Commit 921d3238 authored by Ryan Scott's avatar Ryan Scott Committed by Marge Bot

Ignore unary constraint tuples during typechecking (#17511)

We deliberately avoid defining a magical `Unit%` class, for reasons
that I have expounded upon in the newly added
`Note [Ignore unary constraint tuples]` in `TcHsType`. However, a
sneaky user could try to insert `Unit%` into their program by way of
Template Haskell, leading to the interface-file error observed
in #17511. To avoid this, any time we encounter a unary constraint
tuple during typechecking, we drop the surrounding constraint tuple
application. This is safe to do since `Unit% a` and `a` would be
semantically equivalent (unlike other forms of unary tuples).

Fixes #17511.
parent 843ceb38
Pipeline #13642 canceled with stages
......@@ -821,6 +821,13 @@ data TupleSort
| ConstraintTuple
deriving( Eq, Data )
instance Outputable TupleSort where
ppr ts = text $
case ts of
BoxedTuple -> "BoxedTuple"
UnboxedTuple -> "UnboxedTuple"
ConstraintTuple -> "ConstraintTuple"
tupleSortBoxity :: TupleSort -> Boxity
tupleSortBoxity BoxedTuple = Boxed
tupleSortBoxity UnboxedTuple = Unboxed
......@@ -722,9 +722,9 @@ created in Template Haskell or in, e.g., `deriving` code. There is
nothing special about one-tuples in Core; in particular, they have no
custom pretty-printing, just using `Unit`.
NB (Feb 16): for /constraint/ one-tuples I have 'Unit%' but no class
decl in GHC.Classes, so I think this part may not work properly. But
it's unused I think.
Note that there is *not* a unary constraint tuple, unlike for other forms of
tuples. See [Ignore unary constraint tuples] in TcHsType for more
See also Note [Flattening one-tuples] in MkCore and
Note [Don't flatten tuples from HsSyn] in MkCore.
......@@ -947,30 +947,34 @@ finish_tuple :: HsType GhcRn
-> [TcKind] -- ^ of these kinds
-> TcKind -- ^ expected kind of the whole tuple
-> TcM TcType
finish_tuple rn_ty tup_sort tau_tys tau_kinds exp_kind
= do { traceTc "finish_tuple" (ppr res_kind $$ ppr tau_kinds $$ ppr exp_kind)
; let arg_tys = case tup_sort of
-- See also Note [Unboxed tuple RuntimeRep vars] in TyCon
UnboxedTuple -> tau_reps ++ tau_tys
BoxedTuple -> tau_tys
ConstraintTuple -> tau_tys
; tycon <- case tup_sort of
| arity > mAX_CTUPLE_SIZE
-> failWith (bigConstraintTuple arity)
| otherwise -> tcLookupTyCon (cTupleTyConName arity)
BoxedTuple -> do { let tc = tupleTyCon Boxed arity
; checkWiredInTyCon tc
; return tc }
UnboxedTuple -> return (tupleTyCon Unboxed arity)
; checkExpectedKind rn_ty (mkTyConApp tycon arg_tys) res_kind exp_kind }
finish_tuple rn_ty tup_sort tau_tys tau_kinds exp_kind = do
traceTc "finish_tuple" (ppr tup_sort $$ ppr tau_kinds $$ ppr exp_kind)
case tup_sort of
| [tau_ty] <- tau_tys
-- Drop any uses of 1-tuple constraints here.
-- See Note [Ignore unary constraint tuples]
-> check_expected_kind tau_ty constraintKind
| arity > mAX_CTUPLE_SIZE
-> failWith (bigConstraintTuple arity)
| otherwise
-> do tycon <- tcLookupTyCon (cTupleTyConName arity)
check_expected_kind (mkTyConApp tycon tau_tys) constraintKind
BoxedTuple -> do
let tycon = tupleTyCon Boxed arity
checkWiredInTyCon tycon
check_expected_kind (mkTyConApp tycon tau_tys) liftedTypeKind
UnboxedTuple ->
let tycon = tupleTyCon Unboxed arity
tau_reps = map kindRep tau_kinds
-- See also Note [Unboxed tuple RuntimeRep vars] in TyCon
arg_tys = tau_reps ++ tau_tys
res_kind = unboxedTupleKind tau_reps in
check_expected_kind (mkTyConApp tycon arg_tys) res_kind
arity = length tau_tys
tau_reps = map kindRep tau_kinds
res_kind = case tup_sort of
UnboxedTuple -> unboxedTupleKind tau_reps
BoxedTuple -> liftedTypeKind
ConstraintTuple -> constraintKind
check_expected_kind ty act_kind =
checkExpectedKind rn_ty ty act_kind exp_kind
bigConstraintTuple :: Arity -> MsgDoc
bigConstraintTuple arity
......@@ -978,6 +982,46 @@ bigConstraintTuple arity
<+> parens (text "max arity =" <+> int mAX_CTUPLE_SIZE))
2 (text "Instead, use a nested tuple")
Note [Ignore unary constraint tuples]
GHC provides unary tuples and unboxed tuples (see Note [One-tuples] in
TysWiredIn) but does *not* provide unary constraint tuples. Why? First,
recall the definition of a unary tuple data type:
data Unit a = Unit a
Note that `Unit a` is *not* the same thing as `a`, since Unit is boxed and
lazy. Therefore, the presence of `Unit` matters semantically. On the other
hand, suppose we had a unary constraint tuple:
class a => Unit% a
This compiles down a newtype (i.e., a cast) in Core, so `Unit% a` is
semantically equivalent to `a`. Therefore, a 1-tuple constraint would have
no user-visible impact, nor would it allow you to express anything that
you couldn't otherwise.
We could simply add Unit% for consistency with tuples (Unit) and unboxed
tuples (Unit#), but that would require even more magic to wire in another
magical class, so we opt not to do so. We must be careful, however, since
one can try to sneak in uses of unary constraint tuples through Template
Haskell, such as in this program (from #17511):
f :: $(pure (ForallT [] [TupleT 1 `AppT` (ConT ''Show `AppT` ConT ''Int)]
(ConT ''String)))
-- f :: Unit% (Show Int) => String
f = "abc"
This use of `TupleT 1` will produce an HsBoxedOrConstraintTuple of arity 1,
and since it is used in a Constraint position, GHC will attempt to treat
it as thought it were a constraint tuple, which can potentially lead to
trouble if one attempts to look up the name of a constraint tuple of arity
1 (as it won't exist). To avoid this trouble, we simply take any unary
constraint tuples discovered when typechecking and drop them—i.e., treat
"Unit% a" as though the user had written "a". This is always safe to do
since the two constraints should be semantically equivalent.
{- *********************************************************************
* *
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MonoLocalBinds #-}
{-# LANGUAGE TemplateHaskell #-}
module T17511 where
import Language.Haskell.TH
f :: $(pure (ForallT [] [TupleT 1 `AppT` (ConT ''Show `AppT` ConT ''Int)] (ConT ''String)))
f = "abc"
......@@ -494,4 +494,5 @@ test('T17394', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
test('T17379a', normal, compile_fail, [''])
test('T17379b', normal, compile_fail, [''])
test('T17461', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
test('T17511', normal, compile, [''])
test('TH_PprStar', normal, compile, ['-v0 -dsuppress-uniques'])
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