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Monads in JavaScript
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var extend = function(a, b) { | |
for (var i in b) | |
a[i] = b[i]; | |
return a; | |
}; | |
var fluent = function(f) { | |
return function() { | |
var clone = extend(Object.create(null), this); | |
f.apply(clone, arguments); | |
return clone; | |
}; | |
}; | |
var toArray = function(x) { | |
return Array.prototype.slice.call(x); | |
}; |
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var Either = { | |
unit: function(x) { | |
return this.right(x); | |
}, | |
bind: function(f) { | |
if (this.isLeft) { | |
return this.left(this.l); | |
} else if (this.isRight) { | |
return f.call(this, this.r); | |
} | |
}, | |
left: fluent(function(l) { | |
this.l = l; | |
this.isLeft = true; | |
this.toString = function() { | |
return "Left: " + JSON.stringify(this.l); | |
}; | |
this.valueOf = function() { | |
return this.l; | |
}; | |
}), | |
right: fluent(function(r) { | |
this.r = r; | |
this.isRight = true; | |
this.toString = function() { | |
return "Right: " + JSON.stringify(this.r); | |
}; | |
this.valueOf = function() { | |
return this.r; | |
}; | |
}) | |
}; | |
var div = function(x, y) { | |
if (y === 0) { | |
return Either.left('division by 0'); | |
} | |
return Either.right(x/y); | |
}; | |
var result = div(4,2).bind(function(x) { | |
return div(x,2).bind(function(y) { | |
return this.unit(x + y); | |
}); | |
}); | |
console.log(result.toString()); //=> Right: 3 | |
var result = div(4,2).bind(function(x) { | |
return div(x,0).bind(function(y) { | |
return this.unit(x + y); | |
}); | |
}); | |
console.log(result.toString()); //=> Left: division by 0 |
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var List = { | |
unit: fluent(function() { | |
this.x = toArray(arguments); | |
}), | |
bind: function(f) { | |
var fx = this.x.map(f.bind(this)); | |
var a = fx[0]; | |
for (var i=1; i<fx.length; i++) | |
a.x = a.x.concat(fx[i].x); | |
return a; | |
}, | |
lift: function(f) { | |
return function(x) { | |
return List.unit(f(x)); | |
} | |
}, | |
toString: function() { | |
return 'List: '+ JSON.stringify(this.x); | |
}, | |
valueOf: function() { | |
return this.x; | |
} | |
}; | |
var add1 = function(x) { | |
return x + 1; | |
}; | |
// lift | |
var result = List.unit(1,2,3).bind(List.lift(add1)); | |
console.log(result.toString()); //=> List: [2,3,4] | |
// do | |
var result = List.unit(1,2).bind(function(x) { | |
return this.unit(3,4).bind(function(y) { | |
return this.unit(x + y); | |
}); | |
}); | |
console.log(result.toString()); //=> List: [4,5,5,6] |
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var Maybe = { | |
unit: function(x) { | |
return this.just(x); | |
}, | |
bind: function(f) { | |
if (this.isNothing) { | |
return this.nothing(); | |
} else if (this.isJust) { | |
return f.call(this, this.x); | |
} | |
}, | |
lift: function(f) { | |
var self = this; | |
return function() { | |
if (~toArray(arguments).indexOf(null)) { | |
return self.nothing(); | |
} | |
return self.just(f.apply(self, arguments)); | |
}; | |
}, | |
nothing: fluent(function() { | |
this.x = null; | |
this.isNothing = true; | |
this.toString = function() { | |
return 'Nothing'; | |
}; | |
}), | |
just: fluent(function(x) { | |
this.x = x; | |
this.isJust = true; | |
this.toString = function() { | |
return 'Just: '+ JSON.stringify(this.x); | |
}; | |
}), | |
valueOf: function() { | |
return this.x; | |
} | |
}; | |
var by2 = function(x) { | |
return x * 2; | |
}; | |
var add = function(x, y) { | |
return x + y; | |
}; | |
var result = Maybe.unit(2).bind(Maybe.lift(by2)); | |
console.log(result.toString()); //=> Just: 4 | |
var addM = Maybe.lift(add); | |
var result = addM(1,2).bind(function(x) { | |
return addM(x,3).bind(function(y) { | |
return this.unit(x + y); | |
}); | |
}); | |
console.log(result.toString()); //=> Just: 9 | |
var result = addM(1,2).bind(function(x) { | |
return addM(x,null).bind(function(y) { | |
return this.unit(x + y); | |
}); | |
}); | |
console.log(result.toString()); //=> Nothing |
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var Writer = { | |
unit: fluent(function(x, w) { | |
this.w = w == null ? [] : [w]; | |
this.x = x; | |
}), | |
bind: function(f) { | |
var w = this.w | |
, fx = f.call(this, this.x); | |
fx.w = w.concat(fx.w); | |
return fx; | |
}, | |
tell: fluent(function(w) { | |
this.w = [w]; | |
}), | |
toString: function() { | |
return 'Writer: ('+ this.x +', '+ JSON.stringify(this.w) +')'; | |
}, | |
valueOf: function() { | |
return this.x; | |
} | |
}; | |
var add = function(x, y) { | |
return Writer.unit(x + y, 'add '+ x +' plus '+ y); | |
}; | |
var result = add(1,2).bind(function(x) { | |
return add(x,2).bind(function(y) { | |
return this.tell('multiply '+ x +' by '+ y).bind(function() { | |
return this.unit(x * y); | |
}); | |
}); | |
}); | |
console.log(result.toString()); | |
//^ Writer (15, ["add 1 plus 2", "add 3 plus 2", "multiply 3 by 5"]) |
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