How to instantiate a map with multiple functions

[Ali Çehreli via Digitalmars-d-learn]

On 12/26/2015 05:26 PM, Karthikeyan wrote:

 > if I need to map on a array of tuples will that work with the tuple being
 > unpacked or do I need to get it as single element and do unpacking 
myself?

Unfortunately, there is no automatic unpacking of tuples.

The only exception that I know is when tuples are elements of a range 
(but not a proper slice, in which case the first element is the 
automatic element index).

import std.stdio;
import std.typecons;
import std.range;
import std.algorithm;

void main() {
     auto range = 5.iota.map!(i => tuple(2 * i, i * i));

     // automatic tuple expansion:
     foreach (twice, square; range) {
         writefln("twice: %s, square: %s", twice, square);
     }
}

Prints:

twice: 0, square: 0
twice: 2, square: 1
twice: 4, square: 4
twice: 6, square: 9
twice: 8, square: 16

The problem is when the same elements are inside a slice:

import std.stdio;
import std.typecons;
import std.range;
import std.algorithm;

void main() {
     auto range = [ tuple(0, 0), tuple(2, 1) ];

     foreach (twice, square; range) {
         writefln("twice: %s, square: %s", twice, square);
     }
}

Now 'twice' is the automatic index, and 'square' is the entire element 
(i.e. the tuple):

twice: 0, square: Tuple!(int, int)(0, 0)
twice: 1, square: Tuple!(int, int)(2, 1)

Ali