What is Expressiveness in a Computer Language

[Darren New]

Andreas Rossberg wrote:
> AFAICT, ADT describes a type whose values can only be accessed by a 
> certain fixed set of operations. 

No. AFAIU, an ADT defines the type based on the operations. The stack 
holding the integers 1 and 2 is the value (push(2, push(1, empty()))). 
There's no "internal" representation. The values and operations are 
defined by preconditions and postconditions.

Both a stack and a queue could be written in most languages as "values 
that can only be accessed by a fixed set of operations" having the same 
possible internal representations and the same function signatures. 
They're far from the same type, because they're not abstract. The part 
you can't see from outside the implementation is exactly the part that 
defines how it works.

Granted, it's a common mistake to write that some piece of C++ code 
implements a stack ADT.

For example, an actual ADT for a stack (and a set) is shown on
http://www.cs.unc.edu/~stotts/danish/web/userman/umadt.html
Note that the "axia" for the stack type completely define its operation 
and semantics. There is no other "implementation" involved. And in LOTOS 
(which uses ACT.1 or ACT.ONE (I forget which)) as its type, this is 
actually how you'd write the code for a stack, and then the compiler 
would crunch a while to figure out a corresponding implementation.

I suspect "ADT" is by now so corrupted that it's useful to use a 
different term, such as "algebraic type" or some such.

> Classes qualify for that, as long as they provide proper encapsulation.

Nope.

> OK, I admit that I exaggerated slightly. Although currently I'm indeed 
> able to mostly work with the more pleasant among languages. :-)

Yah. :-)

> (Btw, Pascal did not have it either, AFAIK)

I'm pretty sure in Pascal you could say

Type Apple = Integer; Orange = Integer;
and then vars of type apple and orange were not interchangable.

-- 
   Darren New / San Diego, CA, USA (PST)
     My Bath Fu is strong, as I have
     studied under the Showerin' Monks.