typeclass2.js

var traits = {};
var insts = {};

function trait(name, methods) {
    traits[name] = methods;
    insts[name] = {};
}

function struct(name, params, impls, body) {
    function findTraitName(x) {
        for (var traitName in traits) {
            if (x in traits[traitName]) return traitName;
        }
    }

    for (var x in body) {
        // インターフェイスを取得
        var inst = insts[findTraitName(x)];
        if (!(name in inst)) inst[name] = {};
        inst[name][x] = body[x];
    }

    return function() {
        var i = 0;
        for (var x in params) {
            this[x] = arguments[i];
            i++;
        }
        this.typeName = name;
    };
}

trait("Eq1", {
    eq1: ["fun", ["self", "self"], "bool"]
});
trait("Eq2", {
    eq2: ["fun", ["self", "self"], "bool"]
});

var N = struct("N", {
    "x": "int"
}, ["Eq1", "Eq2"], {
    "eq1": function(s, a) {
        return s.x == a.x;
    },
    "eq2": function(s, a) {
        return s.x == a.x;
    }
});

var i = new N(20);
var j = new N(20);
console.log(i.x);
console.log(insts.Eq1[i.typeName].eq1(i, j));
console.log(insts.Eq2[i.typeName].eq2(i, j));