Type hinting for predicates

[dubiousdavid]

Predicates should have a ^boolean type hint so that they can be properly optimized by the ClojureScript compiler.

[aria42]

Can you elaborate with an example? What's the difference in generated js? Happy to do this if I just understand what the gains might be.

[dubiousdavid]

I learned this after watching Fogus' talk on ClojureScript. Also, you'll notice that all predicates in the cljs.core have this type hint. I can't remember exactly how it works.

On Apr 2, 2013, at 1:20 PM, Aria Haghighi notifications@github.com wrote:

Can you elaborate with an example? What's the difference in generated js? Happy to do this if I just understand what the gains might be.

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[cpetzold]

Here's an example:

(defn ^boolean typed? [x] x)
(defn not-typed? [x] x)
(when (typed? true))
(when (not-typed? true))

.. compiles to

typed_QMARK_ = function typed_QMARK_(x) {
  return x
};
not_typed_QMARK_ = function not_typed_QMARK_(x) {
  return x
};
if(typed_QMARK_.call(null, true)) {
}else {
}
if(cljs.core.truth_(not_typed_QMARK_.call(null, true))) {
}else {
}

You'll notice that in the when, typed? isn't required to be wrapped in cljs.core.truth_.

[aria42]

I'd accept a pull request with the desired changes.