www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - Tuple unpacking at the called function

The slides of the "Information Rich Programming with F# 3.0" talk 
of Lang.NEXT 2012:

In the slide 6 titled "Simplicity Functional data" they compare 
some F# functions to similar C# ones.

It shows a bit of syntax that makes tuple usage more handy, more 
readable and shorter. I have discussed it a bit here:


The F# code shown on that slide:

let rotations (x, y, z) =
     [ (x, y, z);
       (z, x, y);
       (y, z, x) ]



ReadOnlyCollection<Tuple<T,T,T>> Rotations<T>
    (Tuple<T,T,T> t)
   new ReadOnlyCollection<int>
    (new Tuple<T,T,T>[]
      { new Tuple<T,T,T>(t.Item1,t.Item2,t.Item3);
        new Tuple<T,T,T>(t.Item3,t.Item1,t.Item2);
        new Tuple<T,T,T>(t.Item2,t.Item3,t.Item1); });


Similar D code:

import std.typecons;

auto rotations(T)(in Tuple!(T,T,T) t) pure nothrow {
     return [tuple(t[0], t[1], t[2]),
             tuple(t[2], t[0], t[1]),
             tuple(t[1], t[2], t[0])];
void main() {
     auto r = rotations(tuple(1, 2, 3));


Two Python2.6 versions:

def rotations((x, y, z)):
     return [(x, y, z), (z, x, y), (y, z, x)]

rotations = lambda (x, y, z): [(x, y, z),
                                (z, x, y),
                                (y, z, x)]

print rotations((1, 2, 3))


In Haskell there is even an optional syntax to give names to both 
the parts and the whole tuple:

twohead s (x : xs) = x : s

main = print $ twohead [1,2,3]

That outputs:


While there is a good enough D syntax to support tuple unpacking 
at the return point of a function, I don't know what syntax to 
use in D for the naming of tuple parts in the function signature 
(the same syntax is later usable for structs in switch-case).

Some possibilities:

This seems to works with all structs:
auto rotations(T)(Tuple!(T, T, T)(x, y, z)) {

auto rotations(T)(Tuple!(T, T, T) t(x, y, z)) {

Or the same syntax as the normal tuple unpacking syntax:
auto rotations(T)(auto (x, y, z)) {
auto rotations(T)((int x, int y, int z)) {

Or even:
auto rotations(T)(Tuple!(T x, T y, T z)) {

Please take a look at the issue 6544 for more info.

Apr 12 2012