Lazy evaluation in JavaScript
function delay(expressionAsFunction) { | |
var result; | |
var isEvaluated = false; | |
return function () { | |
if (!isEvaluated) | |
result = expressionAsFunction(); | |
return result; | |
}; | |
} | |
function force(promise) { | |
return promise(); | |
} | |
function cons(car, cdr) { | |
return [car, cdr]; | |
} | |
function next(n) { | |
return cons(n, delay(function () {return next(n + 1);})); | |
} | |
function head(stream) { | |
return stream[0]; | |
} | |
function tail(stream) { | |
return force(stream[1]); | |
} | |
var stream = next(0); | |
console.log(stream); | |
console.log(head(tail(tail(stream)))); //==> 2 |
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Except that you never set isEvaluated, so expressionAsFunction is called every time the promise is forced, not just the first time. |
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Wow, LISP is better readable :-)
But good example ;-)