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;
else
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.
