Question

Sometimes you have a function that will work for flat arguments. For example:

send(player,message)

But what if, instead, you have collections of players / messages?

message = ['Welcome!','Check our site for events.']
players = [Player1,Player2,Player3]

Writting for-loops will reduce readability and won’t work if you don’t know statically if your argument is a collection or object. Rewritting the function is sometimes not viable or too laborous and promotes duplicate code. What is a simplier solution?

Answer 1

You can write a decorator that will transform your function into a function that will take the cartesian product of it’s own arguments and call the original function on it.

function send(player,message) {
    console.log('To ',player,': ',message);
}
cartesian(send)(['Player1','Player2','Player3'],['Welcome!','Check our site.']);
//Output:
//To Player1 : Welcome!
//To Player1 : Check our site.
//To Player2 : Welcome!
//To Player2 : Check our site.
//To Player3 : Welcome!
//To Player3 : Check our site.

This implements the decorator (“cartesian“) on Javascript:

function cartesian_product(arr){
    //cartesian_product( [[1,2],[3,4]] ) = [[1,3],[1,3],[2,3],[2,4]]

    function partial_product(arr,i){
        //partial_product([[1,2],3],0) = [[1,3],[2,3]]
        var result = []
        for (j=0; j<arr[i].length; ++j){
            arr_changed = arr.slice();
            arr_changed.splice(i,1,arr[i][j]);
            result.push(arr_changed);
        };
        return result;
    };

    var result = [arr.slice()];
    for (var x=0; x<arr.length; ++x){
        for (var y=0; y<result.length; ++y){
            if (result[y][x] instanceof Array) {
                result.splice.apply(result,[y,1].concat(partial_product(result[y],x)));
            }
        }
    }
    return result;
};

function cartesian(func){
    //cartesian(func)([1,2],[3,4]) = [func([1,3]),func([1,4]),func([2,3]),func([2,4])]
    _this = this;
    return function(){
        var args_list = cartesian_product(Array.prototype.slice.call(arguments));
        var return_values = []
        for (var i=0; i<args_list.length; ++i){
            return_values.push(func.apply(_this,args_list[i]))
        }
        return return_values;
    }
}