = L loc (idName builder_id)
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                            , fun_matches = match_group'
                            , fun_co_fn   = idHsWrapper
                            , bind_fvs    = placeHolderNamesTc
                            , fun_tick    = [] }

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             sig = completeSigFromId (PatSynCtxt name) builder_id
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       ; (builder_binds, _) <- tcPolyCheck emptyPragEnv sig (noLoc bind)
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       ; traceTc "tcPatSynBuilderBind }" $ ppr builder_binds
       ; return builder_binds }
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  | otherwise = panic "tcPatSynBuilderBind"  -- Both cases dealt with
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  where
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    mb_match_group
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       = case dir of
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           ExplicitBidirectional explicit_mg -> Right explicit_mg
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           ImplicitBidirectional             -> fmap mk_mg (tcPatToExpr args lpat)
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           Unidirectional -> panic "tcPatSynBuilderBind"
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    mk_mg :: LHsExpr Name -> MatchGroup Name (LHsExpr Name)
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    mk_mg body = mkMatchGroup Generated [builder_match]
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             where
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               builder_args  = [L loc (VarPat (L loc n)) | L loc n <- args]
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               builder_match = mkMatch (FunRhs (L loc name) Prefix)
                                       builder_args body
                                       (noLoc EmptyLocalBinds)
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    args = case details of
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              PrefixPatSyn args     -> args
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              InfixPatSyn arg1 arg2 -> [arg1, arg2]
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              RecordPatSyn args     -> map recordPatSynPatVar args
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    add_dummy_arg :: MatchGroup Name (LHsExpr Name)
                  -> MatchGroup Name (LHsExpr Name)
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    add_dummy_arg mg@(MG { mg_alts = L l [L loc match@(Match { m_pats = pats })] })
      = mg { mg_alts = L l [L loc (match { m_pats = nlWildPatName : pats })] }
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    add_dummy_arg other_mg = pprPanic "add_dummy_arg" $
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                             pprMatches other_mg
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tcPatSynBuilderOcc :: PatSyn -> TcM (HsExpr TcId, TcSigmaType)
-- monadic only for failure
tcPatSynBuilderOcc ps
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  | Just (builder_id, add_void_arg) <- builder
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  , let builder_expr = HsVar (noLoc builder_id)
        builder_ty   = idType builder_id
  = return $
    if add_void_arg
    then ( HsApp (noLoc $ builder_expr) (nlHsVar voidPrimId)
         , tcFunResultTy builder_ty )
    else (builder_expr, builder_ty)
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  | otherwise  -- Unidirectional
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  = nonBidirectionalErr name
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  where
    name    = patSynName ps
    builder = patSynBuilder ps
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add_void :: Bool -> Type -> Type
add_void need_dummy_arg ty
  | need_dummy_arg = mkFunTy voidPrimTy ty
  | otherwise      = ty

tcPatToExpr :: [Located Name] -> LPat Name -> Either MsgDoc (LHsExpr Name)
-- Given a /pattern/, return an /expression/ that builds a value
-- that matches the pattern.  E.g. if the pattern is (Just [x]),
-- the expression is (Just [x]).  They look the same, but the
-- input uses constructors from HsPat and the output uses constructors
-- from HsExpr.
--
-- Returns (Left r) if the pattern is not invertible, for reason r.
-- See Note [Builder for a bidirectional pattern synonym]
tcPatToExpr args pat = go pat
  where
    lhsVars = mkNameSet (map unLoc args)

    -- Make a prefix con for prefix and infix patterns for simplicity
    mkPrefixConExpr :: Located Name -> [LPat Name] -> Either MsgDoc (HsExpr Name)
    mkPrefixConExpr lcon@(L loc _) pats
      = do { exprs <- mapM go pats
           ; return (foldl (\x y -> HsApp (L loc x) y)
                           (HsVar lcon) exprs) }

    mkRecordConExpr :: Located Name -> HsRecFields Name (LPat Name)
                    -> Either MsgDoc (HsExpr Name)
    mkRecordConExpr con fields
      = do { exprFields <- mapM go fields
           ; return (RecordCon con PlaceHolder noPostTcExpr exprFields) }

    go :: LPat Name -> Either MsgDoc (LHsExpr Name)
    go (L loc p) = L loc <$> go1 p

    go1 :: Pat Name -> Either MsgDoc (HsExpr Name)
    go1 (ConPatIn con info)
      = case info of
          PrefixCon ps  -> mkPrefixConExpr con ps
          InfixCon l r  -> mkPrefixConExpr con [l,r]
          RecCon fields -> mkRecordConExpr con fields

    go1 (SigPatIn pat _) = go1 (unLoc pat)
        -- See Note [Type signatures and the builder expression]

    go1 (VarPat (L l var))
        | var `elemNameSet` lhsVars
        = return $ HsVar (L l var)
        | otherwise
        = Left (quotes (ppr var) <+> text "is not bound by the LHS of the pattern synonym")
    go1 (ParPat pat)                = fmap HsPar $ go pat
    go1 (LazyPat pat)               = go1 (unLoc pat)
    go1 (BangPat pat)               = go1 (unLoc pat)
    go1 (PArrPat pats ptt)          = do { exprs <- mapM go pats
                                         ; return $ ExplicitPArr ptt exprs }
    go1 (ListPat pats ptt reb)      = do { exprs <- mapM go pats
                                         ; return $ ExplicitList ptt (fmap snd reb) exprs }
    go1 (TuplePat pats box _)       = do { exprs <- mapM go pats
                                         ; return $ ExplicitTuple
                                              (map (noLoc . Present) exprs) box }
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    go1 (SumPat pat alt arity _)    = do { expr <- go1 (unLoc pat)
                                         ; return $ ExplicitSum alt arity (noLoc expr) PlaceHolder
                                         }
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    go1 (LitPat lit)                = return $ HsLit lit
    go1 (NPat (L _ n) mb_neg _ _)
        | Just neg <- mb_neg        = return $ unLoc $ nlHsSyntaxApps neg [noLoc (HsOverLit n)]
        | otherwise                 = return $ HsOverLit n
    go1 (ConPatOut{})               = panic "ConPatOut in output of renamer"
    go1 (SigPatOut{})               = panic "SigPatOut in output of renamer"
    go1 (CoPat{})                   = panic "CoPat in output of renamer"
    go1 p = Left (text "pattern" <+> quotes (ppr p) <+> text "is not invertible")

{- Note [Builder for a bidirectional pattern synonym]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For a bidirectional pattern synonym we need to produce an /expression/
that matches the supplied /pattern/, given values for the arguments
of the pattern synoymy.  For example
  pattern F x y = (Just x, [y])
The 'builder' for F looks like
  $builderF x y = (Just x, [y])

We can't always do this:
 * Some patterns aren't invertible; e.g. view patterns
      pattern F x = (reverse -> x:_)

 * The RHS pattern might bind more variables than the pattern
   synonym, so again we can't invert it
      pattern F x = (x,y)

 * Ditto wildcards
      pattern F x = (x,_)


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Note [Redundant constraints for builder]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The builder can have redundant constraints, which are awkard to eliminate.
Consider
   pattern P = Just 34
To match against this pattern we need (Eq a, Num a).  But to build
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(Just 34) we need only (Num a).  Fortunately instTcSigFromId sets
sig_warn_redundant to False.
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************************************************************************
*                                                                      *
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         Helper functions
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*                                                                      *
************************************************************************
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Note [As-patterns in pattern synonym definitions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The rationale for rejecting as-patterns in pattern synonym definitions
is that an as-pattern would introduce nonindependent pattern synonym
arguments, e.g. given a pattern synonym like:
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        pattern K x y = x@(Just y)

one could write a nonsensical function like

        f (K Nothing x) = ...
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or
        g (K (Just True) False) = ...
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Note [Type signatures and the builder expression]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   pattern L x = Left x :: Either [a] [b]

In tc{Infer/Check}PatSynDecl we will check that the pattern has the
specified type.  We check the pattern *as a pattern*, so the type
signature is a pattern signature, and so brings 'a' and 'b' into
scope.  But we don't have a way to bind 'a, b' in the LHS, as we do
'x', say.  Nevertheless, the sigature may be useful to constrain
the type.

When making the binding for the *builder*, though, we don't want
  $buildL x = Left x :: Either [a] [b]
because that wil either mean (forall a b. Either [a] [b]), or we'll
get a complaint that 'a' and 'b' are out of scope. (Actually the
latter; Trac #9867.)  No, the job of the signature is done, so when
converting the pattern to an expression (for the builder RHS) we
simply discard the signature.
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Note [Record PatSyn Desugaring]
-------------------------------
It is important that prov_theta comes before req_theta as this ordering is used
when desugaring record pattern synonym updates.

Any change to this ordering should make sure to change deSugar/DsExpr.hs if you
want to avoid difficult to decipher core lint errors!
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 -}
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tcCheckPatSynPat :: LPat Name -> TcM ()
tcCheckPatSynPat = go
  where
    go :: LPat Name -> TcM ()
    go = addLocM go1

    go1 :: Pat Name -> TcM ()
    go1   (ConPatIn _ info)   = mapM_ go (hsConPatArgs info)
    go1   VarPat{}            = return ()
    go1   WildPat{}           = return ()
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    go1 p@(AsPat _ _)         = asPatInPatSynErr p
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    go1   (LazyPat pat)       = go pat
    go1   (ParPat pat)        = go pat
    go1   (BangPat pat)       = go pat
    go1   (PArrPat pats _)    = mapM_ go pats
    go1   (ListPat pats _ _)  = mapM_ go pats
    go1   (TuplePat pats _ _) = mapM_ go pats
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    go1   (SumPat pat _ _ _)  = go pat
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    go1   LitPat{}            = return ()
    go1   NPat{}              = return ()
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    go1   (SigPatIn pat _)    = go pat
    go1   (ViewPat _ pat _)   = go pat
    go1 p@SplicePat{}         = thInPatSynErr p
    go1 p@NPlusKPat{}         = nPlusKPatInPatSynErr p
    go1   ConPatOut{}         = panic "ConPatOut in output of renamer"
    go1   SigPatOut{}         = panic "SigPatOut in output of renamer"
    go1   CoPat{}             = panic "CoPat in output of renamer"

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asPatInPatSynErr :: (OutputableBndrId name) => Pat name -> TcM a
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asPatInPatSynErr pat
  = failWithTc $
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    hang (text "Pattern synonym definition cannot contain as-patterns (@):")
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       2 (ppr pat)

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thInPatSynErr :: (OutputableBndrId name) => Pat name -> TcM a
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thInPatSynErr pat
  = failWithTc $
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    hang (text "Pattern synonym definition cannot contain Template Haskell:")
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       2 (ppr pat)

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nPlusKPatInPatSynErr :: (OutputableBndrId name) => Pat name -> TcM a
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nPlusKPatInPatSynErr pat
  = failWithTc $
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    hang (text "Pattern synonym definition cannot contain n+k-pattern:")
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       2 (ppr pat)

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nonBidirectionalErr :: Outputable name => name -> TcM a
nonBidirectionalErr name = failWithTc $
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    text "non-bidirectional pattern synonym"
    <+> quotes (ppr name) <+> text "used in an expression"
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-- Walk the whole pattern and for all ConPatOuts, collect the
-- existentially-bound type variables and evidence binding variables.
--
-- These are used in computing the type of a pattern synonym and also
-- in generating matcher functions, since success continuations need
-- to be passed these pattern-bound evidences.
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tcCollectEx
  :: LPat Id
  -> ( ([Var], VarSet) -- Existentially-bound type variables as a
                       -- deterministically ordered list and a set.
                       -- See Note [Deterministic FV] in FV
     , [EvVar]
     )
tcCollectEx pat = let (fv, evs) = go pat in (fvVarListVarSet fv, evs)
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  where
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    go :: LPat Id -> (FV, [EvVar])
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    go = go1 . unLoc

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    go1 :: Pat Id -> (FV, [EvVar])
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    go1 (LazyPat p)         = go p
    go1 (AsPat _ p)         = go p
    go1 (ParPat p)          = go p
    go1 (BangPat p)         = go p
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    go1 (ListPat ps _ _)    = mergeMany . map go $ ps
    go1 (TuplePat ps _ _)   = mergeMany . map go $ ps
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    go1 (SumPat p _ _ _)    = go p
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    go1 (PArrPat ps _)      = mergeMany . map go $ ps
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    go1 (ViewPat _ p _)     = go p
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    go1 con@ConPatOut{}     = merge (FV.mkFVs (pat_tvs con), pat_dicts con) $
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                                 goConDetails $ pat_args con
    go1 (SigPatOut p _)     = go p
    go1 (CoPat _ p _)       = go1 p
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    go1 (NPlusKPat n k _ geq subtract _)
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      = pprPanic "TODO: NPlusKPat" $ ppr n $$ ppr k $$ ppr geq $$ ppr subtract
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    go1 _                   = empty
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    goConDetails :: HsConPatDetails Id -> (FV, [EvVar])
    goConDetails (PrefixCon ps) = mergeMany . map go $ ps
    goConDetails (InfixCon p1 p2) = go p1 `merge` go p2
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    goConDetails (RecCon HsRecFields{ rec_flds = flds })
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      = mergeMany . map goRecFd $ flds
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    goRecFd :: LHsRecField Id (LPat Id) -> (FV, [EvVar])
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    goRecFd (L _ HsRecField{ hsRecFieldArg = p }) = go p
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    merge (vs1, evs1) (vs2, evs2) = (vs1 `unionFV` vs2, evs1 ++ evs2)
    mergeMany = foldr merge empty
    empty = (emptyFV, [])