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.tra...@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
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> ghc-devs@haskell.org
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