monad.js

"use strict";

// count : Number
var count = 0;

function p(obj) {
    console.log(obj);
}

// sumcountup5 : Number, Number -> Countup(Number) 副作用なし
function sum_countup5(x, y) {
    return {value: x + y, countup: 5}
}

// length_countup: String -> Countup(Number) 副作用なし
function length_countup(s) {
    var len = s.length;
    return {value: len, countup: len};
}

// countup: Numer -> Countup(Void) 副作用なし
function countup(n) {
    return {value: undefined, countup: n};
}

// CountupMain: Countup(T) -> T 副作用あり
function CountupMain(countupRequest) {
    count += countupRequest.countup;
    return countupRequest.value;
}

p(count);
p(sum_countup5(1, 3));
p(count);

p(length_countup("Hello,World."));
p(count);

/*
* このような型を持つ関数が欲しい
* Countup(Void) countup_ex(Countup(Number))
* Countup(Number) sum_countup5_ex( Countup(Number), Countup(Number) )
*/

// countup_ex: Countup(Number) -> Countup(Void)
function countup_ex(countup_n) {
    return {value: undefined, countup: countup_n.countup + countup_n.value};
}

// sum_countup5_ex: Countup(Number), Countup(Number) -> Countup(Number)
function sum_countup5_ex(countup_x, countup_y) {
    return {value: countup_x.value + countup_y.value,
	    countup: countup_x.countup + countup_y.countup + 5};
}

// 整数値からCountup(Number) 型のデータを作る関数を作っておく

// noeffect: Number -> Countup(Number)
function noeffect(n) {
    return {value:n, countup:0};
}

var hoge = countup_ex( sum_countup5_ex( noeffect(1), length_countup("Hello, world!") ))
p(hoge);

// ext: ( T -> Countup(S) ) -> ( Countup(T) -> Countup(S) )
function ext(fun) {
    return function(/*...*/) {
	var valueArgs = [];
	var countupTotal = 0;
	for ( var i =0; i < arguments.length; i++ ) {
	    var arg = arguments[i];
	    valueArgs[i] = arg.value;
	    countupTotal += arg.countup;
	}
	var result = fun.apply(null, valueArgs);
	result.countup = result.countup + countupTotal;
	return result;
    };
}