merging map/filter/reduce/... in D

[Ivan Kazmenko via Digitalmars-d-learn]

On Friday, 29 January 2016 at 07:17:04 UTC, glathoud wrote:
> I have the impression that function implementations are not 
> merged:
>
>     return fun0(fun1(a));
>
> For example, fun1(a) outputs a temporary array, which is then 
> used as input for fun0. Merging the implementations of fun0 and 
> fun1 would eliminate the need for a temporary array.

If fun1(a) indeed eagerly returns a temporary array, you are 
right.  Still, if the author of fun1 cares to be generic enough, 
it usually returns a lazy range: a struct with front, empty and 
popFront.  Whenever fun0 requests the next element of the range, 
fun1 calculates it and gives it away (returns front and calls 
popFront).

Note that all this - which function calls which other function - 
is known at compile time.  To merge the actual code of fun0 and 
popFront of fun1's return value is then the job for the optimizer 
pass, it's basically inlining.

Note that a temporary array can theoretically also be optimized 
out by an advanced optimizer.

Ivan Kazmenko.