[Haskell-cafe] Scoped data declarations

[Sebastian Sylvan]

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