"Simon Marlow" <[EMAIL PROTECTED]> wrote,
> > With the attached file, on both Linux and Solaris, I get the following
> >
> > ---- Cut here -------
> > # ghci --interactive
> > ___ ___ _
> > / _ \ /\ /\/ __(_)
> > / /_\// /_/ / / | | GHC Interactive, version 5.04.1,
> > for Haskell 98.
> > / /_\\/ __ / /___| | http://www.haskell.org/ghc/
> > \____/\/ /_/\____/|_| Type :? for help.
> >
> > Loading package base ... linking ... done.
> > Loading package haskell98 ... linking ... done.
> > Prelude> :load GHCBug
> > Compiling GHCBug ( GHCBug.hs, interpreted )
> > Ok, modules loaded: GHCBug.
> > *GHCBug> show (read (show (Commit 1 "1" Nothing)) :: Command)
> > "*** Exception: Prelude.read: no parse
> > ----- Cut here ------------
> >
> > NB (1) this seems to occur in ghc generally, not just in
> > ghci. (2) Read/Show being fairly
> > important to us, I'm afraid this means we will have to avoid
> > using ghc5.04.1. I hope ghc5.04.2 comes along soon.
>
> It turns out there's a bug in the Read instance for Maybe. It's rather
> unfortunate that there's no workaround other than defining your own
> Maybe type.
Luckily, there is a workaround (discovered by Tom Moertel on
#haskell). If you include field names in the data
definition for which you derive Show and Read, it seems to
work fine.
So, in George's example
module GHCBug where
data Command =
NewLocation
| Commit {a :: Int, b :: String, c :: (Maybe String)}
| Retrieve String String
| ListVersions String
deriving (Read,Show)
should be all that is needed.
It seems that as soon as the value of the Maybe type is
delimited by a punctuation symbol or bracket, all is fine.
Cheers,
Manuel
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