Re: [stack] function/object ambiguity + quotation alternative

On Feb 26, 2009, at 2:57 PM, Christopher Diggins wrote:

>>> the evaluator would not abide naked functions on the stack; it  
>>> would apply repeatedly until that was not the case.
>
> Then you have a first order language. I don't really see what that
> buys you. Why not allow functions on the stack?

I think you're misunderstanding what Stevan was saying. Stevan is not  
suggesting anything that would eliminate quotations or make a language  
first order.

Joy and XY have "open" quotations. You can modify them as if they were  
lists. This causes two problems.

The first is that the language becomes non-extensional. For example,  
say you know that 'a b == c d'. In Joy and XY, it does not follow that  
'[a b] == [c d]' as it does in Cat.

The second problem is that, because you can take "things" out of a  
quotation, you end up with weird problems. Factor has this problem too  
with its array literals; or at least I'm pretty sure it does.

Take the program '5 [dup] first' for example. The result of this is '5  
dup'. However, the 'dup' here is *not* a function the same way 'first'  
was; it's a function object. Accordingly, the result of evaluation '5  
[dup]' is a stack with two elements; the first is the value 5 and the  
second is the value 'dup'.

What Stevan was suggesting was to evaluate "unquoted function objects"  
like 'dup' in the example above. In this system '5 [dup] first' would  
reduce to '5 dup', but then further reduce to '5 5' without the need  
to explicitly re-quote 'dup' and call 'i'.

> It seems to me that higher-order languages are only a positive thing.

Certain forms of reasoning are a lot easier without higher order  
functions. It's a tradeoff.

- John