• cal (8/8) Apr 06 2013 Should the code below print anything (it currently doesn't)? I'm
• bearophile (41/49) Apr 06 2013 The code is working as requested. But there are problems at
• cal (4/10) Apr 06 2013 Yes, I understand, the example was just a reduced bit of code.
• bearophile (6/7) Apr 06 2013 Whenever you can, don't create side effects inside a map(). It

"cal" <callumenator gmail.com> writes:

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();
}

"bearophile" <bearophileHUGS lycos.com> writes:

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

"cal" <callumenator gmail.com> writes:

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

"bearophile" <bearophileHUGS lycos.com> writes:

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