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@iahu
Created Sep 26, 2014
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a Maybe Monad
function Maybe() {
Object.freeze(this);
}
function Just (x) {
this.toString = function () { return "Just " + x.toString(); };
this.just = x;
Object.freeze(this);
}
Just.prototype = new Maybe();
Just.prototype.constructor = Just;
function Nothing () {
this.toString = function () { return "Nothing"; };
Object.freeze(this);
}
Nothing.prototype = new Maybe();
Nothing.prototype.constructor = Nothing;
Maybe.unit = function (x) {
return new Just(x);
};
Maybe.bind = function (f) {
return function (a) {
if ( a instanceof Just ) {
return f( a.just );
} else if (a instanceof Nothing) {
return new Nothing();
} else {
throw Error("argument is not a Maybe instance.");
}
};
};
Maybe.lift = function (f) {
return function (x) {
try {
return new Just( f(x) );
} catch(e) {
return new Nothing();
}
};
};
Maybe.do = function (m) {
var fns = Array.prototype.slice.call(arguments, 1);
fns.forEach(function(fn) {
m = Maybe.bind( fn )(m);
});
return m;
};
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