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@puffnfresh
Last active December 12, 2015 02:28
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Creation of unions, with automatic value constructors and an automatic fold encoding.
// Boilerplate
function objectUnion(definer) {
var defined = 0, length = 0, isDefined = false, definitions, key;
definitions = definer(function() {
var names = arguments, fold;
if(isDefined) {
throw new TypeError('This objectUnion has already been defined');
}
function Ctor() {}
Ctor.prototype = wrapped.prototype;
function wrapped() {
var instance;
if(!(this instanceof wrapped)) {
instance = new Ctor();
wrapped.apply(instance, arguments);
return instance;
}
if(arguments.length != names.length) {
throw new TypeError("Expected " + names.length + " arguments, got " + arguments.length);
}
for(i = 0; i < names.length; i++) {
this[names[i]] = arguments[i];
}
}
wrapped.prototype.fold = (function(index) {
return function() {
var args = [], i;
if(arguments.length != defined) {
throw new TypeError("Expected " + defined + " arguments, got " + arguments.length);
}
for(i = 0; i < names.length; i++) {
args.push(this[names[i]]);
}
return arguments[index].apply(this, args);
};
})(defined);
defined++;
return wrapped;
});
for(key in definitions) {
length++;
}
if(length != defined) {
throw new Error("objectUnion's define function was called without result being returned");
}
isDefined = true;
return definitions;
}
// AST
var AST = objectUnion(function(define) {
return {
Let: define('name', 'left', 'right'),
StrLit: define('value'),
NumLit: define('value'),
BoolLit: define('value'),
UndefinedLit: define()
};
});
function astToString(ast) {
return ast.fold(
function(name, left, right) {
return name + ' := ' + astToString(left) + ' ' + astToString(right);
},
function(value) {
return JSON.stringify(value);
},
function(value) {
return value;
},
function(value) {
return value;
},
function() {
return 'undefined';
}
);
}
console.log(astToString(AST.Let('brian', AST.StrLit('Brian McKenna'), AST.NumLit(22))));
// List
var List = objectUnion(function(define) {
return {
Cons: define('car', 'cdr'),
Nil: define()
};
});
function listToArray(list) {
return list.fold(
function(car, cdr) {
return [car].concat(listToArray(cdr));
},
function() {
return [];
}
);
}
console.log(listToArray(List.Cons(1, List.Cons(2, List.Nil()))));
// [0, 1]
// Stream
var Stream = objectUnion(function(define) {
return {
Cons: define('car', 'cdr'),
Nil: define()
};
});
function fibonacci(a, b) {
function inner(a, b) {
var c = a + b;
return Stream.Cons(c, function() {
return inner(b, c);
});
}
return Stream.Cons(a, function() {
return Stream.Cons(b, function() {
return inner(a, b);
});
});
}
function streamToArray(stream, limit) {
if(!limit) return [];
return stream.fold(
function(car, cdr) {
return [car].concat(streamToArray(cdr(), limit - 1));
},
function() {
return [];
}
);
}
console.log(streamToArray(fibonacci(0, 1), 20));
// [ 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181 ]
@julienrf
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julienrf commented Feb 3, 2013

List is enough :) Interesting!

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