[Issue 10712] Compiletime foreach loop

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=10712


monarchdodra at gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |monarchdodra at gmail.com


--- Comment #1 from monarchdodra at gmail.com 2013-07-25 03:12:47 PDT ---
(In reply to comment #0)
> Sometimes it's neccessary to organize compiletime loop/unroll another loop for
> better perfomance.
> 
> I'd make that simple template:
> 
> template IndexTuple(int e, int s = 0, T...) {
>     static if(s == e)
>         alias IndexTuple = T;
>     else
>         static if(s > e)
>             alias IndexTuple = IndexTuple!(e, s - 1, T, s);
>         else
>             alias IndexTuple = IndexTuple!(e, s + 1, T, s);
> }
> 
> Use:
> foreach(idx; IndexTuple!10)
> writeln(idx); // prints 0, 1, ..., 9
> 
> foreach(idx; IndexTuple!(10, 2))
> writeln(idx); // prints 2, 3, ..., 9
> 
> foreach(idx; IndexTuple!-10)
> writeln(idx); // prints 0, -1, ..., -9

It think this already exists in phobos somewhere, though I don't remember where
though, and it probably wasn't good for public use.

In any case, I think the useage should more closely resemble what iota does.
For example:
IndexTuple!(2, 10)) vs IndexTuple!(10, 2))
or
IndexTuple!(0, -10, -1)) vs IndexTuple!(0, -10))

Arguably, iota accepts "iota(10)", but I (and others) think that is a retarded
idea, when typing "iota(0, 10)", is just and easy. I think we should avoid
making the same mistakes.

Also, it should be parameterizable on iteration type. Here is a rough sketch of
an implementation that does this.

import std.stdio, std.traits;

template IndexTuple(alias l, alias h)
{
    alias IndexTuple = IndexTupleImpl!(l, h, 1);
}
template IndexTuple(alias l, alias h, alias inc)
{
    alias IndexTuple = IndexTupleImpl!(l, h, inc);
}

template IndexTupleImpl(alias l, alias h, alias inc, T...)
{
    alias E = CommonType!(l, h, inc);
    static if (inc == 0)
        static assert(0, "increment must be non-0");
    else static if (inc > 0 && l >= h)
        alias IndexTupleImpl = T;
    else static if(inc < 0 && l <= h)
        alias IndexTupleImpl = T;
    else
        alias IndexTupleImpl = IndexTupleImpl!(cast(E)(l + inc), h, inc, T, l);
}

void main()
{
    foreach(idx; IndexTuple!(0, 0))
    write(idx, ' '); // prints 
    writeln();

    foreach(idx; IndexTuple!(0, 10))
    write(idx, ' '); // prints 0, 1, ..., 9
    writeln();

    foreach(idx; IndexTuple!(2, 10))
    write(idx, ' '); // prints 2, 3, ..., 9
    writeln();

    foreach(idx; IndexTuple!(0, -10, -1))
    write(idx, ' '); // prints 0, -1, ..., -9
    writeln();

    foreach_reverse(idx; IndexTuple!(-9, 1))
    write(idx, ' '); // prints 0, -1, ..., -9
    writeln();

    foreach(idx; IndexTuple!(0.5, 10))
    write(idx, ' '); // prints 0.5, 1.5, ..., 9.5
    writeln();

    foreach(idx; IndexTuple!(0, 1, 0.1))
    write(idx, ' '); // prints 0 0.1 ... 0.9
    writeln();

    foreach(idx; IndexTuple!('a', cast(char)('z' + 1), cast(char)1))
    write(idx, ' '); // prints a b ... z
    writeln();
}

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