[Axiom-developer] Re: [Aldor-l] exports and constants

[Ralf Hemmecke]

> > Now let's make things a bit more complicated. Would you be able to guess
> > the output of the following program?
> >
> > ---BEGIN aaa7.as
> > #include "aldor"
> > #include "aldorio"
> >
> > define CatA: Category == with;
> > define CatX: Category == with;
> >
> > A: Join(CatX, CatA) == add;
> > X: CatX == A add;  stdout << (X has CatA) << newline;
> > Y: CatX == A;      stdout << (Y has CatA) << newline;
> > #if WITHZ
> > Z: CatA == Y;      stdout << (Z has CatA) << newline;
> > #endif
> > ---END aaa7.as
> >
> > Right.
> >
> >  >aldor -grun -laldor aaa7.as
> > F
> > T
> >
> >  >aldor -DWITHZ -grun -laldor aaa7.as
> > "aaa7.as", line 11: Z: CatA == Y;      stdout << (Z has CatA) << newline;
> >                      ...........^
> > [L11 C12] #1 (Error) There are 0 meanings for `Y' in this context.
> > The possible types were:
> >       Y: CatX, a local
> >    The context requires an expression of type CatA.
> >
> > So the compiler now thinks that the type of Y is just CatX. OK, but then
> > why can the program print "T" in the previous compilation?

> Because Y has static type CatX and dynamic type with{ CatA; CatX; }.
> The compiler uses the static type to check if Y can be used in "Z: CatA 
> == Y".

OK. If the static type is used for type satisfactions, I can live with that.

> The "has" checks the dynamic type.

So "has" is in fact a weak way to reflect about domains. But surely, we 
need true reflections.

Ralf