A while back, somebody raised the topic of implementing a lazy range for traversing/flattening a tree structure on #d.

The obvious (in terms of Phobos primitives) solution would be something along the lines of this:
---
struct Node(T) {
   T val;
   Node!T[] children;
}

auto flatten(T)(Node!T root) {
   import std.algorithm, std.range;
   return only(root.val).chain(map!flatten(root.children).joiner);
}

void main() {
   alias N = Node!int;
   auto a = N(1, [
      N(2, [
         N(3, [
            N(4)
         ])
      ]),
      N(5)
   ]);

   import std.stdio;
   writeln(a.flatten); // [1, 2, 3, 4, 5]
}
---

But of course, that piece of code does not compile with current DMD, as the return type of flatten() can't refer to the function itself (during name mangling).

Now, one way around this would be to add an array() call at the end of the return statement, which hides the type of map!flatten, but at the cost of many unnecessary memory allocations. A second option would be to use inputRangeObject to convert the return value to ForwardRange!T (well, that is if it actually worked, due to an implementation bug it leads to a runtime crash).

But can you think of a more simple/elegant workaround?

(Note aside: Obviously, the fact that the code relies on recursion might be an issue, and a simple opApply-based solution with a worklist stack would likely perform better. Still, I think it's a simple, yet interesting problem.)

David

On 10/31/2013 12:09 PM, David Nadlinger wrote:
> A while back, somebody raised the topic of implementing a lazy range for
> traversing/flattening a tree structure on #d.
>
> The obvious (in terms of Phobos primitives) solution would be something
> along the lines of this:
> ---
> struct Node(T) {
>     T val;
>     Node!T[] children;
> }
>
> auto flatten(T)(Node!T root) {
>     import std.algorithm, std.range;
>     return only(root.val).chain(map!flatten(root.children).joiner);
> }
>
> void main() {
>     alias N = Node!int;
>     auto a = N(1, [
>        N(2, [
>           N(3, [
>              N(4)
>           ])
>        ]),
>        N(5)
>     ]);
>
>     import std.stdio;
>     writeln(a.flatten); // [1, 2, 3, 4, 5]
> }
> ---
>
> But of course, that piece of code does not compile with current DMD, as
> the return type of flatten() can't refer to the function itself (during
> name mangling).
>
> Now, one way around this would be to add an array() call at the end of
> the return statement, which hides the type of map!flatten, but at the
> cost of many unnecessary memory allocations. A second option would be to
> use inputRangeObject to convert the return value to ForwardRange!T
> (well, that is if it actually worked, due to an implementation bug it
> leads to a runtime crash).
>
> But can you think of a more simple/elegant workaround?
>
> (Note aside: Obviously, the fact that the code relies on recursion might
> be an issue, and a simple opApply-based solution with a worklist stack
> would likely perform better. Still, I think it's a simple, yet
> interesting problem.)
>
> David

Y Combinator? (No, I have not solved it yet. :) )

  http://rosettacode.org/wiki/Y_combinator#D

Ali

On 10/31/2013 02:19 PM, Ali Çehreli wrote:

> Y Combinator? (No, I have not solved it yet. :) )
>
>    http://rosettacode.org/wiki/Y_combinator#D

Ok, I was actually trying to find the following one:


https://github.com/gecko0307/atrium/blob/master/dlib/functional/combinators.d

Ali

> But can you think of a more simple/elegant workaround?
>
>
Is a standard lazy struct range authorized?

On Friday, 1 November 2013 at 09:34:20 UTC, Philippe Sigaud wrote:
>> But can you think of a more simple/elegant workaround?
>>
>>
> Is a standard lazy struct range authorized?

Sure. This wasn't intended as an actual challenge, as I don't have a "right" answer myself (or a prize, for that matter). ;)

I just thought I'd be interested to see what the best solution we can find in terms of conciseness is.

David

On 11/1/13, David Nadlinger <code@klickverbot.at> wrote:
> I just thought I'd be interested to see what the best solution we can find in terms of conciseness is.

Note that I've already asked this in D.learn: http://forum.dlang.org/thread/mailman.43.1383090512.9546.digitalmars-d-learn@puremagic.com

On 10/31/2013 10:19 PM, Ali Çehreli wrote:
> ...
>
> Y Combinator? (No, I have not solved it yet. :) )
>
>    http://rosettacode.org/wiki/Y_combinator#D
>
> Ali
>

Oh my god, my eyes!

auto y(S,T...)(S delegate(T) delegate(S delegate(T)) f){
    struct F{ S delegate(T) delegate(F) f; alias f this; }
    return (x=>x(x))(F(x=>f((T v)=>x(x)(v))));
}