• Tetiana (29/29) Mar 14 2017 I'd like to evaluate values lazily and then process them one by
• Era Scarecrow (7/8) Mar 14 2017 Just looking at the Documentation, Generator is an InputRange
• H. S. Teoh via Digitalmars-d-learn (13/23) Mar 14 2017 You probably need to cache the generated elements so that they can be

Tetiana <tetiana.khot gmail.com> writes:

I'd like to evaluate values lazily and then process them one by 
one, using e.g. `map`. One of the steps requires processing the 
values in chunks. I was thinking of using a Generator for lazy 
evaluation, but I can't seem to make it work with the `chunks`.

How can I make `std.range.chunks` work with a generator?


import std.stdio;
import std.concurrency: Generator, yield;
import std.range:chunks;
import std.algorithm:map;


void main()
{

     auto gen = new Generator!int(
     {
         foreach (i; 1 .. 10)
             yield(i);
     });


     foreach (n; chunks(gen, 2))
     {
         writeln(n);
     }
}

Build fails with the following error:

source/app.d(18,23): Error: template std.range.chunks cannot 
deduce function from argument types !()(Generator!int, int), 
candidates are:
/usr/include/dmd/phobos/std/range/package.d(6940,15):        
std.range.chunks(Source)(Source source, size_t chunkSize) if 
(isForwardRange!Source)
dmd failed with exit code 1.

Era Scarecrow <rtcvb32 yahoo.com> writes:

On Tuesday, 14 March 2017 at 10:00:24 UTC, Tetiana wrote:
 Build fails with the following error:

  Just looking at the Documentation, Generator is an InputRange 
and chunks requires a ForwardRange (able to use save 
functionality).

  So the API doesn't match up more or less.

https://dlang.org/phobos/std_concurrency.html#.Generator
https://dlang.org/phobos/std_range.html#chunks

"H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:

On Tue, Mar 14, 2017 at 12:30:13PM +0000, Era Scarecrow via Digitalmars-d-learn
wrote:
 On Tuesday, 14 March 2017 at 10:00:24 UTC, Tetiana wrote:
 Build fails with the following error:

 
 Just looking at the Documentation, Generator is an InputRange and
 chunks requires a ForwardRange (able to use save functionality).
 
  So the API doesn't match up more or less.
 
 https://dlang.org/phobos/std_concurrency.html#.Generator
 https://dlang.org/phobos/std_range.html#chunks

You probably need to cache the generated elements so that they can be
reused.

Or you could try using std.algorithm.iteration.chunkBy instead, which
*can* work with mere input ranges (that aren't necessarily forward
ranges).  Note, though, that you would only be able to evaluate each
element once, and calling popFront on the outer range will discard all
elements in the current chunk.  This is generally OK for single-pass
algorithms, but if you ever need to backtrack or process chunk elements
out-of-order, it won't work and you'll need to cache.


T

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