What is Expressiveness in a Computer Language

[Andreas Rossberg]

Rob Thorpe wrote:
> 
> I think this should make it clear.  If I have a "xyz" in lisp I know it
> is a string.
> If I have "xyz" in an untyped language like assembler it may be
> anything, two pointers in binary, an integer, a bitfield.  There is no
> data at compile time or runtime to tell what it is, the programmer has
> to remember.

You have to distinguish between values (at the level of language 
semantics) and their low-level representation (at the implementation 
level). In a high-level language, the latter should be completely 
immaterial to the semantics, and hence not interesting for the discussion.

>>No, variables are insignificant in this context. You can consider a
>>language without variables at all (such languages exist, and they can
>>even be Turing-complete) and still have evaluation, values, and a
>>non-trivial type system.
> 
> Hmm.  You're right, ML is no-where in my definition since it has no
> variables.

Um, it has. Mind you, it has no /mutable/ variables, but that was not 
even what I was talking about.

>>>But the value itself has no type
>>
>>You mean that the type of the value is not represented at runtime? True,
>>but that's simply because the type system is static. It's not the same
>>as saying it has no type.
> 
> Well, is it even represented at compile time?
> The compiler doesn't know in general what values will exist at runtime,
> it knows only what types variables have.  Sometimes it only has partial
> knowledge and sometimes the programmer deliberately overrides it.  From
> what knowledge it you could say it know what types values will have.

Again, variables are insignificant. From the structure of an expression 
the type system derives the type of the resulting value. An expression 
may contain variables, and then the type system generally must know (or 
be able to derive) their types too, but that's a separate issue. Most 
values are anonymous. Nevertheless their types are known.

> Unfortunately it's often necessary to break static type systems.

Your definitely using the wrong static language then. ;-)

- Andreas