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[GHC] #6044: Regression error: Kind variables don't work inside of kind constructors in type families

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[GHC] #6044: Regression error: Kind variables don't work inside of kind constructors in type families

#6044: Regression error: Kind variables don't work inside of kind constructors in
type families
---------------------------------------+------------------------------------
Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.5 | Keywords: PolyKinds TypeFamilies
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------------+------------------------------------
Many thanks for the quick bug fixes around kind variables recently.

With the newest build (7.5.20120425), the following code fails:

{{{
{-# LANGUAGE PolyKinds, DataKinds, TypeFamilies, KindSignatures #-}

type family Foo (a :: k) :: Maybe k
type instance Foo a = Just a
}}}

The error is:

{{{
Kind mis-match
The first argument of `Just' should have kind `k0',
but `a' has kind `k'
In the type `Just a'
In the type instance declaration for `Foo'
}}}

The above code compiles without error on, e.g., 7.5.20120329.

I think it's worth noting that the following compiles fine, which
surprised me given the error above:

{{{
type family Id (a :: k) :: k
type instance Id a = a
}}}

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6044>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler

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GHC

Re: [GHC] #6044: Regression error: Kind variables don't work inside of kind constructors in type families

#6044: Regression error: Kind variables don't work inside of kind constructors in
type families
----------------------------------------+-----------------------------------
Reporter: goldfire | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.5
Resolution: fixed | Keywords: PolyKinds TypeFamilies
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Difficulty: Unknown
Testcase: polykinds/T6044 | Blockedby:
Blocking: | Related:
----------------------------------------+-----------------------------------
Changes (by simonpj):

* status: new => closed
* difficulty: => Unknown
* resolution: => fixed
* testcase: => polykinds/T6044

Comment:

Turns out to be the same trivial bug as #6020. Fixed by
{{{
commit 4cc882650af6a4fadb30706f21e987edb846bcc3
Author: Simon Peyton Jones <[hidden email]>
Date: Thu Apr 26 09:29:15 2012 +0100

Fix a one-character typo (kv1 should be kv2!)

Fixes Trac #6020, #6044

>---------------------------------------------------------------

compiler/typecheck/TcUnify.lhs | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/compiler/typecheck/TcUnify.lhs
b/compiler/typecheck/TcUnify.lhs index 1729dcd..6e4d128 100644
--- a/compiler/typecheck/TcUnify.lhs
+++ b/compiler/typecheck/TcUnify.lhs
@@ -1119,7 +1119,7 @@ uKVar isFlipped unify_kind eq_res kv1 k2
| TyVarTy kv2 <- k2, kv1 == kv2
= return eq_res

- | TyVarTy kv2 <- k2, isTcTyVar kv1, isMetaTyVar kv2
+ | TyVarTy kv2 <- k2, isTcTyVar kv2, isMetaTyVar kv2
= uKVar (not isFlipped) unify_kind eq_res kv2 (TyVarTy kv1)

| otherwise = if isFlipped
}}}

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6044#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler

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