PatternSynonyms + ScopedTypeVariables triggers an internal error
When trying to give a type signature with a free type variable to a pattern in a pattern synonym, GHC dies with an internal error. Given the file
{-# LANGUAGE PatternSynonyms, ScopedTypeVariables #-}
pattern Nil = [] :: [a]
the resulting error is
Example.hs:2:22: GHC internal error: ‘a’ is not in scope during type checking, but it passed the renamer …
tcl_env of environment: []
In an expression type signature: [a]
In the expression: [] :: [a]
In an equation for ‘$WNil’: $WNil = [] :: [a]
Compilation failed.
The fix is to add RankNTypes
and an explicit forall
:
{-# LANGUAGE PatternSynonyms, ScopedTypeVariables, RankNTypes #-}
pattern Nil = [] :: forall a. [a]
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |