What is Expressiveness in a Computer Language

[David Hopwood]

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.

"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.

-- 
David Hopwood <david.nospam.hopwood at blueyonder.co.uk>