Bugs item #1200592, was opened at 2005-05-12 05:34
Message generated for change (Tracker Item Submitted) made by Item Submitter
You can respond by visiting:
https://sourceforge.net/tracker/?func=detail&atid=108032&aid=1200592&group_id=8032
Category: Compiler
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: GHC fails to pass dictionary in a rank-2 situation.
Initial Comment:
Hello,
The following code leads to a run- or compile-time error.
\begin{code}
{-# OPTIONS -fglasgow-exts #-}
module Main () where
foo :: (forall m. Monad m => m a) -> IO a
foo = id . id
main :: IO ()
main = foo (return ())
\end{code}
GHC translates `foo' effectively to an identity
function, failing to pass a dictionary to the argument.
foo :: %forall a . (%forall (m::(*->*)) . ZCTMonad m ->
m a)
-> IO a =
\ @ a ->
zi @ (IO a) @ (IO a)
@ (IO a) (id @ (IO a))
(id @ (IO a));
This doesn't typecheck, therefore
$ ./ghc-6.5.20050510 -O IsolateBug.hs
ghc-6.5.20050510: panic! (the `impossible' happened,
GHC version 6.5.20050510):
No match in record selector Var.idInfo
Please report it as a compiler bug to
glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.
$ ghc-6.5.20050510 IsolateBug.hs
$ ./a.out
zsh: segmentation fault ./a.out
The bug still occurs when we give the compiler a little
bit more type information
> foo = (id :: IO a -> IO a) . (id :: IO a -> IO a)
, however
> foo = id . id :: IO a -> IO a
and
> foo = id
behave correctly.
Tested with ghc-6.2.2, ghc-6.4 and the latest ghc-6.5
snapshot.
-- Thomas Jäger <[EMAIL PROTECTED]>
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