What is Expressiveness in a Computer Language

[Marshall]

Rob Warnock wrote:
> Marshall <marshall.spight at gmail.com> wrote:
> >
> > Can you be more explicit about what "latent types" means?
> > I'm sorry to say it's not at all natural or intuitive to me.
> > Are you referring to the types in the programmers head,
> > or the ones at runtime, or what?
>
> Here's what the Scheme Standard has to say:
>
>     http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-4.html
>     1.1  Semantics
>     ...
>     Scheme has latent as opposed to manifest types. Types are assoc-
>     iated with values (also called objects) rather than with variables.
>     (Some authors refer to languages with latent types as weakly typed
>     or dynamically typed languages.) Other languages with latent types
>     are APL, Snobol, and other dialects of Lisp. Languages with manifest
>     types (sometimes referred to as strongly typed or statically typed
>     languages) include Algol 60, Pascal, and C.
>
> To me, the word "latent" means that when handed a value of unknown type
> at runtime, I can look at it or perform TYPE-OF on it or TYPECASE or
> something and thereby discover its actual type at the moment[1], whereas
> "manifest" means that types[2] are lexically apparent in the code.

Hmmm. If I read the R5RS text correctly, it is simply doing the
either/or thing that often happens with "static/dynamic" only
using different terms. I don't see any difference between
"latent" and "dynamic." Also, this phrase "types associated with
values instead of variables" that I'm starting to see a lot is
beginning to freak me out: the implication is that other languages
have types associated with variables and not values, which
doesn't describe anything I can think of.

In your followup paragraph, you've contrasted runtime type
introspection, vs. explicit type declarations, which seem
orthorgonal to me. (Not that you said they weren't.)


Marshall