inferred type of an implicit parameter rejected (associated type)
The following module:
{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
module Test where
class C t where
type TF t
ttt :: TF t -> t
b :: (C t, ?x :: TF t) => t
b = ttt ?x compiles fine with ghc 6.12.3 but ghc 7.0.3 says:
Could not deduce (?x::TF t)
arising from a use of implicit parameter `?x'
from the context (C t, ?x::TF t)
bound by the type signature for b :: (C t, ?x::TF t) => t
at Test.hs:13:1-10
In the first argument of `ttt', namely `?x'
In the expression: ttt ?x
In an equation for `b': b = ttt ?xMoreover, when I comment out the type declaration for b, ghc 7.0.3 compiles it and the inferred type for b is identical to the one that was commented out:
*Test> :t b
b :: (C t, ?x::TF t) => tTrac metadata
| Trac field | Value |
|---|---|
| Version | 7.0.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |