id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
1214,Iface type variable out of scope,igloo,simonpj,"Christophe Poucet reports the below bug; the 6.6 branch is broken, but the HEAD works.

----

While working some code I stumbled into a bug with GHC. Basically I have a
GADT that has a phantom-type for all but one of it's data constructors. When
I try to store a record-type in that tag it gives me a problem due to
interfaces when I recompile something against the interface file containing
the GADT.  Namely, it complains that the type variable is out of scope.

The code in question (after treeing to find out what the actual problem was,
namely the combination of GADTs with records) is the following:
{{{
-- AST.hs
{-# OPTIONS_GHC -fglasgow-exts #-}
module AST where

data Exp_;

data AST :: * -> * -> * where
  Var   :: String -> AST Exp_ tag
  Tag   :: tag    -> AST a tag -> AST a tag
}}}
{{{
-- Main.hs
module Main where
import AST

data Foo = Foo { bar :: Int }

convert :: AST a tag -> AST a Foo
convert t = case t of
  Var v     -> Tag (Foo 42) $ Var v
  Tag t e   -> Tag (Foo 42) $ convert e

main = return ()
}}}

The bug is easily replicated with the following commands:

{{{
> ghc --make Main.hs
  -- Works fine
> touch Main.hs
> ghc --make Main.hs
  --   The interface for `main:AST'
  --   Declaration for AST
  --   Constructor AST.Tag:
  --     Iface type variable out of scope:  tag
  --   Cannot continue after interface file error
> rm -f Main.hi Main.o AST.hi AST.o
> ghc --make Main.hs
  -- Works fine
}}}",bug,closed,normal,6.8.1,Compiler (Type checker),6.6,fixed,,,Unknown/Multiple,Unknown/Multiple,,Unknown,gadt23,,,