purescript-promises
An alternative effect monad for PureScript.
Use this for easy interop with existing promise-based JavaScript libraries.
Usage
With monadic do notation, or with promise-chaining notation. The following are
equivalent:
myApply :: forall a b. Promise (a -> b) -> Promise a -> Promise b
myApply pab pa = do
ab <- pab
a <- pa
pure (ab a)
myApplyChained :: forall a b. Promise (a -> b) -> Promise a -> Promise b
myApplyChained pab pa = pab # then' \ ab -> pa # then' \ a -> resolve (ab a)in fact, if you squint a little, myApplyChained looks like the following JavaScript:
var myApplyChained = function (pab, pa) {
pab.then(function (ab) {
pa.then(function (a) {
return Promise.resolve(ab(a));
});
});
}Also see the tests folder.
eagerness
While promises are eager,
this library provides the Deferred typeclass to ensure promises don't prematurely
run their side-effects until safely consumed with runPromise, or the nonstandard
done.
In fact, not only are promises eager, but they're eager about being eager. They really want to run:
delay example
promDelay :: Deferred => Promise Unit
promDelay = do
p1
p2
p3
where
p1 = do
Console.log "one"
Promise.delay (Milliseconds 1000.0) unit
Console.log "two"
p2 = do
Promise.delay (Milliseconds 1000.0) unit
Console.log "three"
p3 = do
Promise.delay (Milliseconds 1000.0) unit
Console.log "four"this will output one, wait one second, then four three two.
In order to obtain the desired behavior of waiting one second between
each log, it's necessary to add type annotations to p1, p2 and
p3:
p1 :: Deferred => Promise Unit
p1 = do ...parallel (<*>)
promApply :: Deferred => Promise Unit
promApply = p1 *> p2 *> p3
where
p1 :: Deferred => Promise Unit
p1 = do
Console.log "<*> is"
Promise.delay (Milliseconds 1000.0) unit
Console.log "done"
p2 :: Deferred => Promise Unit
p2 = do
Promise.delay (Milliseconds 3000.0) unit
Console.log "parallel"
p3 :: Deferred => Promise Unit
p3 = do
Promise.delay (Milliseconds 2000.0) unit
Console.log "in"Note that difference (between this example and the last) that we're using (*>)
instead of implicit (>>=)s. And even though we added the Deferred constraint,
it will still take 3 seconds to run total -- not 6, as it would be using do notation.
FFI example
exports.myPromise = new Promise(function (resolve, reject) {
resolve(5);
});foreign import myPromise :: Promise Int
doSomething :: Deferred => Promise Unit
doSomething = do
p <- myPromise
Console.logShow p
main :: Effect Unit
main
= runPromise
(const (log "success callback"))
(const (error "error callback"))
doSomethingInstallation
bower install --save purescript-promises