Sure, here it is.
https://gitlab.haskell.org/ghc/ghc/issues/16614
On Mon, Apr 29, 2019 at 11:46 PM Matthew Pickering <
matthewtpicker...@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.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
> > _______________________________________________
> > ghc-devs mailing list
> > ghc-devs@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
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