So the no_gen flag decides whether the pattern-bound variables should
have exactly the type in the type signature (when not generalising) or
the instantiated version (when generalising)
%************************************************************************
%* *
The main worker functions
%* *
%************************************************************************
Note [Nesting]
~~~~~~~~~~~~~~
tcPat takes a "thing inside" over which the pattern 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 tcMultiple
\begin{code}
type Checker inp out = forall r.
inp
-> PatEnv
-> TcM r
-> TcM (out, r)
tcMultiple :: Checker inp out -> Checker [inp] [out]
tcMultiple tc_pat args penv thing_inside
= do { err_ctxt <- getErrCtxt
; let loop _ []
= do { res <- thing_inside
; return ([], res) }
loop penv (arg:args)
= do { (p', (ps', res))
<- tc_pat arg penv $
setErrCtxt err_ctxt $
loop penv args
; return (p':ps', res) }
; loop penv args }
tc_lpat :: LPat Name
-> TcSigmaType
-> PatEnv
-> TcM a
-> TcM (LPat TcId, a)
tc_lpat (L span pat) pat_ty penv thing_inside
= setSrcSpan span $
do { (pat', res) <- maybeWrapPatCtxt pat (tc_pat penv pat pat_ty)
thing_inside
; return (L span pat', res) }
tc_lpats :: PatEnv
-> [LPat Name] -> [TcSigmaType]
-> TcM a
-> TcM ([LPat TcId], a)
tc_lpats penv pats tys thing_inside
= tcMultiple (\(p,t) -> tc_lpat p t)
(zipEqual "tc_lpats" pats tys)
penv thing_inside
tc_pat :: PatEnv
-> Pat Name
-> TcSigmaType
-> TcM a
-> TcM (Pat TcId,
a)
tc_pat penv (VarPat name) pat_ty thing_inside
= do { (coi, id) <- tcPatBndr penv name pat_ty
; res <- tcExtendIdEnv1 name id thing_inside
; return (mkHsWrapPatCo coi (VarPat id) pat_ty, res) }
tc_pat penv (ParPat pat) pat_ty thing_inside
= do { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
; return (ParPat pat', res) }
tc_pat penv (BangPat pat) pat_ty thing_inside
= do { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
; return (BangPat pat', res) }
tc_pat penv lpat@(LazyPat pat) pat_ty thing_inside
= do { (pat', (res, pat_ct))
<- tc_lpat pat pat_ty (makeLazy penv) $
captureConstraints thing_inside
; emitConstraints pat_ct
; when (any (isUnLiftedType . idType) $ collectPatBinders pat') $
lazyUnliftedPatErr lpat
; pat_ty' <- newFlexiTyVarTy liftedTypeKind
; _ <- unifyType pat_ty pat_ty'
; return (LazyPat pat', res) }
tc_pat _ p@(QuasiQuotePat _) _ _
= pprPanic "Should never see QuasiQuotePat in type checker" (ppr p)
tc_pat _ (WildPat _) pat_ty thing_inside
= do { checkUnboxedTuple pat_ty $
ptext (sLit "A wild-card pattern")
; res <- thing_inside
; return (WildPat pat_ty, res) }
tc_pat penv (AsPat (L nm_loc name) pat) pat_ty thing_inside
= do { (coi, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
; (pat', res) <- tcExtendIdEnv1 name bndr_id $
tc_lpat pat (idType bndr_id) penv thing_inside
; return (mkHsWrapPatCo coi (AsPat (L nm_loc bndr_id) pat') pat_ty, res) }
tc_pat penv vpat@(ViewPat expr pat _) overall_pat_ty thing_inside
= do { checkUnboxedTuple overall_pat_ty $
ptext (sLit "The view pattern") <+> ppr vpat
; (expr',expr'_inferred) <- tcInferRho expr
; (expr_coi, pat_ty) <- tcInfer $ \ pat_ty ->
unifyPatType expr'_inferred (mkFunTy overall_pat_ty pat_ty)
; (pat', res) <- tc_lpat pat pat_ty penv thing_inside
; return (ViewPat (mkLHsWrapCo expr_coi expr') pat' overall_pat_ty, res) }
tc_pat penv (SigPatIn pat sig_ty) pat_ty thing_inside
= do { (inner_ty, tv_binds, wrap) <- tcPatSig (patSigCtxt penv) sig_ty pat_ty
; (pat', res) <- tcExtendTyVarEnv2 tv_binds $
tc_lpat pat inner_ty penv thing_inside
; return (mkHsWrapPat wrap (SigPatOut pat' inner_ty) pat_ty, res) }
tc_pat penv (ListPat pats _) pat_ty thing_inside
= do { (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTy pat_ty
; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
pats penv thing_inside
; return (mkHsWrapPat coi (ListPat pats' elt_ty) pat_ty, res)
}
tc_pat penv (PArrPat pats _) pat_ty thing_inside
= do { (coi, elt_ty) <- matchExpectedPatTy matchExpectedPArrTy pat_ty
; (pats', res) <- tcMultiple (\p -> tc_lpat p elt_ty)
pats penv thing_inside
; return (mkHsWrapPat coi (PArrPat pats' elt_ty) pat_ty, res)
}
tc_pat penv (TuplePat pats boxity _) pat_ty thing_inside
= do { let tc = tupleTyCon boxity (length pats)
; (coi, arg_tys) <- matchExpectedPatTy (matchExpectedTyConApp tc) pat_ty
; (pats', res) <- tc_lpats penv pats arg_tys thing_inside
; let pat_ty' = mkTyConApp tc arg_tys
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 )
return (mkHsWrapPat coi possibly_mangled_result pat_ty, res)
}
tc_pat penv (ConPatIn con arg_pats) pat_ty thing_inside
= tcConPat penv con pat_ty arg_pats thing_inside
tc_pat _ (LitPat simple_lit) pat_ty thing_inside
= do { let lit_ty = hsLitType simple_lit
; coi <- unifyPatType lit_ty pat_ty
; res <- thing_inside
; return ( mkHsWrapPatCo coi (LitPat simple_lit) pat_ty
, res) }
tc_pat _ (NPat over_lit mb_neg eq) pat_ty thing_inside
= do { let orig = LiteralOrigin over_lit
; lit' <- newOverloadedLit orig over_lit pat_ty
; eq' <- tcSyntaxOp orig eq (mkFunTys [pat_ty, pat_ty] boolTy)
; mb_neg' <- case mb_neg of
Nothing -> return Nothing
Just neg ->
do { neg' <- tcSyntaxOp orig neg (mkFunTy pat_ty pat_ty)
; return (Just neg') }
; res <- thing_inside
; return (NPat lit' mb_neg' eq', res) }
tc_pat penv (NPlusKPat (L nm_loc name) lit ge minus) pat_ty thing_inside
= do { (coi, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
; let pat_ty' = idType bndr_id
orig = LiteralOrigin lit
; lit' <- newOverloadedLit orig lit pat_ty'
; ge' <- tcSyntaxOp orig ge (mkFunTys [pat_ty', pat_ty'] boolTy)
; minus' <- tcSyntaxOp orig minus (mkFunTys [pat_ty', pat_ty'] pat_ty')
; let pat' = NPlusKPat (L nm_loc bndr_id) lit' ge' minus'
; icls <- tcLookupClass integralClassName
; instStupidTheta orig [mkClassPred icls [pat_ty']]
; res <- tcExtendIdEnv1 name bndr_id thing_inside
; return (mkHsWrapPatCo coi pat' pat_ty, res) }
tc_pat _ _other_pat _ _ = panic "tc_pat"
unifyPatType :: TcType -> TcType -> TcM Coercion
unifyPatType actual_ty expected_ty
= do { coi <- unifyType actual_ty expected_ty
; return (mkSymCo coi) }
\end{code}
Note [Hopping the LIE in lazy patterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a lazy pattern, we must *not* discharge constraints from the RHS
from dictionaries bound in the pattern. E.g.
f ~(C x) = 3
We can't discharge the Num constraint from dictionaries bound by
the pattern C!
So we have to make the constraints from thing_inside "hop around"
the pattern. Hence the captureConstraints and emitConstraints.
The same thing ensures that equality constraints in a lazy match
are not made available in the RHS of the match. 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!
Finally, a lazy pattern should not bind any existential type variables
because they won't be in scope when we do the desugaring
%************************************************************************
%* *
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 previous 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.
RIP GADT refinement: refinements have been replaced by the use of explicit
equality constraints that are used in conjunction with implication constraints
to express the local scope of GADT refinements.
\begin{code}
tcConPat :: PatEnv -> Located Name
-> TcRhoType
-> HsConPatDetails Name -> TcM a
-> TcM (Pat TcId, a)
tcConPat penv (L con_span con_name) pat_ty arg_pats thing_inside
= do { data_con <- tcLookupDataCon con_name
; let tycon = dataConTyCon data_con
(univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _)
= dataConFullSig data_con
; (wrap, ctxt_res_tys) <- matchExpectedPatTy (matchExpectedConTy tycon) pat_ty
; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys
; checkExistentials ex_tvs penv
; ex_tvs' <- tcInstSuperSkolTyVars ex_tvs
; let pat_ty' = mkTyConApp tycon ctxt_res_tys
tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
(ctxt_res_tys ++ mkTyVarTys ex_tvs')
arg_tys' = substTys tenv arg_tys
; if null ex_tvs && null eq_spec && null theta
then do {
(arg_pats', res) <- tcConArgs data_con arg_tys'
arg_pats penv thing_inside
; let res_pat = ConPatOut { pat_con = L con_span data_con,
pat_tvs = [], pat_dicts = [],
pat_binds = emptyTcEvBinds,
pat_args = arg_pats',
pat_ty = pat_ty' }
; return (mkHsWrapPat wrap res_pat pat_ty, res) }
else do
{ let theta' = substTheta tenv (eqSpecPreds eq_spec ++ theta)
no_equalities = not (any isEqPred theta')
skol_info = case pe_ctxt penv of
LamPat mc -> PatSkol data_con mc
LetPat {} -> UnkSkol
; gadts_on <- xoptM Opt_GADTs
; checkTc (no_equalities || gadts_on)
(ptext (sLit "A pattern match on a GADT requires -XGADTs"))
; given <- newEvVars theta'
; (ev_binds, (arg_pats', res))
<- checkConstraints skol_info ex_tvs' given $
tcConArgs data_con arg_tys' arg_pats penv thing_inside
; let res_pat = ConPatOut { pat_con = L con_span data_con,
pat_tvs = ex_tvs',
pat_dicts = given,
pat_binds = ev_binds,
pat_args = arg_pats',
pat_ty = pat_ty' }
; return (mkHsWrapPat wrap res_pat pat_ty, res)
} }
matchExpectedPatTy :: (TcRhoType -> TcM (Coercion, a))
-> TcRhoType -> TcM (HsWrapper, a)
matchExpectedPatTy inner_match pat_ty
| null tvs && null theta
= do { (coi, res) <- inner_match pat_ty
; return (coToHsWrapper (mkSymCo coi), res) }
| otherwise
= do { (_, tys, subst) <- tcInstTyVars tvs
; wrap1 <- instCall PatOrigin tys (substTheta subst theta)
; (wrap2, arg_tys) <- matchExpectedPatTy inner_match (TcType.substTy subst tau)
; return (wrap2 <.> wrap1 , arg_tys) }
where
(tvs, theta, tau) = tcSplitSigmaTy pat_ty
matchExpectedConTy :: TyCon
-> TcRhoType
-> TcM (Coercion, [TcSigmaType])
matchExpectedConTy data_tc pat_ty
| Just (fam_tc, fam_args, co_tc) <- tyConFamInstSig_maybe data_tc
= do { (_, tys, subst) <- tcInstTyVars (tyConTyVars data_tc)
; coi1 <- unifyType (mkTyConApp fam_tc (substTys subst fam_args)) pat_ty
; let coi2 = mkAxInstCo co_tc tys
; return (mkTransCo (mkSymCo coi2) coi1, tys) }
| otherwise
= matchExpectedTyConApp data_tc pat_ty
\end{code}
Note [Matching constructor patterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose (coi, tys) = matchExpectedConType data_tc pat_ty
* In the simple case, pat_ty = tc tys
* If pat_ty is a polytype, we want to instantiate it
This is like part of a subsumption check. Eg
f :: (forall a. [a]) -> blah
f [] = blah
* In a type family case, suppose we have
data family T a
data instance T (p,q) = A p | B q
Then we'll have internally generated
data T7 p q = A p | B q
axiom coT7 p q :: T (p,q) ~ T7 p q
So if pat_ty = T (ty1,ty2), we return (coi, [ty1,ty2]) such that
coi = coi2 . coi1 : T7 t ~ pat_ty
coi1 : T (ty1,ty2) ~ pat_ty
coi2 : T7 ty1 ty2 ~ T (ty1,ty2)
For families we do all this matching here, not in the unifier,
because we never want a whisper of the data_tycon to appear in
error messages; it's a purely internal thing
\begin{code}
tcConArgs :: DataCon -> [TcSigmaType]
-> Checker (HsConPatDetails Name) (HsConPatDetails Id)
tcConArgs data_con arg_tys (PrefixCon arg_pats) penv thing_inside
= do { checkTc (con_arity == no_of_args)
(arityErr "Constructor" data_con con_arity no_of_args)
; let pats_w_tys = zipEqual "tcConArgs" arg_pats arg_tys
; (arg_pats', res) <- tcMultiple tcConArg pats_w_tys
penv thing_inside
; return (PrefixCon arg_pats', res) }
where
con_arity = dataConSourceArity data_con
no_of_args = length arg_pats
tcConArgs data_con arg_tys (InfixCon p1 p2) penv thing_inside
= do { checkTc (con_arity == 2)
(arityErr "Constructor" data_con con_arity 2)
; let [arg_ty1,arg_ty2] = arg_tys
; ([p1',p2'], res) <- tcMultiple tcConArg [(p1,arg_ty1),(p2,arg_ty2)]
penv thing_inside
; return (InfixCon p1' p2', res) }
where
con_arity = dataConSourceArity data_con
tcConArgs data_con arg_tys (RecCon (HsRecFields rpats dd)) penv thing_inside
= do { (rpats', res) <- tcMultiple tc_field rpats penv thing_inside
; return (RecCon (HsRecFields rpats' dd), res) }
where
tc_field :: Checker (HsRecField FieldLabel (LPat Name)) (HsRecField TcId (LPat TcId))
tc_field (HsRecField field_lbl pat pun) penv thing_inside
= do { (sel_id, pat_ty) <- wrapLocFstM find_field_ty field_lbl
; (pat', res) <- tcConArg (pat, pat_ty) penv thing_inside
; return (HsRecField sel_id pat' pun, res) }
find_field_ty :: FieldLabel -> TcM (Id, TcType)
find_field_ty field_lbl
= case [ty | (f,ty) <- field_tys, f == field_lbl] of
[] -> do { addErrTc (badFieldCon data_con field_lbl)
; bogus_ty <- newFlexiTyVarTy liftedTypeKind
; return (error "Bogus selector Id", bogus_ty) }
(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
tcConArg :: Checker (LPat Name, TcSigmaType) (LPat Id)
tcConArg (arg_pat, arg_ty) penv thing_inside
= tc_lpat arg_pat arg_ty penv thing_inside
\end{code}
\begin{code}
addDataConStupidTheta :: DataCon -> [TcType] -> TcM ()
addDataConStupidTheta data_con inst_tys
| null stupid_theta = return ()
| otherwise = instStupidTheta origin inst_theta
where
origin = OccurrenceOf (dataConName data_con)
stupid_theta = dataConStupidTheta data_con
tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
inst_theta = substTheta tenv stupid_theta
\end{code}
Note [Arrows and patterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~
(Oct 07) Arrow noation has the odd property that it involves
"holes in the scope". For example:
expr :: Arrow a => a () Int
expr = proc (y,z) -> do
x <- term -< y
expr' -< x
Here the 'proc (y,z)' binding scopes over the arrow tails but not the
arrow body (e.g 'term'). As things stand (bogusly) all the
constraints from the proc body are gathered together, so constraints
from 'term' will be seen by the tcPat for (y,z). But we must *not*
bind constraints from 'term' here, becuase the desugarer will not make
these bindings scope over 'term'.
The Right Thing is not to confuse these constraints together. But for
now the Easy Thing is to ensure that we do not have existential or
GADT constraints in a 'proc', and to short-cut the constraint
simplification for such vanilla patterns so that it binds no
constraints. Hence the 'fast path' in tcConPat; but it's also a good
plan for ordinary vanilla patterns to bypass the constraint
simplification step.
%************************************************************************
%* *
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}
%* *
%************************************************************************
{- This was used to improve the error message from
an existential escape. Need to think how to do this.
sigPatCtxt :: [LPat Var] -> [Var] -> [TcType] -> TcType -> TidyEnv
-> TcM (TidyEnv, SDoc)
sigPatCtxt pats 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 2 (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
bound_ids = collectPatsBinders pats
show_ids = filter is_interesting bound_ids
is_interesting id = any (`elemVarSet` varTypeTyVars 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
-}
\begin{code}
maybeWrapPatCtxt :: Pat Name -> (TcM a -> TcM b) -> TcM a -> TcM b
maybeWrapPatCtxt pat tcm thing_inside
| not (worth_wrapping pat) = tcm thing_inside
| otherwise = addErrCtxt msg $ tcm $ popErrCtxt thing_inside
where
worth_wrapping (VarPat {}) = False
worth_wrapping (ParPat {}) = False
worth_wrapping (AsPat {}) = False
worth_wrapping _ = True
msg = hang (ptext (sLit "In the pattern:")) 2 (ppr pat)
checkExistentials :: [TyVar] -> PatEnv -> TcM ()
checkExistentials [] _ = return ()
checkExistentials _ (PE { pe_ctxt = LetPat {}}) = failWithTc existentialLetPat
checkExistentials _ (PE { pe_ctxt = LamPat ProcExpr }) = failWithTc existentialProcPat
checkExistentials _ (PE { pe_lazy = True }) = failWithTc existentialLazyPat
checkExistentials _ _ = return ()
existentialLazyPat :: SDoc
existentialLazyPat
= hang (ptext (sLit "An existential or GADT data constructor cannot be used"))
2 (ptext (sLit "inside a lazy (~) pattern"))
existentialProcPat :: SDoc
existentialProcPat
= ptext (sLit "Proc patterns cannot use existential or GADT data constructors")
existentialLetPat :: SDoc
existentialLetPat
= vcat [text "My brain just exploded",
text "I can't handle pattern bindings for existential or GADT data constructors.",
text "Instead, use a case-expression, or do-notation, to unpack the constructor."]
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:"))
2 (ppr sig_ty)
lazyUnliftedPatErr :: OutputableBndr name => Pat name -> TcM ()
lazyUnliftedPatErr pat
= failWithTc $
hang (ptext (sLit "A lazy (~) pattern cannot contain unlifted types:"))
2 (ppr pat)
unboxedTupleErr :: SDoc -> Type -> SDoc
unboxedTupleErr what ty
= hang (what <+> ptext (sLit "cannot have an unboxed tuple type:"))
2 (ppr ty)
\end{code}