[GHC] #8651: 'Untouchable' error when using type function in class constraint in rank-2 type
#8651: 'Untouchable' error when using type function in class constraint in rank-2 type -------------------------------------------+------------------------------- Reporter: sbarclay | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: #8644 #7594 | Test Case: | Blocking: -------------------------------------------+------------------------------- I noticed there are some cases that no longer compile after the recent fix for #8644, such as the following: {{{#!haskell {-# 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: {{{#!haskell type Id a = a }}} I guess that the problem is caused by equalities of the form t ~ [Char] introduced by the type family instance. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8651 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8651: 'Untouchable' error when using type function in class constraint in rank-2
type
--------------------------------------------+------------------------------
Reporter: sbarclay | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets: #8644
| #7594
--------------------------------------------+------------------------------
Comment (by Simon Peyton Jones
#8651: 'Untouchable' error when using type function in class constraint in rank-2
type
--------------------------------------------+------------------------------
Reporter: sbarclay | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets: #8644
| #7594
--------------------------------------------+------------------------------
Comment (by Simon Peyton Jones
#8651: 'Untouchable' error when using type function in class constraint in rank-2 type -------------------------------------------------+------------------------- Reporter: sbarclay | Owner: Type: bug | Status: Priority: normal | closed Component: Compiler (Type checker) | Milestone: Resolution: fixed | Version: 7.7 Operating System: Unknown/Multiple | Keywords: Type of failure: None/Unknown | Architecture: Test Case: | Unknown/Multiple indexed_types/should_compile/T8651 | Difficulty: Blocking: | Unknown | Blocked By: | Related Tickets: #8644 | #7594 -------------------------------------------------+------------------------- Changes (by simonpj): * status: new => closed * testcase: => indexed_types/should_compile/T8651 * resolution: => fixed Comment: Very excellent catch, thank you. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8651#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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