'Untouchable' error in list comprehension
Hi,
I think this program should compile but it doesn't:
{-# LANGUAGE GADTs #-}
data X a where X :: X Int
foo :: Bool
foo = null [ () | X <- [] ]
It fails with:
Couldn't match expected type `a0' with actual type `()'
`a0' is untouchable
inside the constraints (t_g ~ Int)
bound at a pattern with constructor
X :: X Int,
in a pattern binding in
list comprehension
In the pattern: X
In a stmt of a list comprehension: X <- []
In the first argument of `null', namely `[() | X <- []]'
If I remove the type declaration GHC compiles without errors and happily infers foo :: Bool
.
Trac metadata
Trac field | Value |
---|---|
Version | 7.6.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |