Should the code below print anything (it currently doesn't)? I'm not sure I understand map:

import std.stdio, std.algorithm;	

void main() {
    int[] a = [1,2,3,4,5];
    auto r = a.map!( i => writeln(i) );
    while(!r.empty) r.popFront();
}

cal:

> Should the code below print anything (it currently doesn't)? I'm not sure I understand map:
>
> import std.stdio, std.algorithm;	
>
> void main() {
>     int[] a = [1,2,3,4,5];
>     auto r = a.map!( i => writeln(i) );
>     while(!r.empty) r.popFront();
> }

The code is working as requested. But there are problems at various levels.

First, to see the side effects you have also to call front:

import std.stdio, std.algorithm;
void main() {
    auto a = [1, 2, 3, 4, 5];
    auto r = a.map!(x => writeln(x));
    while(!r.empty) {
        r.front;
        r.popFront;
    }
}

That prints:
1
2
3
4
5


But this is wrong still. map() is a higher order function, it's meant to take a function and an iterable and produce a new lazy iterable that contains the ordered results of applying the given function on each item. writeln() returns nothing (void, it's a type with just one value, void itself).

So one more meaningful way to do that is:

import std.stdio, std.algorithm, std.conv;
void main() {
    auto a = [1, 2, 3, 4, 5];
    a.map!text.writeln;
}

That prints:

["1", "2", "3", "4", "5"]

But writeln is well designed, so it's able to take a range, so this is better:

import std.stdio, std.algorithm;
void main() {
    auto a = [1, 2, 3, 4, 5];
    a.writeln;
}

Bye,
bearophile

On Saturday, 6 April 2013 at 21:40:37 UTC, bearophile wrote:
> First, to see the side effects you have also to call front:

Ah thank you, I did not realize a call to front was required.

> But this is wrong still. map() is a higher order function, it's meant to take a function and an iterable and produce a new lazy iterable that contains the ordered results of applying the given function on each item. writeln() returns nothing (void, it's a type with just one value, void itself).

Yes, I understand, the example was just a reduced bit of code.

Thanks again

cal:

> Ah thank you, I did not realize a call to front was required.

Whenever you can, don't create side effects inside a map(). It will not work reliably. A higher order function like map is not meant to be used that way.

Bye,
bearophile