[Aldor-l] exports and constants

[Bill Page]

On July 21, 2006 4:06 AM Ralf Hemmecke wrote:
> ...
> Christian Aistleitner wrote:
> > 
> > I'd consider that a bug in comparison of exports. Replacing 
> > your (*) line by
> > 
> >   X: CatX == if true then (A at CatX) else (B at CatX);
> > 
> > gives a working program with output
> > 
> >   X has CatA: T
> >   X has CatB: F
> >   X has CatX: T
> > 
> > So the problem (wild guess) is that the compiler Has 
> > problems with seeing that the if statement gives CatX in
> > both branches of the if  statement. Mainly because the
> > types of A and B aro not equal. However, you can hint the
> > compiler. My code is telling him "The if part gives 
> > CatX and the else part gives CatX". Then the compiler can 
> > infer, that the whole "if" statement gives CatX. And it is
> > at least the type of X (which is CatX). So it matches.
> 
> COOL! So without the "add" the compiler gives X the type of 
> what it sees on the right of "==". Now you declare that through
> "@CatX" to be CatX. But, hey, why then you get
> 
>     X has CatA: T
> 
> ??? That looks like cheating.

Ralf, I seems to me you might be using the wrong definition of
"type". In Aldor when we write:

  X: CatX

it means explicitly that the type of X is CatX. And we are
saying that it is the compiler's job to check (at compile time)
that this is necessarily always the case.

But 'X has CatA' is something different. I think Christian
referred to it earlier as something like the "dynamic type"?
But in the Aldor User's Guide this is just called
"type satisfaction".

Regards,
Bill Page.