[ ghc-Bugs-1186853 ] Kind error cccasionally reported

Bugs item #1186853, was opened at 2005-04-20 18:10
Message generated for change (Comment added) made by simonpj
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Category: Compiler
Group: 6.4
>Status: Closed
>Resolution: Invalid
Priority: 5
Submitted By: daniel (jcpetruzza)
Assigned to: Nobody/Anonymous (nobody)
Summary: Kind error cccasionally reported

Initial Comment:
Consider an example made of the following two files:

1) TypeDefinition.hs:
module TypeDefinition (PublicType)
where

import Control.Monad.State

type PublicType a b = a String b
-- type PrivateType = PublicType StateT IO

2) Main.hs
module Main
where

import Control.Monad.State
import TypeDefinition(PublicType)

type MyType = PublicType StateT IO

main :: IO ()
main = putStrLn "Hello, world"

If I try to build Main, I get the following error:

bash-2.05b$ ghc --make Main.hs
Chasing modules from: Main.hs
Compiling TypeDefinition   ( ./TypeDefinition.hs,
./TypeDefinition.o )
Compiling Main             ( Main.hs, Main.o )

Main.hs:7:37:
    Kind error: `StateT' is not applied to enough type
arguments
    In the type synonym declaration for `MyType'

However, if one uncomments the commented line in
TypeDefinition.hs, everything builds ok (even when
PrivateType is not visible from Main)

I'm using GHC 6.4 on Mandrake 10.1

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>Comment By: Simon Peyton Jones (simonpj)
Date: 2005-04-25 08:20

Message:
Logged In: YES 
user_id=50165

Simon M is exactly right. It's not a bug.

Simon PJ

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Comment By: Simon Marlow (simonmar)
Date: 2005-04-21 10:14

Message:
Logged In: YES 
user_id=48280

This looks strange, but I think it's the right behaviour.  

In the absence of any other information about the type
variable 'b' in your definition of  PublicType, it defaults
to kind *.  When you uncomment the PrivateType definition,
it is instantiated with kind (*->*), so the compiler infers
that as the kind for b.

I'll leave the bug open until Simon gets back, just in case
I'm wrong!

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