What is Expressiveness in a Computer Language

[Marshall]

David Hopwood wrote:
> Marshall wrote:
> > David Hopwood wrote:
> >>Marshall wrote:
> >>
> >>>The real question is, are there some programs that we
> >>>can't write *at all* in a statically typed language, because
> >>>they'll *never* be typable?
> >>
> >>In a statically typed language that has a "dynamic" type, all
> >>dynamically typed programs are straightforwardly expressible.
> >
> > So, how does this "dynamic" type work?
>
> <http://citeseer.ist.psu.edu/abadi89dynamic.html>
>
> > It can't simply be the "any" type, because that type has no/few
> > functions defined on it.
>
> It isn't. From the abstract of the above paper:
>
>   [...] even in statically typed languages, there is often the need to
>   deal with data whose type cannot be determined at compile time. To handle
>   such situations safely, we propose to add a type Dynamic whose values are
>   pairs of a value v and a type tag T where v has the type denoted by T.
>   Instances of Dynamic are built with an explicit tagging construct and
>   inspected with a type safe typecase construct.

Well, all this says is that the type "dynamic" is a way to explicitly
indicate the inclusion of rtti. But that doesn't address my objection;
if a typesafe typecase construct is required, it's not like using
a dynamic language. They don't require typecase to inspect values
before one can, say, invoke a function.


> "Gradual typing" as described in
> <http://www.cs.rice.edu/~jgs3847/pubs/pubs/2006/siek06:_gradual.pdf> is
> another alternative. The difference between gradual typing and a
> "dynamic" type is one of convenience rather than expressiveness --
> gradual typing does not require explicit tagging and typecase constructs.

Perhaps this is the one I should read; it sounds closer to what I'm
talking about.


Marshall