[Haskell-cafe] Weird interaction between literate haskell, ghci and OverloadedStrings

[Bas van Dijk]

On 3 December 2011 11:19, Erik de Castro Lopo <mle+hs at mega-nerd.com> wrote:
> Joachim Breitner wrote:
>
>> it does not seem to be related to literate haskell, if I copy the code
>> from your file into a .hs without the "> ", ghci still does not activate
>> the OverloadedStrings extension when loading the file.
>
> I hadn't noticed that.
>
>> I’d consider this a bug until the developers explain why this should or
>> cannot be different, and suggest you file it as such.
>
> I agree. I've lodged a bug report here:
>
>    http://hackage.haskell.org/trac/ghc/ticket/5673

I think it's very dangerous if language extensions "leak" from modules
by default. For example if someone creates a library and needs to use
some unsafe language extensions like:

{-# LANGUAGE UndecidableInstances, OverlappingInstances, IncoherentInstances #-}
module SomeLib where ...

You surely don't want to silently enable these in some unsuspecting client:

module MyFirstHaskellModule where
import SomeLib
...

I can imagine having a pragma for explicitly exporting language extensions:

{-# EXPORT_LANGUAGE OverloadedStrings #-}

Cheers,

Bas