On Wed, 2006-10-04 at 00:53 -0400, Peter Tanski wrote:
> On Oct 3, 2006, at 11:47 PM, skaller wrote:
>
> > On Mon, 2006-10-02 at 09:40 -0400, Peter Tanski wrote:
> >
> >>> Typeclasses and instances have to be toplevel: that is,
> >>> they can be nested in modules but NOT in functions.
> >>
> >> It wouldn't make sense if typeclasses were nested in functions.
> >
> > Why not?
> >
> > They're allowed in programs .. and a program is just a function.
>
> I knew you were going to say that :)
Well of course you did .. that's why you made the original
assertion :)
> Typeclasses exist to scope over
> types and in a functional world types only make sense when they scope
> over multiple functions. If you had a single function with no nested
> functions--the compiler may actually create nested functions, such as
> the GNU extension to C, as a separate function anyway--defining a
> type within the function for even a few procedures would not make any
> real a difference.
That's probably right.
> You can imagine the scenario: a typedef within a
> function would really operate only as syntactic sugar since you would
> have to (1) create the type and (2) destroy the type before returning
> from the function itself.
> As a practical matter, in a language with
> functional features and in any "functional" language function-nested
> types would be very difficult (if not impossible) to support.
That's funny because Felix *already* supports function nested types:
//------------------------------------------------------
#import <flx.flxh>
fun f(x:int)= {
union x_t = | X of int;
fun pr(a:x_t): int = {
return
match a with
| X ?i => i
| _ => 0
endmatch
;
}
val a = X x;
return a,the pr;
}
val v,d = f 1;
print$ d v; endl;
//-----------------------------------------------------
[This doesn't work in current SVN due to a bug I will shortly fix,
it does work on my box with the wrong code temporarily commented out]
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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