[Aldor-l] [fricas-devel] Re: "has" in the interpreter

[Ralf Hemmecke]

> I do not see any immediate need for reflection (self description) in a
> mathematical library  [snip]

Good.

> Programmers might use reflection in order to avoid having to consider
> the most general cases. Look at the following code in
> 'src/algebra/numsolve.spad.pamphlet':

I'd say that is not the best piece of code.

>        numeric(r:K):F ==
>          K is I =>
>            F is Float => r::I::Float
>            F is RN    => r::I::RN
>            F is CF    => r::I::CF
>            F is GRN   => r::I::GRN
>          K is GI =>
>            gr:GI := r::GI
>            F is GRN => complex(real(gr)::RN,imag(gr)::RN)$GRN
>            F is CF  => convert(gr)
>          error "case not handled"

K is I => r::I::F

for the first part would probably have worked fine and even covers more 
cases.

But it shows that 'is'/'has' might lead to lots of branches and thus the 
code doesn't scale very well.

But look at

mygcd(R: EuclideanDomain)(a: R, b: R): R == {
   R has Field => if zero? a and zero? b then 0 else 1;
   while not zero? b repeat { (a, b) := (b, a rem b) }
   return a;
}

That I consider a case where 'has' is useful.

I actually wonder what will happen if I define two functions:

mygcd(R: Field)(a: R, b: R): R == {
   if zero? a and zero? b then 0 else 1;
}
mygcd(R: EuclideanDomain)(a: R, b: R): R == {
   while not zero? b repeat { (a, b) := (b, a rem b) }
   return a;
}

and then try to say:

Q ==> Fraction Integer;
g: (Q, Q) -> Q := mygcd Q

Should a compiler reject it or simply take the most specific code? Note, 
that the compiler already knows that 'Q has Field'.

Even for Aldor I don't think there is a specification for that. (Can 
somebody prove me wrong?)

> So my conclusion in the end is that the use of reflection is (almost
> always) undesirable and usually indicates the presence of some other
> deficiency in the language.

+1

Ralf