[ghc-hetmet.git] / compiler / typecheck / TcPat.lhs

%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%

TcPat: Typechecking patterns

\begin{code}
module TcPat ( tcLetPat, tcLamPat, tcLamPats, tcOverloadedLit,
               addDataConStupidTheta, badFieldCon, polyPatSig ) where

#include "HsVersions.h"

import {-# SOURCE #-}   TcExpr( tcSyntaxOp )

import HsSyn
import TcHsSyn
import TcRnMonad
import Inst
import Id
import Var
import CoreFVs
import Name
import TcSimplify
import TcEnv
import TcMType
import TcType
import VarSet
import TcUnify
import TcHsType
import TysWiredIn
import TcGadt
import Type
import StaticFlags
import TyCon
import DataCon
import PrelNames
import BasicTypes hiding (SuccessFlag(..))
import SrcLoc
import ErrUtils
import Util
import Maybes
import Outputable
import FastString
\end{code}


%************************************************************************
%*                                                                      *
                External interface
%*                                                                      *
%************************************************************************

\begin{code}
tcLetPat :: (Name -> Maybe TcRhoType)
         -> LPat Name -> BoxySigmaType 
         -> TcM a
         -> TcM (LPat TcId, a)
tcLetPat sig_fn pat pat_ty thing_inside
  = do  { let init_state = PS { pat_ctxt = LetPat sig_fn, 
                                pat_reft = emptyRefinement }
        ; (pat', ex_tvs, res) <- tc_lpat pat pat_ty init_state (\ _ -> thing_inside)

        -- Don't know how to deal with pattern-bound existentials yet
        ; checkTc (null ex_tvs) (existentialExplode pat)

        ; return (pat', res) }

-----------------
tcLamPats :: [LPat Name]                                -- Patterns,
          -> [BoxySigmaType]                            --   and their types
          -> BoxyRhoType                                -- Result type,
          -> ((Refinement, BoxyRhoType) -> TcM a)       --   and the checker for the body
          -> TcM ([LPat TcId], a)

-- This is the externally-callable wrapper function
-- Typecheck the patterns, extend the environment to bind the variables,
-- do the thing inside, use any existentially-bound dictionaries to 
-- discharge parts of the returning LIE, and deal with pattern type
-- signatures

--   1. Initialise the PatState
--   2. Check the patterns
--   3. Apply the refinement to the environment and result type
--   4. Check the body
--   5. Check that no existentials escape

tcLamPats pats tys res_ty thing_inside
  = tc_lam_pats (zipEqual "tcLamPats" pats tys)
                (emptyRefinement, res_ty) thing_inside

tcLamPat :: LPat Name -> BoxySigmaType 
         -> (Refinement,BoxyRhoType)            -- Result type
         -> ((Refinement,BoxyRhoType) -> TcM a) -- Checker for body, given its result type
         -> TcM (LPat TcId, a)
tcLamPat pat pat_ty res_ty thing_inside
  = do  { ([pat'],thing) <- tc_lam_pats [(pat, pat_ty)] res_ty thing_inside
        ; return (pat', thing) }

-----------------
tc_lam_pats :: [(LPat Name,BoxySigmaType)]
            -> (Refinement,BoxyRhoType)                 -- Result type
            -> ((Refinement,BoxyRhoType) -> TcM a)      -- Checker for body, given its result type
            -> TcM ([LPat TcId], a)
tc_lam_pats pat_ty_prs (reft, res_ty) thing_inside 
  =  do { let init_state = PS { pat_ctxt = LamPat, pat_reft = reft }

        ; (pats', ex_tvs, res) <- tcMultiple tc_lpat_pr pat_ty_prs init_state $ \ pstate' ->
                                  refineEnvironment (pat_reft pstate') $
                                  thing_inside (pat_reft pstate', res_ty)

        ; let tys = map snd pat_ty_prs
        ; tcCheckExistentialPat pats' ex_tvs tys res_ty

        ; returnM (pats', res) }


-----------------
tcCheckExistentialPat :: [LPat TcId]            -- Patterns (just for error message)
                      -> [TcTyVar]              -- Existentially quantified tyvars bound by pattern
                      -> [BoxySigmaType]        -- Types of the patterns
                      -> BoxyRhoType            -- Type of the body of the match
                                                -- Tyvars in either of these must not escape
                      -> TcM ()
-- NB: we *must* pass "pats_tys" not just "body_ty" to tcCheckExistentialPat
-- For example, we must reject this program:
--      data C = forall a. C (a -> Int) 
--      f (C g) x = g x
-- Here, result_ty will be simply Int, but expected_ty is (C -> a -> Int).

tcCheckExistentialPat pats [] pat_tys body_ty
  = return ()   -- Short cut for case when there are no existentials

tcCheckExistentialPat pats ex_tvs pat_tys body_ty
  = addErrCtxtM (sigPatCtxt (collectPatsBinders pats) ex_tvs pat_tys body_ty)   $
    checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs

data PatState = PS {
        pat_ctxt :: PatCtxt,
        pat_reft :: Refinement  -- Binds rigid TcTyVars to their refinements
  }

data PatCtxt 
  = LamPat 
  | LetPat (Name -> Maybe TcRhoType)    -- Used for let(rec) bindings

patSigCtxt :: PatState -> UserTypeCtxt
patSigCtxt (PS { pat_ctxt = LetPat _ }) = BindPatSigCtxt
patSigCtxt other                        = LamPatSigCtxt
\end{code}



%************************************************************************
%*                                                                      *
                Binders
%*                                                                      *
%************************************************************************

\begin{code}
tcPatBndr :: PatState -> Name -> BoxySigmaType -> TcM TcId
tcPatBndr (PS { pat_ctxt = LamPat }) bndr_name pat_ty
  = do  { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
                -- We have an undecorated binder, so we do rule ABS1,
                -- by unboxing the boxy type, forcing any un-filled-in
                -- boxes to become monotypes
                -- NB that pat_ty' can still be a polytype:
                --      data T = MkT (forall a. a->a)
                --      f t = case t of { MkT g -> ... }
                -- Here, the 'g' must get type (forall a. a->a) from the
                -- MkT context
        ; return (Id.mkLocalId bndr_name pat_ty') }

tcPatBndr (PS { pat_ctxt = LetPat lookup_sig }) bndr_name pat_ty
  | Just mono_ty <- lookup_sig bndr_name
  = do  { mono_name <- newLocalName bndr_name
        ; boxyUnify mono_ty pat_ty
        ; return (Id.mkLocalId mono_name mono_ty) }

  | otherwise
  = do  { pat_ty' <- unBoxPatBndrType pat_ty bndr_name
        ; mono_name <- newLocalName bndr_name
        ; return (Id.mkLocalId mono_name pat_ty') }


-------------------
bindInstsOfPatId :: TcId -> TcM a -> TcM (a, LHsBinds TcId)
bindInstsOfPatId id thing_inside
  | not (isOverloadedTy (idType id))
  = do { res <- thing_inside; return (res, emptyLHsBinds) }
  | otherwise
  = do  { (res, lie) <- getLIE thing_inside
        ; binds <- bindInstsOfLocalFuns lie [id]
        ; return (res, binds) }

-------------------
unBoxPatBndrType  ty name = unBoxArgType ty (ptext SLIT("The variable") <+> quotes (ppr name))
unBoxWildCardType ty      = unBoxArgType ty (ptext SLIT("A wild-card pattern"))

unBoxArgType :: BoxyType -> SDoc -> TcM TcType
-- In addition to calling unbox, unBoxArgType ensures that the type is of ArgTypeKind; 
-- that is, it can't be an unboxed tuple.  For example, 
--      case (f x) of r -> ...
-- should fail if 'f' returns an unboxed tuple.
unBoxArgType ty pp_this
  = do  { ty' <- unBox ty       -- Returns a zonked type

        -- Neither conditional is strictly necesssary (the unify alone will do)
        -- but they improve error messages, and allocate fewer tyvars
        ; if isUnboxedTupleType ty' then
                failWithTc msg
          else if isSubArgTypeKind (typeKind ty') then
                return ty'
          else do       -- OpenTypeKind, so constrain it
        { ty2 <- newFlexiTyVarTy argTypeKind
        ; unifyType ty' ty2
        ; return ty' }}
  where
    msg = pp_this <+> ptext SLIT("cannot be bound to an unboxed tuple")
\end{code}


%************************************************************************
%*                                                                      *
                The main worker functions
%*                                                                      *
%************************************************************************

Note [Nesting]
~~~~~~~~~~~~~~
tcPat takes a "thing inside" over which the patter scopes.  This is partly
so that tcPat can extend the environment for the thing_inside, but also 
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
want the error-context for the pattern to scope over the RHS. 
Hence the getErrCtxt/setErrCtxt stuff in tc_lpats.

\begin{code}
--------------------
type Checker inp out =  forall r.
                          inp
                       -> PatState
                       -> (PatState -> TcM r)
                       -> TcM (out, [TcTyVar], r)

tcMultiple :: Checker inp out -> Checker [inp] [out]
tcMultiple tc_pat args pstate thing_inside
  = do  { err_ctxt <- getErrCtxt
        ; let loop pstate []
                = do { res <- thing_inside pstate
                     ; return ([], [], res) }

              loop pstate (arg:args)
                = do { (p', p_tvs, (ps', ps_tvs, res)) 
                                <- tc_pat arg pstate $ \ pstate' ->
                                   setErrCtxt err_ctxt $
                                   loop pstate' args
                -- setErrCtxt: restore context before doing the next pattern
                -- See note [Nesting] above
                                
                     ; return (p':ps', p_tvs ++ ps_tvs, res) }

        ; loop pstate args }

--------------------
tc_lpat_pr :: (LPat Name, BoxySigmaType)
           -> PatState
           -> (PatState -> TcM a)
           -> TcM (LPat TcId, [TcTyVar], a)
tc_lpat_pr (pat, ty) = tc_lpat pat ty

tc_lpat :: LPat Name 
        -> BoxySigmaType
        -> PatState
        -> (PatState -> TcM a)
        -> TcM (LPat TcId, [TcTyVar], a)
tc_lpat (L span pat) pat_ty pstate thing_inside
  = setSrcSpan span               $
    maybeAddErrCtxt (patCtxt pat) $
    do  { let mb_reft = refineType (pat_reft pstate) pat_ty
              pat_ty' = case mb_reft of { Just (_, ty') -> ty'; Nothing -> pat_ty }

                -- Make sure the result type reflects the current refinement
                -- We must do this here, so that it correctly ``sees'' all
                -- the refinements to the left.  Example:
                -- Suppose C :: forall a. T a -> a -> Foo
                -- Pattern      C a p1 True
                -- So p1 might refine 'a' to True, and the True 
                -- pattern had better see it.

        ; (pat', tvs, res) <- tc_pat pstate pat pat_ty' thing_inside
        ; let final_pat = case mb_reft of
                                Nothing     -> pat'
                                Just (co,_) -> CoPat (WpCo co) pat' pat_ty
        ; return (L span final_pat, tvs, res) }

--------------------
tc_pat  :: PatState
        -> Pat Name -> BoxySigmaType    -- Fully refined result type
        -> (PatState -> TcM a)  -- Thing inside
        -> TcM (Pat TcId,       -- Translated pattern
                [TcTyVar],      -- Existential binders
                a)              -- Result of thing inside

tc_pat pstate (VarPat name) pat_ty thing_inside
  = do  { id <- tcPatBndr pstate name pat_ty
        ; (res, binds) <- bindInstsOfPatId id $
                          tcExtendIdEnv1 name id $
                          (traceTc (text "binding" <+> ppr name <+> ppr (idType id))
                           >> thing_inside pstate)
        ; let pat' | isEmptyLHsBinds binds = VarPat id
                   | otherwise             = VarPatOut id binds
        ; return (pat', [], res) }

tc_pat pstate (ParPat pat) pat_ty thing_inside
  = do  { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
        ; return (ParPat pat', tvs, res) }

tc_pat pstate (BangPat pat) pat_ty thing_inside
  = do  { (pat', tvs, res) <- tc_lpat pat pat_ty pstate thing_inside
        ; return (BangPat pat', tvs, res) }

-- There's a wrinkle with irrefutable patterns, namely that we
-- must not propagate type refinement from them.  For example
--      data T a where { T1 :: Int -> T Int; ... }
--      f :: T a -> Int -> a
--      f ~(T1 i) y = y
-- It's obviously not sound to refine a to Int in the right
-- hand side, because the arugment might not match T1 at all!
--
-- Nor should a lazy pattern bind any existential type variables
-- because they won't be in scope when we do the desugaring
tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
  = do  { (pat', pat_tvs, res) <- tc_lpat pat pat_ty pstate $ \ _ ->
                                  thing_inside pstate
                                        -- Ignore refined pstate',
                                        -- revert to pstate
        -- Check no existentials
        ; if (null pat_tvs) then return ()
          else lazyPatErr lpat pat_tvs

        -- Check that the pattern has a lifted type
        ; pat_tv <- newBoxyTyVar liftedTypeKind
        ; boxyUnify pat_ty (mkTyVarTy pat_tv)

        ; return (LazyPat pat', [], res) }

tc_pat pstate (WildPat _) pat_ty thing_inside
  = do  { pat_ty' <- unBoxWildCardType pat_ty   -- Make sure it's filled in with monotypes
        ; res <- thing_inside pstate
        ; return (WildPat pat_ty', [], res) }

tc_pat pstate (AsPat (L nm_loc name) pat) pat_ty thing_inside
  = do  { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
        ; (pat', tvs, res) <- tcExtendIdEnv1 name bndr_id $
                              tc_lpat pat (idType bndr_id) pstate 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 (AsPat (L nm_loc bndr_id) pat', tvs, res) }

-- Type signatures in patterns
-- See Note [Pattern coercions] below
tc_pat pstate (SigPatIn pat sig_ty) pat_ty thing_inside
  = do  { (inner_ty, tv_binds) <- tcPatSig (patSigCtxt pstate) sig_ty pat_ty
        ; (pat', tvs, res) <- tcExtendTyVarEnv2 tv_binds $
                              tc_lpat pat inner_ty pstate thing_inside
        ; return (SigPatOut pat' inner_ty, tvs, res) }

tc_pat pstate pat@(TypePat ty) pat_ty thing_inside
  = failWithTc (badTypePat pat)

------------------------
-- Lists, tuples, arrays
tc_pat pstate (ListPat pats _) pat_ty thing_inside
  = do  { elt_ty <- boxySplitListTy pat_ty
        ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
                                                pats pstate thing_inside
        ; return (ListPat pats' elt_ty, pats_tvs, res) }

tc_pat pstate (PArrPat pats _) pat_ty thing_inside
  = do  { [elt_ty] <- boxySplitTyConApp parrTyCon pat_ty
        ; (pats', pats_tvs, res) <- tcMultiple (\p -> tc_lpat p elt_ty)
                                                pats pstate thing_inside 
        ; ifM (null pats) (zapToMonotype pat_ty)        -- c.f. ExplicitPArr in TcExpr
        ; return (PArrPat pats' elt_ty, pats_tvs, res) }

tc_pat pstate (TuplePat pats boxity _) pat_ty thing_inside
  = do  { arg_tys <- boxySplitTyConApp (tupleTyCon boxity (length pats)) pat_ty
        ; (pats', pats_tvs, res) <- tcMultiple tc_lpat_pr (pats `zip` arg_tys)
                                               pstate thing_inside

        -- 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 unmangled_result = TuplePat pats' boxity pat_ty
              possibly_mangled_result
                | opt_IrrefutableTuples && isBoxed boxity = LazyPat (noLoc unmangled_result)
                | otherwise                               = unmangled_result

        ; ASSERT( length arg_tys == length pats )       -- Syntactically enforced
          return (possibly_mangled_result, pats_tvs, res) }

------------------------
-- Data constructors
tc_pat pstate pat_in@(ConPatIn (L con_span con_name) arg_pats) pat_ty thing_inside
  = do  { data_con <- tcLookupDataCon con_name
        ; let tycon = dataConTyCon data_con
        ; tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside }

------------------------
-- Literal patterns
tc_pat pstate (LitPat simple_lit) pat_ty thing_inside
  = do  { boxyUnify (hsLitType simple_lit) pat_ty
        ; res <- thing_inside pstate
        ; returnM (LitPat simple_lit, [], res) }

------------------------
-- Overloaded patterns: n, and n+k
tc_pat pstate pat@(NPat over_lit mb_neg eq _) pat_ty thing_inside
  = do  { let orig = LiteralOrigin over_lit
        ; lit'    <- tcOverloadedLit 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 pstate
        ; returnM (NPat lit' mb_neg' eq' pat_ty, [], res) }

tc_pat pstate pat@(NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
  = do  { bndr_id <- setSrcSpan nm_loc (tcPatBndr pstate name pat_ty)
        ; let pat_ty' = idType bndr_id
              orig    = LiteralOrigin lit
        ; lit' <- tcOverloadedLit orig lit pat_ty'

        -- 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')

        -- 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']]     
    
        ; res <- tcExtendIdEnv1 name bndr_id (thing_inside pstate)
        ; returnM (NPlusKPat (L nm_loc bndr_id) lit' ge' minus', [], res) }

tc_pat _ _other_pat _ _ = panic "tc_pat"        -- DictPat, ConPatOut, SigPatOut, VarPatOut
\end{code}


%************************************************************************
%*                                                                      *
        Most of the work for constructors is here
        (the rest is in the ConPatIn case of tc_pat)
%*                                                                      *
%************************************************************************

[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 -> ...

As explained by [Wrappers for data instance tycons] in MkIds.lhs, the
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}

In other words, boxySplitTyConAppWithFamily implicitly takes the coercion 

  Co123Map a b v :: {Map (a, b) v :=: :R123Map a b v}

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.

Now it might appear seem as if we could have used the existing GADT type
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.

In fact, if we have a data instance declaration defining a GADT, eq_spec will
be non-empty and we will get a mixture of global instantiations and local
refinement from a single match.  This neatly reflects that, as soon as we
have constrained the type of the scrutinee to the required type index, all
further type refinement is local to the alternative.

\begin{code}
--      Running example:
-- MkT :: forall a b c. (a:=:[b]) => b -> c -> T a
--       with scrutinee of type (T ty)

tcConPat :: PatState -> SrcSpan -> DataCon -> TyCon 
         -> BoxySigmaType       -- Type of the pattern
         -> HsConDetails Name (LPat Name) -> (PatState -> TcM a)
         -> TcM (Pat TcId, [TcTyVar], a)
tcConPat pstate con_span data_con tycon pat_ty arg_pats thing_inside
  = do  { span <- getSrcSpanM   -- Span for the whole pattern
        ; let (univ_tvs, ex_tvs, eq_spec, theta, arg_tys) = dataConFullSig data_con
              skol_info = PatSkol data_con span
              origin    = SigOrigin skol_info

          -- Instantiate the constructor type variables [a->ty]
        ; ctxt_res_tys <- boxySplitTyConAppWithFamily tycon pat_ty
        ; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs
        ; let tenv     = zipTopTvSubst (univ_tvs ++ ex_tvs)
                                      (ctxt_res_tys ++ mkTyVarTys ex_tvs')
              eq_spec' = substEqSpec tenv eq_spec
              theta'   = substTheta  tenv theta
              arg_tys' = substTys    tenv arg_tys

        ; co_vars <- newCoVars eq_spec' -- Make coercion variables
        ; pstate' <- refineAlt data_con pstate ex_tvs' co_vars pat_ty

        ; ((arg_pats', inner_tvs, res), lie_req) <- getLIE $
                tcConArgs data_con arg_tys' arg_pats pstate' thing_inside

        ; loc <- getInstLoc origin
        ; dicts <- newDictBndrs loc theta'
        ; dict_binds <- tcSimplifyCheck doc ex_tvs' dicts lie_req

        ; addDataConStupidTheta data_con ctxt_res_tys

        ; return
            (unwrapFamInstScrutinee tycon ctxt_res_tys $
               ConPatOut { pat_con = L con_span data_con, 
                           pat_tvs = ex_tvs' ++ co_vars,
                           pat_dicts = map instToId dicts, 
                           pat_binds = dict_binds,
                           pat_args = arg_pats', pat_ty = pat_ty },
             ex_tvs' ++ inner_tvs, res)
        }
  where
    doc = ptext SLIT("existential context for") <+> quotes (ppr data_con)

    -- Split against the family tycon if the pattern constructor belongs to a
    -- representation tycon.
    --
    boxySplitTyConAppWithFamily tycon pat_ty =
      traceTc traceMsg >>
      case tyConFamInst_maybe tycon of
        Nothing                   -> boxySplitTyConApp tycon pat_ty
        Just (fam_tycon, instTys) -> 
          do { scrutinee_arg_tys <- boxySplitTyConApp fam_tycon pat_ty
             ; (_, freshTvs, subst) <- tcInstTyVars (tyConTyVars tycon)
             ; boxyUnifyList (substTys subst instTys) scrutinee_arg_tys
             ; return freshTvs
             }
      where
        traceMsg = sep [ text "tcConPat:boxySplitTyConAppWithFamily:" <+>
                         ppr tycon <+> ppr pat_ty
                       , text "  family instance:" <+> 
                         ppr (tyConFamInst_maybe tycon)
                       ]

    -- Wraps the pattern (which must be a ConPatOut pattern) in a coercion
    -- pattern if the tycon is an instance of a family.
    --
    unwrapFamInstScrutinee :: TyCon -> [Type] -> Pat Id -> Pat Id
    unwrapFamInstScrutinee tycon args pat
      | Just co_con <- tyConFamilyCoercion_maybe tycon 
--      , not (isNewTyCon tycon)       -- newtypes are explicitly unwrapped by
                                     -- the desugarer
          -- NB: We can use CoPat directly, rather than mkCoPat, as we know the
          --     coercion is not the identity; mkCoPat is inconvenient as it
          --     wants a located pattern.
      = CoPat (WpCo $ mkTyConApp co_con args)       -- co fam ty to repr ty
              (pat {pat_ty = mkTyConApp tycon args})    -- representation type
              pat_ty                                    -- family inst type
      | otherwise
      = pat


tcConArgs :: DataCon -> [TcSigmaType]
          -> Checker (HsConDetails Name (LPat Name)) 
                     (HsConDetails Id (LPat Id))

tcConArgs data_con arg_tys (PrefixCon arg_pats) pstate thing_inside
  = do  { checkTc (con_arity == no_of_args)     -- Check correct arity
                  (arityErr "Constructor" data_con con_arity no_of_args)
        ; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
        ; (arg_pats', tvs, res) <- tcMultiple tcConArg pats_w_tys
                                              pstate thing_inside 
        ; return (PrefixCon arg_pats', tvs, res) }
  where
    con_arity  = dataConSourceArity data_con
    no_of_args = length arg_pats

tcConArgs data_con [arg_ty1,arg_ty2] (InfixCon p1 p2) pstate thing_inside
  = do  { checkTc (con_arity == 2)      -- Check correct arity
                  (arityErr "Constructor" data_con con_arity 2)
        ; ([p1',p2'], tvs, res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
                                              pstate thing_inside
        ; return (InfixCon p1' p2', tvs, res) }
  where
    con_arity  = dataConSourceArity data_con

tcConArgs data_con other_args (InfixCon p1 p2) pstate thing_inside
  = pprPanic "tcConArgs" (ppr data_con) -- InfixCon always has two arguments

tcConArgs data_con arg_tys (RecCon rpats) pstate thing_inside
  = do  { (rpats', tvs, res) <- tcMultiple tc_field rpats pstate thing_inside
        ; return (RecCon rpats', tvs, res) }
  where
    -- doc comments are typechecked to Nothing here
    tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
    tc_field (HsRecField field_lbl pat _) pstate thing_inside
      = do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
           ; (pat', tvs, res) <- tcConArg (pat, pat_ty) pstate thing_inside
           ; return (mkRecField sel_id pat', tvs, res) }

    find_field_ty :: FieldLabel -> TcM (Id, TcType)
    find_field_ty field_lbl
        = case [ty | (f,ty) <- field_tys, f == field_lbl] of

                -- No matching field; chances are this field label comes from some
                -- other record type (or maybe none).  As well as reporting an
                -- error we still want to typecheck the pattern, principally to
                -- make sure that all the variables it binds are put into the
                -- environment, else the type checker crashes later:
                --      f (R { foo = (a,b) }) = a+b
                -- If foo isn't one of R's fields, we don't want to crash when
                -- typechecking the "a+b".
           [] -> do { addErrTc (badFieldCon data_con field_lbl)
                    ; bogus_ty <- newFlexiTyVarTy liftedTypeKind
                    ; return (error "Bogus selector Id", bogus_ty) }

                -- The normal case, when the field comes from the right constructor
           (pat_ty : extras) -> 
                ASSERT( null extras )
                do { sel_id <- tcLookupField field_lbl
                   ; return (sel_id, pat_ty) }

    field_tys :: [(FieldLabel, TcType)]
    field_tys = zip (dataConFieldLabels data_con) arg_tys
        -- Don't use zipEqual! If the constructor isn't really a record, then
        -- dataConFieldLabels will be empty (and each field in the pattern
        -- will generate an error below).

tcConArg :: Checker (LPat Name, BoxySigmaType) (LPat Id)
tcConArg (arg_pat, arg_ty) pstate thing_inside
  = tc_lpat arg_pat arg_ty pstate thing_inside
        -- NB: the tc_lpat will refine pat_ty if necessary
        --     based on the current pstate, which may include
        --     refinements from peer argument patterns to the left
\end{code}

\begin{code}
addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
-- Instantiate the "stupid theta" of the data con, and throw 
-- the constraints into the constraint set
addDataConStupidTheta data_con inst_tys
  | null stupid_theta = return ()
  | otherwise         = instStupidTheta origin inst_theta
  where
    origin = OccurrenceOf (dataConName data_con)
        -- The origin should always report "occurrence of C"
        -- even when C occurs in a pattern
    stupid_theta = dataConStupidTheta data_con
    tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
    inst_theta = substTheta tenv stupid_theta
\end{code}


%************************************************************************
%*                                                                      *
                Type refinement
%*                                                                      *
%************************************************************************

\begin{code}
refineAlt :: DataCon            -- For tracing only
          -> PatState 
          -> [TcTyVar]          -- Existentials
          -> [CoVar]            -- Equational constraints
          -> BoxySigmaType      -- Pattern type
          -> TcM PatState

refineAlt con pstate ex_tvs [] pat_ty
  = return pstate       -- Common case: no equational constraints

refineAlt con pstate ex_tvs co_vars pat_ty
  | not (isRigidTy pat_ty)
  = failWithTc (nonRigidMatch con)
        -- We are matching against a GADT constructor with non-trivial
        -- constraints, but pattern type is wobbly.  For now we fail.
        -- We can make sense of this, however:
        -- Suppose MkT :: forall a b. (a:=:[b]) => b -> T a
        --      (\x -> case x of { MkT v -> v })
        -- We can infer that x must have type T [c], for some wobbly 'c'
        -- and translate to
        --      (\(x::T [c]) -> case x of
        --                        MkT b (g::([c]:=:[b])) (v::b) -> v `cast` sym g
        -- To implement this, we'd first instantiate the equational
        -- constraints with *wobbly* type variables for the existentials;
        -- then unify these constraints to make pat_ty the right shape;
        -- then proceed exactly as in the rigid case

  | otherwise   -- In the rigid case, we perform type refinement
  = case gadtRefine (pat_reft pstate) ex_tvs co_vars of {
            Failed msg     -> failWithTc (inaccessibleAlt msg) ;
            Succeeded reft -> do { traceTc trace_msg
                                 ; return (pstate { pat_reft = reft }) }
                    -- DO NOT refine the envt right away, because we 
                    -- might be inside a lazy pattern.  Instead, refine pstate
                where
                    
                    trace_msg = text "refineAlt:match" <+> 
                                vcat [ ppr con <+> ppr ex_tvs,
                                       ppr [(v, tyVarKind v) | v <- co_vars],
                                       ppr reft]
        }
\end{code}


%************************************************************************
%*                                                                      *
                Overloaded literals
%*                                                                      *
%************************************************************************

In tcOverloadedLit we convert directly to an Int or Integer if we
know that's what we want.  This may save some time, by not
temporarily generating overloaded literals, but it won't catch all
cases (the rest are caught in lookupInst).

\begin{code}
tcOverloadedLit :: InstOrigin
                 -> HsOverLit Name
                 -> BoxyRhoType
                 -> TcM (HsOverLit TcId)
tcOverloadedLit orig lit@(HsIntegral i fi) res_ty
  | not (fi `isHsVar` fromIntegerName)  -- Do not generate a LitInst for rebindable syntax.  
        -- Reason: If we do, tcSimplify will call lookupInst, which
        --         will call tcSyntaxName, which does unification, 
        --         which tcSimplify doesn't like
        -- ToDo: noLoc sadness
  = do  { integer_ty <- tcMetaTy integerTyConName
        ; fi' <- tcSyntaxOp orig fi (mkFunTy integer_ty res_ty)
        ; return (HsIntegral i (HsApp (noLoc fi') (nlHsLit (HsInteger i integer_ty)))) }

  | Just expr <- shortCutIntLit i res_ty 
  = return (HsIntegral i expr)

  | otherwise
  = do  { expr <- newLitInst orig lit res_ty
        ; return (HsIntegral i expr) }

tcOverloadedLit orig lit@(HsFractional r fr) res_ty
  | not (fr `isHsVar` fromRationalName) -- c.f. HsIntegral case
  = do  { rat_ty <- tcMetaTy rationalTyConName
        ; fr' <- tcSyntaxOp orig fr (mkFunTy rat_ty res_ty)
                -- Overloaded literals must have liftedTypeKind, because
                -- we're instantiating an overloaded function here,
                -- whereas res_ty might be openTypeKind. This was a bug in 6.2.2
                -- However this'll be picked up by tcSyntaxOp if necessary
        ; return (HsFractional r (HsApp (noLoc fr') (nlHsLit (HsRat r rat_ty)))) }

  | Just expr <- shortCutFracLit r res_ty 
  = return (HsFractional r expr)

  | otherwise
  = do  { expr <- newLitInst orig lit res_ty
        ; return (HsFractional r expr) }

newLitInst :: InstOrigin -> HsOverLit Name -> BoxyRhoType -> TcM (HsExpr TcId)
newLitInst orig lit res_ty      -- Make a LitInst
  = do  { loc <- getInstLoc orig
        ; res_tau <- zapToMonotype res_ty
        ; new_uniq <- newUnique
        ; let   lit_nm   = mkSystemVarName new_uniq FSLIT("lit")
                lit_inst = LitInst {tci_name = lit_nm, tci_lit = lit, 
                                    tci_ty = res_tau, tci_loc = loc}
        ; extendLIE lit_inst
        ; return (HsVar (instToId lit_inst)) }
\end{code}


%************************************************************************
%*                                                                      *
                Note [Pattern coercions]
%*                                                                      *
%************************************************************************

In principle, these program would be reasonable:
        
        f :: (forall a. a->a) -> Int
        f (x :: Int->Int) = x 3

        g :: (forall a. [a]) -> Bool
        g [] = True

In both cases, the function type signature restricts what arguments can be passed
in a call (to polymorphic ones).  The pattern type signature then instantiates this
type.  For example, in the first case,  (forall a. a->a) <= Int -> Int, and we
generate the translated term
        f = \x' :: (forall a. a->a).  let x = x' Int in x 3

From a type-system point of view, this is perfectly fine, but it's *very* seldom useful.
And it requires a significant amount of code to implement, becuase we need to decorate
the translated pattern with coercion functions (generated from the subsumption check 
by tcSub).  

So for now I'm just insisting on type *equality* in patterns.  No subsumption. 

Old notes about desugaring, at a time when pattern coercions were handled:

A SigPat is a type coercion and must be handled one at at time.  We can't
combine them unless the type of the pattern inside is identical, and we don't
bother to check for that.  For example:

        data T = T1 Int | T2 Bool
        f :: (forall a. a -> a) -> T -> t
        f (g::Int->Int)   (T1 i) = T1 (g i)
        f (g::Bool->Bool) (T2 b) = T2 (g b)

We desugar this as follows:

        f = \ g::(forall a. a->a) t::T ->
            let gi = g Int
            in case t of { T1 i -> T1 (gi i)
                           other ->
            let gb = g Bool
            in case t of { T2 b -> T2 (gb b)
                           other -> fail }}

Note that we do not treat the first column of patterns as a
column of variables, because the coerced variables (gi, gb)
would be of different types.  So we get rather grotty code.
But I don't think this is a common case, and if it was we could
doubtless improve it.

Meanwhile, the strategy is:
        * treat each SigPat coercion (always non-identity coercions)
                as a separate block
        * deal with the stuff inside, and then wrap a binding round
                the result to bind the new variable (gi, gb, etc)


%************************************************************************
%*                                                                      *
\subsection{Errors and contexts}
%*                                                                      *
%************************************************************************

\begin{code}
patCtxt :: Pat Name -> Maybe Message    -- Not all patterns are worth pushing a context
patCtxt (VarPat _)  = Nothing
patCtxt (ParPat _)  = Nothing
patCtxt (AsPat _ _) = Nothing
patCtxt pat         = Just (hang (ptext SLIT("In the pattern:")) 
                               4 (ppr pat))

-----------------------------------------------

existentialExplode pat
  = hang (vcat [text "My brain just exploded.",
                text "I can't handle pattern bindings for existentially-quantified constructors.",
                text "In the binding group for"])
        4 (ppr pat)

sigPatCtxt bound_ids bound_tvs pat_tys body_ty tidy_env 
  = do  { pat_tys' <- mapM zonkTcType pat_tys
        ; body_ty' <- zonkTcType body_ty
        ; let (env1,  tidy_tys)    = tidyOpenTypes tidy_env (map idType show_ids)
              (env2, tidy_pat_tys) = tidyOpenTypes env1 pat_tys'
              (env3, tidy_body_ty) = tidyOpenType  env2 body_ty'
        ; return (env3,
                 sep [ptext SLIT("When checking an existential match that binds"),
                      nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
                      ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
                      ptext SLIT("The body has type:") <+> ppr tidy_body_ty
                ]) }
  where
    show_ids = filter is_interesting bound_ids
    is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs

    ppr_id id ty = ppr id <+> dcolon <+> ppr ty
        -- Don't zonk the types so we get the separate, un-unified versions

badFieldCon :: DataCon -> Name -> SDoc
badFieldCon con field
  = hsep [ptext SLIT("Constructor") <+> quotes (ppr con),
          ptext SLIT("does not have field"), quotes (ppr field)]

polyPatSig :: TcType -> SDoc
polyPatSig sig_ty
  = hang (ptext SLIT("Illegal polymorphic type signature in pattern:"))
         4 (ppr sig_ty)

badTypePat pat = ptext SLIT("Illegal type pattern") <+> ppr pat

lazyPatErr pat tvs
  = failWithTc $
    hang (ptext SLIT("A lazy (~) pattern connot bind existential type variables"))
       2 (vcat (map pprSkolTvBinding tvs))

nonRigidMatch con
  =  hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
        2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))

inaccessibleAlt msg
  = hang (ptext SLIT("Inaccessible case alternative:")) 2 msg
\end{code}