Pft, dude whatever. This is HN. We don't want lame-o languages like Java, or boring monads like Maybe. We want crazy languages like JavaScript, and the mother of all monads, the continuation monad. Here's an equivalent to what's posted in this article, but more powerful, and in JavaScript:
// Haskell typeclass equivalent is >>=
// which has a signature of:
// m a -> (a -> m b) -> m b
// which in our case, is the more specific:
// Cont a b -> (b -> Cont a c) -> Cont a c
function bind(x, y) {
return function(k, end) {
return x(function(result) { return y(result)(k, end); }, end);
}
}
// 'yes' can also serve as identity for this monad. Haskell monad typeclass
// equivalent is 'return', not to be confused with JavaScript return. if
// this were an applicative functor, we would call it 'pure' instead, even
// though it does the same thing.
// b -> Cont a b
function yes(x) {
return function(k, end) { return k(x); };
}
// bail out.
// a -> Cont a b
function no(x) {
return function(k, end) { return end(x); };
}
// look up a user ID from a name string, in the context of a continuation
// monad. our type signature for this in Haskell would be:
// String -> Cont String UserId
// or something.
// 'UserId' would be a newtype wrapper around Integer, to prevent
// accidentally conflating it with account balance or another numeric type.
function userid_from_name(person_name) {
switch (person_name) {
case "Irek": return yes(1); // we have three user names in our system
case "John": return yes(2);
case "Alex": return yes(3);
case "Nick": return yes(1); // Nick is on the same acct as Irek
default: return no("No account associated with name " + person_name);
}
}
// UserId -> Cont String Balance
// 'Balance' would also be a wrapper around Integer type
function balance_from_userid(userid) {
switch (userid) {
case 1: return yes(1000000); // some amounts for a couple of accounts
case 2: return yes(75000);
default: return no("No balance associated with account #" + userid);
}
}
// Balance -> Cont String Balance
// we could do something fancier here if we liked, like pass the difference
// between the minimum required balance for a loan and the actual balance.
function balance_qualifies_for_loan(balance) {
if (balance > 200000) return yes(balance);
else return no("Insufficient funds for loan, current balance is " + balance);
}
// tada. put it all together.
// String -> String
function name_qualifies_for_loan(person_name) {
return bind(bind(userid_from_name(person_name), balance_from_userid),
balance_qualifies_for_loan)(
function(x) { return "This person qualifies for a loan. Their \
account has a balance of: " + x; },
function(x) { return "Do not issue loan, reason given: " + x; }
);
}
Test some output:
> name_qualifies_for_loan("Irek");
"This person qualifies for a loan. Their account has a balance of:
1000000"
> name_qualifies_for_loan("John");
"Do not issue loan, reason given: Insufficient funds for loan, current
balance is 75000"
> name_qualifies_for_loan("Alex");
"Do not issue loan, reason given: No balance associated with account #3"
> name_qualifies_for_loan("Nick");
"This person qualifies for a loan. Their account has a balance of:
1000000"
> name_qualifies_for_loan("Foo");
"Do not issue loan, reason given: No account associated with name Foo"
This example actually looks a bit contrived... If your language supports exceptions, but doesn't support nice syntax for monads (i.e. you still have to write all the levels with bind and parentheses), why not simply throw() on error? I guess you would normally write something to support function like bindl(person_name, [userid_from_name, balance_from_userid, balance_qualifies_for_loan]) anyways... but that's still not that nice.
The call becomes simpler: balance_qualifies_for_loan(balance_from_userid(userid_from_name(person_name))), as well as functions which just return the value or throw.
You are critiquing this as if I were suggesting people use it in production JavaScript code, which I'm not. It's a demonstration. Did you read the original article? It has an example that's even more contrived (and also incomplete.) You aren't even pointing out the biggest reason not to use monads in JavaScript (no hints, sorry!)
Implementing exceptions with the continuation monad is straightforward. As you can see, that's sort of how it's used here. You can also recreate the State monad, Maybe, and others, deriving each from the continuation monad. I don't recommend this for actual use in JS, because JS lacks tail call optimization, which means any serious use of monads will cause stack overflows straight away.
But it gets crazier: you can implement the continuation monad using JavaScript exceptions. And JavaScript exceptions do cause the stack to reset (with a performance penalty). So if you derive a continuation monad in JS from its built-in exceptions mechanism, you can implement the other monads without breaking the stack.
You can also implement them with a setTimeout(0), which gives the browser's event loop a chance to run so that you don't lock up the browser forever. I did something like this with ArcLite, resetting the stack with a setTimeout(0) after ever top-level definition was parsed, so that it didn't take 30 seconds of unresponsive browser for the standard library to load. It occurred to me afterwards that I could've used this to implement continuations, but by then I was pretty much done with Arc development.
