[fpc-devel] for-in-index loop

[Daniël Mantione]

Op Sat, 26 Jan 2013, schreef Alexander Klenin:

>>> var
>>>   a: array [1..5] of Integer = (1, 2, 9, 4, 5);
>>>
>>> In my proposal, he should write:
>>> var
>>>   v, i: Integer;
>>> begin
>>>   for v in a index i do
>>>     Writeln(i, ' ', v);
>>> end.
>>
>> In this case I just write
>>
>> for i:=low(a) to high(a) do
>>   writeln(i,' ',a[i]);
> Consider these arguments:
> 1) Even for simple arrays, depending on array element type,
>  and optimizer implementation, for-in can be more efficient since it
> can avoid multiplication for element access.

True, but it shouldn't. Loop induction is such a well documented 
optimization that implementation of that optimization (could be a nice job 
for a student) is by far preferable over mutilating the language.

> 2) If "a" is a function of another complex expression returning an
> array, then "for-in" may be much more efficient and compact

Again true, but again easy to avoid: temp variable, with statement, common 
subexpression elimination. Again these are all preferable over mutilating 
the language.

> 3) If "a" is an associative array (so i is a String), then the
> "traditional" form of loop is not even possible.

Disagree. You need to define a mapping between an ordinal type and the 
index type. This is always possible (otherwise for-in would be impossible 
to implement).

> 4) If there is no uniform method for iterating over containers, then
> each container will define its own method,
>  with the end result that the language will be *more* complex for the
> user, who will have to remember all these methods.

This I consider a valid argument, and it affects the fundament of what a 
for loop is. A for loop iterates over the keys only, while the values are 
retrieved by [] syntax. This is perfectly possible even for non-numeric 
keys as we have properties that allow you to do a['elephant'].

The for-in syntax violates this fundamental property by returning values, 
which means there is a lack of a key. The fundamentalistic view then says 
we should not have for-in in Pascal, because it changes the fundamental 
property of a for-loop.

For pragmantic reasons the for-in syntax got supported. However, when 
using it you discover the lack of keys. So you want to repair something 
that's already broken by design.

The question becomes, which is worse? Further mutilation? Or stick to the 
basics?

Daniël