Multifunctions

[Lord Delvin <timm.felden@...>]

As I allways wanted this function, and found out, that it's really
simple to do and I got so many good ideas from this list, I want to
share my code for this:
You need a Class system or you have to slightly rewrite it, depends on
what taste of lua you like most:)
The purpose of this code is to allow different behavior on different
signatures for functions, as function overloading does it for most
static typed languages.
I needed this to do better bindings to my application, as I often dont
know if I will get a vector or 2 or 3 numbers.
Feel free to use or comment it.

Greatings
Timm Felden
---------------------

--multifunction uses the type() function to determine the type of an
argument
--the type __default is a reserved word, so dont use it
multifunction = Class("multifunction");

--construction of a new multifunction
--#1     the multifunction
--#2     the default function, which gets called if type does not match
any function
--#2n-1  signature of the function
--#2n    the function
multifunction.new = function(this, default, ...)
	this.argType = {};
	--set the default function
	this.argType.__default = default;

	--register functions
	local name = "";
	for _,i in pairs({...}) do
		if _ % 2 == 0 then
			this.argType[name] = i;
		else
			name = i;
		end
	end
end

--call searches for matching function
multifunction.__call = function(this, ...)
	--create a type string ...
	local signature = "";
	for _,i in pairs({...}) do
		if _ == 1 then
			signature = type(i);
		else
			signature = signature .. " " .. type(i);
		end
	end

	if this.argType[signature] then
		this.argType[signature](...);
	else
		--we could not find the signature, so call default
		print("default on: " .. signature)
		this.argType.__default(...);
	end
end

--adds a signature and function
--usage: f + {"<type>", f1}
multifunction.__add = function(this, t)
	this.argType[t[1]] = t[2]
	return this;
end

--used to unregister a function by signature
multifunction.__sub = function(this, sig)
	this.argType[sig] = nil;
	return this;
end


print("multi function loaded")

--usage--
do

local vec = function(x, y)
print("v={"..tostring(x)..","..tostring(y).."}") end
local s = function(s) print("string is: " .. s) end
local d = function(any) print(tostring(any)) end
local n = function() print"void" end

local f = multifunction(d, "number number", vec, "string", s)

f(3,4)
f("hallo welt!")
f()
--you can not use nil here as first type, as you can not pass a single nil
f = f + {"", n}
f(nil)
f = f - ""

end