[Haskell-cafe] some ideas for Haskell', from Python

[Bas van Dijk]

On Wed, Jan 14, 2009 at 3:59 PM, Manlio Perillo
<manlio_perillo at libero.it> wrote:
> 2) In Python it is possible to import modules inside a function.
>
>   In Haskell something like:
>
>   joinPath' root name =
>       joinPath [root, name]
>       importing System.FilePath (joinPath)

I just like to point out the dependently typed, Haskell-like,
programming language Agda[1] which has a very nice module system with
the following features:

 * Modules can contain other modules

 * Modules can be locally opened. For example:
   mapMaybe f m = let open Maybe in maybe nothing (just . f) m

 * Renaming of important names: For example:
   open Maybe renaming (Maybe to option; nothing to none; just to some)

 * Parameterized modules: For example:

   module Sort (A : Set) (_<_ : A -> A -> Bool) where
     insert : A -> List A -> List A
     insert y [] = y :: []
     insert y (x :: xs) with x < y
     ... | true  = x :: insert y xs
     ... | false = y :: x :: xs

See section 2.7 of the following Agda tutorial (an open minded Haskell
hacker should be able to read that section on its own):

http://www.cs.chalmers.se/~ulfn/darcs/AFP08/LectureNotes/AgdaIntro.pdf

Hopefully Haskell can borrow some of these ideas sometime.

regards,

Bas

[1] http://wiki.portal.chalmers.se/agda/