On 1/17/19 2:31 PM, H. S. Teoh wrote:
On Thu, Jan 17, 2019 at 06:03:07PM +0000, Paul Backus via 
Digitalmars-d-announce wrote:

Haha, seems D did better than C++ in this respect, but not quite at the
level of Haskell.

The C++ example of a template that takes templates and arguments and
declares another template is a perfect example of why C++ template
syntax is utterly horrible for doing these sorts of things.

Coming back to the D example at the end, I totally agree with the
sentiment that D templates, in spite of their significant improvements
over C++ syntax, ultimately still follow the same recursive model. Yes,
you can use CTFE to achieve the same thing at runtime, but it's not the
same thing, and CTFE cannot manipulate template argument lists (aka
AliasSeq aka whatever it is you call them).  This lack of symmetry
percolates down the entire template system, leading to the necessity of
the hack that Bartosz refers to.

Had template argument lists / AliasSeq been symmetric w.r.t. runtime
list manipulation, we would've been able to write a foreach loop that
manipulates the AliasSeq in the most readable way without needing to
resort to hacks or recursive templates.

well, there was no static foreach for that article (which I admit I didn't read, but I know what you mean).

But it's DEFINITELY not as easy as it could be:

import std.conv;

alias AliasSeq(P...) = P;

template staticMap(alias Transform, Params...)
    alias seq0 = Transform!(Params[0]);
    static foreach(i; 1 .. Params.length)
mixin("alias seq" ~ i.to!string ~ " = AliasSeq!(seq" ~ (i-1).to!string ~ ", Transform!(Params[" ~ i.to!string ~ "]));");
    mixin("alias staticMap = seq" ~ (Params.length-1).to!string ~ ";");

alias Constify(T) = const(T);
void main()
    alias someTypes = AliasSeq!(int, char, bool);
pragma(msg, staticMap!(Constify, someTypes)); // (const(int), const(char), const(bool))

Note, that this would be a LOT easier with string interpolation...

mixin("alias seq${i} = AliasSeq!(seq${i-1}, Transform!(Params[${i}]));".text);


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