On Wednesday, 21 September 2016 at 18:34:13 UTC, Michael Coulombe wrote:
On Wednesday, 21 September 2016 at 17:14:34 UTC, Stefan Koch wrote:
That can and is being fixed.
Templates can only be fixed partially and I am not even sure of that.

I am not suggesting to remove templates.
I just want to raise awareness that they have a rather high cost.
CTFE performance is fixable. Template performance might not.

Thinking about templates and CTFE, is part of the performance issue due to immutability? Compare std.meta.Reverse vs std.algorithm.reverse:

template Reverse(TList...)
    static if (TList.length <= 1)
        alias Reverse = TList;
alias Reverse = AliasSeq!(Reverse!(TList[$/2..$]), Reverse!(TList[0..$/2]));

void reverse(Range)(Range r) if (isRandomAccessRange!Range && hasLength!Range)
    immutable last = r.length-1;
    immutable steps = r.length/2;
    for (size_t i = 0; i < steps; i++)
        r.swapAt(i, last-i);

If we are able to get a bytecode CTFE interpretor, it seems like the same technology could be used to "run" templates like Reverse which only serves to compute a "value" rather than declare a new function or type, and perhaps be able to write a function like reverse which uses mutation and apply it to an AliasSeq in a typesafe way.

This would need to have special compiler support.
We would have to introduce another langauge feature that allows manipulating type-lists without going trough templates. __traits give D insanely powerful introspection abilities by porviding the ability to do a query on types. However the languages only way to work with types right now goes trough templates. We would need something like type-functions that do not relay on template mechanisms.

I am not the right person to do language changes, but I am willing to implement a proposal should I hear of one that could solve this.

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