I'm very new to D, coming from C++ I'm missing parameter pack expansion and fold expressions.

For example, I want to write a function `f(x[0], ..., x[n])` which accepts any number of objects and calls `g(x[0].member, ..., x[n].member)`.

In C++, I would write this:

   template<class ...X>
   void f(X ... x){
      g(x.member...);
   }

Similar transformation can be also done in C++ on types too:

   template<class ...X>
   using F = G<X::Member...>;

How do I do something like this in D?

Basicaly, how do I preform tuple transformations

X = (X[0], ..., X[n]) ->  TX = (X[0].Member, ... ,X[n].Member)
x = (x[0], ..., x[n]) ->  tx = (x[0].member, ... ,x[0].member)

Here is my best shot in D, it kind of does what I want, but I have do define my own map function `myMap`.

   // Helper function
   auto myMap(alias Fun, X...)(X x) {
     import std.typecons;

     static if (X.length > 1)
       return tuple(Fun(x[0])) ~ myMap!(Fun)(x[1 .. $]);
     else
       return tuple(Fun(x[0]));
   }

   // Do the transformation inside
   void foo(X...)(X x){
      // type transformation
      alias MemberOf(T) = T.Member;
      alias XM = staticMap!(MemberOf, X);

      // value transformation, notice the `expand`!
      auto tx  = myMap!(x => x.member)(x).expand;
   }

What is the recommended what to do this in D? Surely, there has to be a standard function which does exactly(probably better) the same thing as what `myMap` does.

On Thursday, 30 May 2019 at 20:20:47 UTC, Tomas wrote:
> How do I do something like this in D?

D's built-in tuples expand automatically when they are used. Library tuples (from phobos std.typecons) have a `.expand` method you can use.

Fun fact: all structs also have a `.tupleof` thing that yields the members. Try this:

struct Foo {int a; string b; }

void test(int, string) {}

Foo foo;
test(foo.tupleof); // works!

> Basicaly, how do I preform tuple transformations

import std.meta;

then staticMap is like what your myMap thing was.

http://dpldocs.info/experimental-docs/std.meta.staticMap.html

On 30.05.19 22:20, Tomas wrote:
> Surely, there has to be a standard function which does exactly(probably better) the same thing as what `myMap` does.

Nope.

On Thursday, 30 May 2019 at 20:20:47 UTC, Tomas wrote:
> I'm very new to D, coming from C++ I'm missing parameter pack expansion and fold expressions.

I also miss the expansion operator and fold expressions.
In D, there is no such thing, and simulating it in a general way has so much friction, I just used string mixin to solve these.

I even thought about writing a DIP, but I think, it wouldn't make it.
In my static indexing DIP [1], there is a suggestion to add an expansion operator, but that would only work on sequences and similar things.

If it would be implemented, you could place your parameter pack into a Tuple-like structure with an opDispatch that returns such a Tuple again, but the contents would be the member components "projection" to the member. It's doable[2], but the lack of indexing the pack is the main blocker.

If you look closer, the DIP proposes to make "pack..." work by replacing it with pack[0], ..., pack[$ - 1] if the indexing and length are there to do it.

The DIP doesn't give you parameter or type packs, it gives you what you need to implement them; still, fold expressions are still not possible. They are rather easy to implement using D's template mechanisms and/or string mixin mechanisms. The projections to the members in [2] are an example of a fold expression.

[1] https://github.com/dlang/DIPs/pull/155
[2] https://run.dlang.io/is/tIxmQS

On Thursday, 30 May 2019 at 20:34:18 UTC, Adam D. Ruppe wrote:
> Foo foo;
> test(foo.tupleof); // works!

This is really neat! I will definitely use this in the future.

> import std.meta;
>
> then staticMap is like what your myMap thing was.
>
> http://dpldocs.info/experimental-docs/std.meta.staticMap.html

I'm already using `staticMap` to transform types but can `staticMap` be used also to transform values? I do not see how.

On Thursday, 30 May 2019 at 20:55:00 UTC, Q. Schroll wrote:
> If it would be implemented, you could place your parameter pack into a Tuple-like structure with an opDispatch that returns such a Tuple again, but the contents would be the member components "projection" to the member. It's doable[2], but the lack of indexing the pack is the main blocker.
>
>  ...
>
> [1] https://github.com/dlang/DIPs/pull/155
> [2] https://run.dlang.io/is/tIxmQS

Thanks! Sounds interesting, but right now it is a bit over my head. It will take me some time digest it.