Cannot bundle pattern synonym with exported data family
I'd like to write this code:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Foo (Sing(.., SCons)) where
data family Sing (a :: k)
data instance Sing (z :: [a]) where
SNil :: Sing '[]
(:%) :: Sing x -> Sing xs -> Sing (x:xs)
pattern SCons :: forall a (z :: [a]). ()
=> forall (x :: a) (xs :: [a]). z ~ (x:xs)
=> Sing x -> Sing xs -> Sing z
pattern SCons x xs = (:%) x xs
{-# COMPLETE SNil, SCons #-}
But alas, GHC will not let me:
$ ghci Bug.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Foo ( Bug.hs, interpreted )
Bug.hs:7:13: error:
• Pattern synonyms can be bundled only with datatypes.
• In the pattern synonym: SCons
In the export: Sing(.., SCons)
|
7 | module Foo (Sing(.., SCons)) where
| ^^^^^^^^^^^^^^^
Failed, 0 modules loaded.
Can this restriction be lifted for data families?
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.1 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | mpickering |
Operating system | |
Architecture |