What is Expressiveness in a Computer Language

[Rob Thorpe]

rossberg at ps.uni-sb.de wrote:
> Rob Thorpe wrote:
> >
> > But it differs from latently typed languages like python, perl or lisp.
> >  In such a language there is no information about the type the variable
> > stores.  The programmer cannot write code to test it, and so can't
> > write functions that issue errors if given arguments of the wrong type.
> >  The programmer must use his or her memory to substitute for that
> > facility.  As far as I can see this is a significant distinction and
> > warrants a further category for latently typed languages.
>
> Take one of these languages. You have a variable that is supposed to
> store functions from int to int. Can you test that a given function
> meets this requirement?

The answer is no for python and perl AFAIK.  Also no for lisp _aux
naturelle_ (you can do it by modifying lisp though of-course, I believe
you can make it entirely statically typed if you want).

But the analogous criticism could be made of statically typed
languages.
Can I make a type in C that can only have values between 1 and 10?
How about a variable that can only hold odd numbers, or, to make it
more difficult, say fibonacci numbers?

> You see, IMO the difference is marginal. From my point of view, the
> fact that you can do such tests *in some very trivial cases*

They are not very trivial for practical programming purposes though,
they are important.

> in the
> languages you mention is an artefact, nothing fundamental.

The cases can be far from trivial.  In lisp a type specifier can mean
something like "argument must be a 10x10 matrix with top corner element
larger than 35"  You can also, for example, easily make predicates that
tell if a value is odd, or a fibonacci number.

The system simply extends in different directions.