Summary of progress

Johannes Waldmann
Tue, 13 Mar 2001 08:50:53 +0100 (MET)

Two remarks on what Malcolm wrote:

> I don't see the need to bring whole namespaces into scope.  
> And even if I did, I wouldn't overload the standard import syntax 
> to mean namespace scoping as well -

Well, the syntax *is* overloaded already :-) `import' serves two purposes: 
a) declare what modules you want to access
b) declare how you want to name entities from these modules.

Java drops a) - technically, you could write Java programs
without any `import's, by using fully qualified names everywhere.
Ada separates a) and b) - it has `with' for a) and `use' for b). (*)

Why do we want a) for Haskell? Sure it's easier to implement
(but that alone would be a bad excuse) the main reason is
that it serves to document the code (namely, its dependecy on other modules),
and allows to enforce some coding standards. (**)

(*) The advantage (in my eyes) of having these concerns separated
is that you can have local `use' statements, i. e. at the top of your module,
you have to `with' all packages that you want to access,
but you can write `use' statements (that bring their namespace into scope)
not only at the top level, but also at local binding groups.
(Is this the same as `open' in ML? - I confess I don't know ML.)

(**) I use this feature in the following way: students send me their homework 
in the form of a Haskell module that contains a definition like

solution :: Push_Down_Automaton
solution =  Push_Down_Automaton { states = .., transitions = .., .. }

and I then (automatically) run a test program that imports their module.
Of course I want to be sure that they don't do some hidden IO stuff
and wipe my hard disk - but this is nicely ensured by Haskell's
type and module system: the typechecker verifies that the type of `solution'
is pure (and not IO something), and I can easily check textually 
that they don't `import IOExts', which would allow `unsafePerformIO'.
(However, I can't simply remove hugs/lib/exts/IOExts.hs 
because `runhugs' wants it - I don't really see why?)

Yes I see that this doesn't necessarily advance the discussion of
the standard library layout. 
-- Johannes Waldmann ---- --
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