Inferred type for pattern synonym has redundant equality constraint
Load this file into GHCi:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
module Bug where
import Data.Type.Equality
pattern Foo = HRefl
$ /opt/ghc/8.2.1/bin/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 Bug ( Bug.hs, interpreted )
Ok, 1 module loaded.
λ> :i Foo
pattern Foo :: () => (* ~ *, b ~ a) => a :~~: b
Notice that the type signature for Foo
has an entirely redundant * ~ *
constraint. The same does not happen if TypeInType
is enabled.
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE TypeInType #-}
module Works where
import Data.Type.Equality
pattern Foo = HRefl
λ> :i Foo
pattern Foo :: forall k2 k1 (a :: k1) (b :: k2). () => (k2 ~ k1,
(b :: k2) ~~ (a :: k1)) => a :~~: b
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |