Opened 11 years ago

Closed 11 years ago

Last modified 9 years ago

#1214 closed bug (fixed)

Iface type variable out of scope

Reported by: igloo Owned by: simonpj
Priority: normal Milestone: 6.8.1
Component: Compiler (Type checker) Version: 6.6
Keywords: Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case: gadt23
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


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

Change History (5)

comment:1 Changed 11 years ago by simonpj

Owner: set to simonpj

Definitely a bug. I will look at it.


comment:2 Changed 11 years ago by simonpj

Resolution: fixed
Status: newclosed

Good report. It was a buglet in the 6.6 branch; I've fixed it.

Ian: the gadt23 test should really be pushed to the 6.6 branch. (But it's hardly worth the bother.)


comment:3 Changed 10 years ago by simonmar


comment:4 Changed 9 years ago by simonmar

Architecture: UnknownUnknown/Multiple

comment:5 Changed 9 years ago by simonmar

Operating System: UnknownUnknown/Multiple
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