On 22 June 2005 08:04, Serge D. Mechveliani wrote:
> Who knows, please, how to work with the program of
>
>
> main = interact (\ s -> shows (read s :: Bool) "\n")
>
>
> in the interpreter ghci ?
>
> This is on ghc-cvs-6-4-branch-June-15-2005
> under Debian Linux, i386-uknown.
>
> When compiled, it works: > ghc --make ReadBug
> > ./a.out
> > True press Enter
> > press Control-d
> > True
>
> And it does not work under ghci:
>
> --------------------------------------------------
>> ghci ReadBug.hs
>
> ___ ___ _
> / _ \ /\ /\/ __(_)
> / /_\// /_/ / / | | GHC Interactive, version 6.4.1, for
> Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/
> \____/\/ /_/\____/|_| Type :? for help.
>
> Loading package base-1.0 ... linking ... done.
> Compiling Main ( ReadBug.hs, interpreted )
> Ok, modules loaded: Main.
> (0.08 secs, 3210432 bytes)
> *Main> main
> True
> ^D*** Exception: Prelude.read: no parse
> *Main>
> ------------------------------------------------
>
>
> Is this a GHC bug?
Not really. When stdin is in non-blocking mode, as it is by default
inside GHCi, then ^D loses its special meaning (this is standard Unix
terminal behaviour, we turn off "cooked" mode on the terminal). In your
example, the ^D character is being sent to the program, and causing a
parse error.
You can re-enable ^D by putting stdin in LineBuffering mode, but then ^D
will also close GHCi, because GHCi is using the same stdin (it might be
possible to work around this, because ^D doesn't really "close" stdin,
but GHCi definitely got confused when I tried it).
Cheers,
Simon
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