Package

purescript-promises

Repository
thimoteus/purescript-promises
License
MIT
Uploaded by
Thimoteus

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 r a b. Promise r (a -> b) -> Promise r a -> Promise r b
myApply pab pa = do
  ab <- pab
  a <- pa
  pure (ab a)

myApplyChained :: forall r a b. Promise r (a -> b) -> Promise r a -> Promise r 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.

Note: 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.

FFI example

exports.myPromise = new Promise(function (resolve, reject) {
  resolve(5);
});
foreign import myPromise :: forall r. Promise r Int

doSomething :: Deferred => Promise (console :: CONSOLE | r) Unit
doSomething = do
  p <- myPromise
  Console.logShow p

main :: forall r. Eff (console :: CONSOLE | r) Unit  
main
  = runPromise
    (const (log "success callback"))
    (const (error "error callback"))
    doSomething

Installation

bower install --save purescript-promises

See also