• Timothee Cour via Digitalmars-d (12/12) Mar 21 2016 given fun(put, a) a lambda that can call $put 0 or more times,
• Seb (3/5) Mar 21 2016 Could you try to point out whats wrong with map & filter?
• Tamas (4/5) Mar 21 2016 It's hard to do stuff like this:
• Nick Treleaven (4/9) Mar 23 2016 Seems doable:
• Tamas (3/13) Mar 23 2016 This one emits only 1 element when a==3. (BTW what is `mapFilter`
• Nick Treleaven (5/20) Mar 24 2016 It combines both functions, the lambda returns an Option type
• thedeemon (9/14) Mar 23 2016 Is it a bug or you forgot 0 here?
• crimaniak (11/16) Mar 23 2016 map: 1 element -> 1 element
• cy (26/27) Mar 25 2016 I wonder if "template emit(alias fun, T)" wouldn't be a better

Timothee Cour via Digitalmars-d <digitalmars-d puremagic.com> writes:

given fun(put, a) a lambda that can call $put 0 or more times,
some_range.emit!fun computes a range formed of all the calls to $put

eg:
assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a);}).equal([1, 9, 25, 49]));

in this case it can be done by combining map and filter but in other cases
emit is more powerful and an equivalent (with map,filter,joiner) can be
inefficient and more complex.

see https://github.com/timotheecour/dtools/blob/master/dtools/util/emit.d

could that be turned into std.algorithm.iteration.emit (after addressing
comments) ?

Any comments would be appreciated (especially regarding whether we can get
rid of the 1st template argument with type deduction)

Seb <seb wilzba.ch> writes:

On Monday, 21 March 2016 at 11:35:49 UTC, Timothee Cour wrote:
 assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a);}).equal([1, 
 9, 25, 49]))

Could you try to point out whats wrong with map & filter?

assert(9.iota.filter!"a%2".map!"a*a".equal([1, 9, 25, 49])

Tamas <user dlang.io> writes:

On Monday, 21 March 2016 at 11:48:52 UTC, Seb wrote:
 Could you try to point out whats wrong with map & filter?

It's hard to do stuff like this:

assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a); if(a%3==0) 
put(a);}).equal([1,9,3,25,6,49]));

Nick Treleaven <ntrel-pub mybtinternet.com> writes:

On Monday, 21 March 2016 at 23:09:27 UTC, Tamas wrote:
 On Monday, 21 March 2016 at 11:48:52 UTC, Seb wrote:
 Could you try to point out whats wrong with map & filter?

 It's hard to do stuff like this:

 assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a); if(a%3==0) 
 put(a);}).equal([1,9,3,25,6,49]));

Seems doable:
mapFilter!((a){if(a%2) return some(a*a); if(a%3==0) return 
some(a); return none;})

Tamas <user dlang.io> writes:

On Wednesday, 23 March 2016 at 17:29:55 UTC, Nick Treleaven wrote:
 On Monday, 21 March 2016 at 23:09:27 UTC, Tamas wrote:
 On Monday, 21 March 2016 at 11:48:52 UTC, Seb wrote:
 Could you try to point out whats wrong with map & filter?

 It's hard to do stuff like this:

 assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a); if(a%3==0) 
 put(a);}).equal([1,9,3,25,6,49]));

 Seems doable:
 mapFilter!((a){if(a%2) return some(a*a); if(a%3==0) return 
 some(a); return none;})

This one emits only 1 element when a==3. (BTW what is `mapFilter` 
and `some`?)

Nick Treleaven <ntrel-pub mybtinternet.com> writes:

On Wednesday, 23 March 2016 at 17:43:07 UTC, Tamas wrote:
 On Wednesday, 23 March 2016 at 17:29:55 UTC, Nick Treleaven 
 wrote:
 On Monday, 21 March 2016 at 23:09:27 UTC, Tamas wrote:
 On Monday, 21 March 2016 at 11:48:52 UTC, Seb wrote:
 Could you try to point out whats wrong with map & filter?

 It's hard to do stuff like this:

 assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a); if(a%3==0) 
 put(a);}).equal([1,9,3,25,6,49]));

 Seems doable:
 mapFilter!((a){if(a%2) return some(a*a); if(a%3==0) return 
 some(a); return none;})

 This one emits only 1 element when a==3.

Oops, thanks.

 (BTW what is `mapFilter` and `some`?)

It combines both functions, the lambda returns an Option type 
representing 0 or 1 element:
http://forum.dlang.org/post/ncom7m$1ebr$1 digitalmars.com

thedeemon <dlang thedeemon.com> writes:

On Monday, 21 March 2016 at 23:09:27 UTC, Tamas wrote:
 On Monday, 21 March 2016 at 11:48:52 UTC, Seb wrote:
 Could you try to point out whats wrong with map & filter?

 It's hard to do stuff like this:

 assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a); if(a%3==0) 
 put(a);}).equal([1,9,3,25,6,49]));

Is it a bug or you forgot 0 here?

How it looks with Phobos today:

     auto r = new Generator!int({
         9.iota.each!((a) { if (a % 2) yield(a*a); if (a % 3 == 0) 
yield(a); });
     });
     assert(r.equal([0,1,9,3,25,6,49]));

Probably much less efficient, of course.

crimaniak <crimaniak gmail.com> writes:

On Monday, 21 March 2016 at 11:48:52 UTC, Seb wrote:
 On Monday, 21 March 2016 at 11:35:49 UTC, Timothee Cour wrote:
 assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a);}).equal([1, 
 9, 25, 49]))


I support idea to have such feature, sometimes it really need.

 Could you try to point out whats wrong with map & filter?

 assert(9.iota.filter!"a%2".map!"a*a".equal([1, 9, 25, 49])

map: 1 element -> 1 element
filter: 1 element -> 0..1 element(s)

emit: 1 element -> 0..infinite elements

This feature is more generic then map() and filter() together. 
But proposed implementation is not good. I think this is good 
place for generator function with yield.
If map() will accept generators and process it as expected, 
different from ordinary functions, then additional emit() method 
is not needed. And filter() too :)

cy <dlang verge.info.tm> writes:

 template emit(T, alias fun)

I wonder if "template emit(alias fun, T)" wouldn't be a better 
idea. That way you could leave T open for type inference, even if 
you specify a function. D doesn't support reordering template 
arguments, so it's important to put the ones "most used" at the 
beginning.

Anyway, it looks neat. I hope you've looked at 
std.algorithm.iteration.reduce though? It does the same thing 
that emit does, and it's an ancient algorithm.

"fold" is just reduce with the arguments more convenient for D's 
syntax. But...


assert(9.iota.emit!(int,(put,a){if(a%2) put(a*a);}).equal([1, 9, 
25, 49]));
=>
int[] intderp;
assert(9.iota.fold!((result,a) { if(a%2) return result ~ [a*a]; 
else return result;})(intderp) == ([1,9,25,49]));

What would be neat is a "reduce" that could produce an infinite 
range as a result. All I can do with reduce and fold is produce 
an eagerly evaluated growing array in memory, or something like 
chain(chain(chain(chain(chain(...))))) which uses up memory just 
from creating all those iterators. Like, I don't know how to use 
fold! to do this:

   auto natural_numbers = sequence!"n"();
   auto oddsquared = natural_numbers.filter!"a%2==1".map!"a*a"; // 
fold?
   assert(take(oddsquared,4).equal([1, 9, 25, 49]));