Generic swap

Generic swap
You are encouraged to solve this task according to the task description, using any language you may know.

Many statically typed languages provide a generic programming capability. In C++ this capability is called templates. In Ada it is called generics. Such generic capabilities are simply the natural approach to programming for dynamically typed languages.

This task asks you to create a generic swap method that can be used for a wide variety of data types. If your solution language is statically typed please describe the way your language provides genericity. (This is actually a simple example of Parametric Polymorphism)

Ada

The generic parameters for an Ada generic procedure are defined in a procedure specification, while the algorithm is defined in a procedure body. The first code snippet is the procedure instantiation. The second code snippet is the procedure body.

generic
   type Swap_Type is private; -- Generic parameter
procedure Generic_Swap(Left : in out Swap_Type; Right : in out Swap_Type);

procedure Generic_Swap(Left : in out Swap_Type; Right : in out Swap_Type) is
   Temp : Swap_Type := Left;
begin
   Left := Right;
   Right := Temp;
end Generic_Swap;

C

This has a restriction that a and b must be the same size. I think you could do better with preprocessor voodoo, but I am not enlightened enough to use it. I'm being cute here, so please don't hate me too much, and if a C guru could check it, I'd appreciate it (I tried it and it worked for me). It won't work for something like a linked list if you want all references to middle nodes to be translated, but it'll swap the heads just fine.

 void swap(void *a, void *b, size_t size)
 {
   char *ca, *cb;
   int i;
   ca = (char *)a;
   cb = (char *)b;
   for(i=0;i<size;*(ca+i)^=*(cb+i),*(cb+i)^=*(ca+i),*(ca+i)^=*(cb+i),++i);
 }

You could also do it with a third buffer but then it wouldn't work for extremely large void pointers. It'd be faster to use a larger pointer than char *, but I wanted to keep it simple.

C++

Generic programming in C++ is provided through templates. Templates in C++ are quite powerful: They form a Turing-complete compile-time sub-language. However, that power isn't needed for swap. Note that the C++ standard library already provides a swap function which contains optimized implementations for standard library types; thus it's advisable to use that instead of a self-written variant like the one below.

While the standard allows to separate declaration and definition of templates into different files using the export keyword, most compilers (including the most used ones) don't implement that. Therefore in practice, templates declared in header files also have to be defined there.

The implementation of the swap function template is straightforward:

template<typename T> void swap(T& left, T& right)
{
  T tmp(left);
  left = right;
  right = tmp;
}

D

The solution for D is quite similar to that for C++:

 void swap(T)(ref T left, ref T right) {
   auto temp = left;
   left = right;
   right = temp;
 }

Common Lisp

(rotatef a b)

(psetq a b b a)

E

(slots)

def swap(&left, &right) {
  def t := left
  left := right
  right := t
}

(functional)

def swap([left, right]) {
  return [right, left]
}

Haskell

The type signature, the first line, is optional; it may be inferred.

swap :: (a, b) -> (b, a)
swap (x, y) = (y, x)

Java

public class Swapper<T>{

//more code probably

  public void swap(T first, T second){
     T temp = first;
     first = second;
     second = temp;
  }
}

MAXScript

swap a b

OCaml

let swap (x, y) = (y, x)

Perl

sub swap {
  my $t = $_[0];
  $_[0] = $_[1];
  $_[1] = $t;
}

Python

Python has support for swapping built in:

a, b = b, a

But the task calls for a "generic swap method" to be written, so here it is:

def swap(a, b):
    return b, a

Seed7

A generic template to generate swap functions is defined with:

const proc: generate_swap (in type: aType) is func
  begin

    const proc: swap (inout aType: left, inout aType: right) is func
      local
        var aType: temp is aType.value;
      begin
        temp := left;
        left := right;
        right := temp;
      end func;

  end func;

An instance of a swap function can be generated with:

generate_swap(integer);
generate_swap(string);

A swap function can be called with:

swap(a, b);