Type Annotations in the Presence of Injective Type Families, Bidirectional Pattern Synonyms, and Data Kinds
Hello GHC Devs, I've found a case in which annotating a bidirectional pattern synonym with a type signature causes the typechecker to reject an otherwise typeable program. Before filing a bug report I want to check that I'm thinking about this correctly. Consider this module: {-# LANGUAGE TypeFamilies , TypeFamilyDependencies , DataKinds , TypeOperators , GADTs , FlexibleContexts , PatternSynonyms #-} module Strange where data Expr a where Tuple :: NTup ts -> Expr (Flatten ts) data NTup (ts :: [*]) where Unit :: NTup '[] Pair :: Expr a -> NTup ts -> NTup (a ': ts) type family Flatten ts = r | r -> ts where Flatten '[] = () Flatten '[a, b] = (a, b) pattern P a b = Pair a (Pair b Unit) fstExpr :: Expr (a, b) -> (Expr a, Expr b) fstExpr (Tuple (P x y)) = (x, y) The idea is that an NTup '[a, b, c ...] should be isomorphic to an (a, b, c, ...), but removes a lot of repetitive code for dealing with Exprs of tuples. This module compiles with GHC 8.6.4 and GHC infers the expected type for P. P :: Expr a -> Expr a1 -> NTup '[a, a1] However, annotating P with this type causes typechecking fstExpr to fail with: Strange.hs:26:17: error: • Could not deduce: ts ~ '[a, b] from the context: (a, b) ~ Flatten ts bound by a pattern with constructor: Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten ts), in an equation for ‘fstExpr’ at Strange.hs:26:10-22 ‘ts’ is a rigid type variable bound by a pattern with constructor: Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten ts), in an equation for ‘fstExpr’ at Strange.hs:26:10-22 Expected type: NTup ts Actual type: NTup '[a, b] • In the pattern: P x y In the pattern: Tuple (P x y) In an equation for ‘fstExpr’: fstExpr (Tuple (P x y)) = (x, y) • Relevant bindings include fstExpr :: Expr (a, b) -> (Expr a, Expr b) (bound at Strange.hs:26:1) | 26 | fstExpr (Tuple (P x y)) = (x, y) | ^^^^^ It seems that providing a type signature causes GHC to forget the injectivity of Flatten (i.e (a, b) ~ Flatten ts implies ts ~ '[a, b]). Is this a bug, some limitation of pattern synonyms, or am I missing something? Thanks as always, Travis
Please can you open a bug report anyway? It is easier to discuss it there than on the mailing list. Matt On Tue, Apr 30, 2019 at 5:25 AM Travis Whitaker <pi.boy.travis@gmail.com> wrote:
Hello GHC Devs,
I've found a case in which annotating a bidirectional pattern synonym with a type signature causes the typechecker to reject an otherwise typeable program. Before filing a bug report I want to check that I'm thinking about this correctly. Consider this module:
{-# LANGUAGE TypeFamilies , TypeFamilyDependencies , DataKinds , TypeOperators , GADTs , FlexibleContexts , PatternSynonyms #-}
module Strange where
data Expr a where Tuple :: NTup ts -> Expr (Flatten ts)
data NTup (ts :: [*]) where Unit :: NTup '[] Pair :: Expr a -> NTup ts -> NTup (a ': ts)
type family Flatten ts = r | r -> ts where Flatten '[] = () Flatten '[a, b] = (a, b)
pattern P a b = Pair a (Pair b Unit)
fstExpr :: Expr (a, b) -> (Expr a, Expr b) fstExpr (Tuple (P x y)) = (x, y)
The idea is that an NTup '[a, b, c ...] should be isomorphic to an (a, b, c, ...), but removes a lot of repetitive code for dealing with Exprs of tuples. This module compiles with GHC 8.6.4 and GHC infers the expected type for P.
P :: Expr a -> Expr a1 -> NTup '[a, a1]
However, annotating P with this type causes typechecking fstExpr to fail with:
Strange.hs:26:17: error: • Could not deduce: ts ~ '[a, b] from the context: (a, b) ~ Flatten ts bound by a pattern with constructor: Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten ts), in an equation for ‘fstExpr’ at Strange.hs:26:10-22 ‘ts’ is a rigid type variable bound by a pattern with constructor: Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten ts), in an equation for ‘fstExpr’ at Strange.hs:26:10-22 Expected type: NTup ts Actual type: NTup '[a, b] • In the pattern: P x y In the pattern: Tuple (P x y) In an equation for ‘fstExpr’: fstExpr (Tuple (P x y)) = (x, y) • Relevant bindings include fstExpr :: Expr (a, b) -> (Expr a, Expr b) (bound at Strange.hs:26:1) | 26 | fstExpr (Tuple (P x y)) = (x, y) | ^^^^^
It seems that providing a type signature causes GHC to forget the injectivity of Flatten (i.e (a, b) ~ Flatten ts implies ts ~ '[a, b]). Is this a bug, some limitation of pattern synonyms, or am I missing something?
Thanks as always,
Travis _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Sure, here it is. https://gitlab.haskell.org/ghc/ghc/issues/16614 On Mon, Apr 29, 2019 at 11:46 PM Matthew Pickering < matthewtpickering@gmail.com> wrote:
Please can you open a bug report anyway? It is easier to discuss it there than on the mailing list.
Matt
On Tue, Apr 30, 2019 at 5:25 AM Travis Whitaker <pi.boy.travis@gmail.com> wrote:
Hello GHC Devs,
I've found a case in which annotating a bidirectional pattern synonym
with a type signature causes the typechecker to reject an otherwise typeable program. Before filing a bug report I want to check that I'm thinking about this correctly. Consider this module:
{-# LANGUAGE TypeFamilies , TypeFamilyDependencies , DataKinds , TypeOperators , GADTs , FlexibleContexts , PatternSynonyms #-}
module Strange where
data Expr a where Tuple :: NTup ts -> Expr (Flatten ts)
data NTup (ts :: [*]) where Unit :: NTup '[] Pair :: Expr a -> NTup ts -> NTup (a ': ts)
type family Flatten ts = r | r -> ts where Flatten '[] = () Flatten '[a, b] = (a, b)
pattern P a b = Pair a (Pair b Unit)
fstExpr :: Expr (a, b) -> (Expr a, Expr b) fstExpr (Tuple (P x y)) = (x, y)
The idea is that an NTup '[a, b, c ...] should be isomorphic to an (a,
b, c, ...), but removes a lot of repetitive code for dealing with Exprs of tuples. This module compiles with GHC 8.6.4 and GHC infers the expected type for P.
P :: Expr a -> Expr a1 -> NTup '[a, a1]
However, annotating P with this type causes typechecking fstExpr to fail
with:
Strange.hs:26:17: error: • Could not deduce: ts ~ '[a, b] from the context: (a, b) ~ Flatten ts bound by a pattern with constructor: Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten
ts),
in an equation for ‘fstExpr’ at Strange.hs:26:10-22 ‘ts’ is a rigid type variable bound by a pattern with constructor: Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten ts), in an equation for ‘fstExpr’ at Strange.hs:26:10-22 Expected type: NTup ts Actual type: NTup '[a, b] • In the pattern: P x y In the pattern: Tuple (P x y) In an equation for ‘fstExpr’: fstExpr (Tuple (P x y)) = (x, y) • Relevant bindings include fstExpr :: Expr (a, b) -> (Expr a, Expr b) (bound at Strange.hs:26:1) | 26 | fstExpr (Tuple (P x y)) = (x, y) | ^^^^^
It seems that providing a type signature causes GHC to forget the
injectivity of Flatten (i.e (a, b) ~ Flatten ts implies ts ~ '[a, b]). Is this a bug, some limitation of pattern synonyms, or am I missing something?
Thanks as always,
Travis _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
participants (2)
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Matthew Pickering -
Travis Whitaker