[fpc-devel] for-in-index loop

[Alexander Klenin]

On Sat, Jan 26, 2013 at 5:12 AM, Daniël Mantione
<daniel.mantione at freepascal.org> wrote:
>> 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.
I argee. Still, for the general container such an optimization is
hard-to-impossible to implement.

>> 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.
"with" statement of of course not help (try it).
CSE might help, but is tricky across function boundaries -- function
must be first proven to be pure,
which is non-trivial in Pascal.
temp variable is, of course, always a solution -- but so is using while loop
or even labels and goto. I am tying to make Pascal higher-level language,
as opposed to, e.g. C, which is determined to stay at low level.

>> 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).
Please try to define a mapping between String and Integer :)

>> 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'].
First, for-in loop is not a for-loop. It is unfortunate that due to the need to
avoid new reserved words, "for" war chosen instead of more logical
"foreach" or even "for each".
But nevertheless, they should not be confused.
Similarly, I have chosen "index" instead of more logical "key",
which tricked some developers into thinking that I am talking about
indeger indexes only.
Second, accessing the associative array element via key has non-trivial cost,
which is avoided by using foreach-type loops.

> 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.
"fundamentalistic" is a right word here.

> 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.
I do not see why it is broken -- only because it is contrary to the
fundamentalist view?

--
Alexander S. Klenin