'Untouchable' error when using type function in class constraint in rank-2 type
I noticed there are some cases that no longer compile after the recent fix for #8644 (closed), such as the following:
{-# LANGUAGE RankNTypes, FlexibleContexts, TypeFamilies #-}
import Data.Monoid
type family Id a
type instance Id a = a
--type instance Id [a] = [Id a]
foo :: (Monoid (Id String) => r) -> r
foo x = x
main :: IO ()
main = print $ foo "Hello"
Attempting to compile this on HEAD produces the same error in 'main' as reported in the earlier ticket:
Couldn't match expected type ‛s0’ with actual type ‛[Char]’
‛s0’ is untouchable
However, it compiles fine if the commented-out type family instance is used instead. It also compiles fine if the type family is replaced with an ordinary type synonym:
type Id a = a
I guess that the problem is caused by equalities of the form t ~ [Char] introduced by the type family instance.
Trac metadata
Trac field | Value |
---|---|
Version | 7.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |