Re: [stack] Properties of Concatenative langauges and Forth

On Jan 2, 2009, at 5:50 PM, Christopher Diggins wrote:

>> I think macros and compiling words get a pass.
>
> Forth without compile-time words isn't Turing-complete as far as I can
> tell. I don't think you can just give it a pass. Can you even
> construct a conditional expression in Forth without compile-time
> words?

I was considering Forth's conditional as a functional form with the  
syntax 'IF <expr> THEN <expr> ELSE'. The expressions within the form  
are still completely concatenative. It's true that IF, THEN, and ELSE  
are not functions, but neither are [ and ] which are used for forming  
quotations. Furthermore, 'IF <expr> THEN <expr> ELSE' does indeed  
denote a function. I see no issue with a concatenative language  
offering the functional form of condition.

>> The concatenative requirement is sensibly limited to expressions
>> within the language. In certain cases, it might be required to expand
>> the macros to get the concatenative expression.
>
> Macros sure, but your earlier statement included compile-time words.

Nearly all languages have "compile-time" words, including Cat. Is Cat  
not concatenative because it has 'define'? Of course not, because  
'define' is not part of the expression language. The same goes for  
things in Forth like ':', ';', etc. There may be other examples in  
Forth that break this, but I'm not enough of a Forth programmer to know.

> Joy is a nearly pure example of a special class of programming
> languages. If you remove the side-effects, you are left with a  
> calculus
> of stacks. See my other post about the evaluation of a concatenative
> language, to get a sense of why Joy is special.

That's a pretty big "if". Unfortunately, Joy is not pure enough to  
allow reduction in an arbitrary order in the sense you're describing.  
I agree that your description is correct if side-effects are barred.

- John