[Haskell-cafe] Feature Proposal: GHC Flag for implicit external Prelude

L Corbijn aspergesoepje at gmail.com
Wed Jul 16 09:46:02 UTC 2014

Another problem with making it (cabal) global is that you have to
separate the alternative Prelude (and the modules it depends on) from
the rest of the modules. Otherwise you would have an circular
dependency by importing the new Prelude in the new Prelude you are

For project specific preludes I can think of two options. Separate
into a package/library, which will probably be more trouble than it
would solve in the first place. Alternatively there could be a
mechanism for excluding the automatic new Prelude import, which would
make things only more complicated. Either way it seems that it would
make things only more difficult if you want a prelude tailored to a
specific project.


On Tue, Jul 15, 2014 at 7:30 PM, Michael Orlitzky <michael at orlitzky.com> wrote:
> On 07/14/2014 11:23 PM, Andrew Gibiansky wrote:
>> Hey all,
>> I would like to propose a very minor flag to add to GHC. I would like
>> GHC to have a --with-prelude flag, which would specify an alternate
>> Prelude to use instead of the default Haskell prelude.
>> This would have an effect similar to -XNoImplicitPrelude and an additional
>> import MyNewPrelude
>> in the source file. It might be a /little/ different semantically, as a
>> qualified import would disable the original implicit import, just like
>> it does with the default Haskell prelude.
> To play devil's advocate, I don't think this is a good idea. It moves
> information that is fundamentally local (which library imports a module
> requires) into global state: magic flags that must be passed to the
> build system for anything to work.
> It would be super annoying for an {IDE, coworker, ...} to have to
> analyze your build system just to figure out which functions are in scope.
> We went down this road once before with -fglasgow-exts. Eventually, we
> all realized it was much better to place the required extensions in
> pragmas at the top of the file.
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