[Haskell-cafe] Scoped data declarations

[Christophe Poucet]

Hello,

Well one specific example where this would be useful is for lambdabot and
similar systems.  Additionally this could be useful for experimenting in any
interpreter such as hugs or ghci.

Regards

On 6/26/06, Sebastian Sylvan <sylvan at student.chalmers.se> wrote:
>
> On 6/23/06, Christophe Poucet <christophe.poucet at gmail.com> wrote:
> > Dear,
> >
> > Yesterday, while discussing with Cale and SamB on I suddenly came up
> with
> > the crazy idea of scoped data declarations.  After some brief discussion
> to
> > check the validity, I finally came to the conclusion that they should be
> > feasible. In addition, I don't think that they would require a high
> amount
> > of changes in current compilers.
> >
> > Basically if you have something like:
> >
> > module Main where
> > foo = let data Foo = Foo deriving Show in Foo\
> > main :: IO ()
> > main = print foo
> >
> > One can see this as having an extra hidden module that defines Foo but
> that
> > does not export it.  The only change that is then required is that while
> > compiling Foo, the hidden-ness of Foo must be removed.
> >
> > For instance, if one were to load this into, say, ghci (this is fictive
> of
> > course):
> > # ghci Main.hs
> > > :t foo
> > foo :: Codeloc2.Foo
> >
> > There were initially some objections to this, because it is no longer
> > feasible to actually write the type of the function foo.  But if one
> looks
> > at current GHC, this objection is already there:
> >
> > module A(foo) where
> > data Foo = Foo deriving Show
> > foo = Foo
> >
> > module Main where
> > import A
> > main = print foo
> >
> > As Excedrin then pointed out, importing this Main into ghci, gives
> > foo :: Foo.Foo
> >
> > And this notation can not be written in Main either, because Foo is
> hidden
> > in A.
> >
> > Therefore, I would like to note that scoped data declarations are just
> like
> > hidden data-declarations with two extra requirements:
> > 1) Generate source-location-based submodule names
> > 2) Add an extra import rule for those hidden modules in the
> subexpressions
> > of where the data-declaration is being originally defined.
> >
> > Comments are welcome, of course :)
>
>
> I'm not sure I understand why this is something we need. Do you have
> any examples where this would be useful?
>
>
> /S
>
> --
> Sebastian Sylvan
> +46(0)736-818655
> UIN: 44640862
>
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