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Promise applicative example
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| // Apply a function in a Promise to a value in a Promise once both resolve (used by `promiseLift` below) | |
| // :: Promise (a -> b) -> Promise a -> Promise b | |
| var promiseAp = R.curry((promiseFn, promiseArg) => | |
| promiseFn.then(fn => promiseArg.then(arg => fn(arg))) | |
| ); | |
| // Lift a plain function to accept Promise arguments | |
| // :: (a -> ...n -> b) -> Promise a -> ...Promise n -> Promise b | |
| var promiseLift = (fn) => { | |
| var run = R.curryN(fn.length, (firstArg) => | |
| R.reduce( | |
| promiseAp, | |
| firstArg.then(R.curry(fn)), // `fn` must be curried for partial application | |
| R.drop(1, arguments) | |
| )); | |
| // Apply any arguments provided along with `fn` | |
| return arguments.length > 1 ? R.apply(run, R.drop(1, arguments)) : run; | |
| }; | |
| // Example function for creating a Promise that executes a given `fn` after some `ms` delay | |
| var delay = R.curry((ms, fn) => new Promise(res => setTimeout(res, ms)).then(fn)); | |
| // Some example Promises | |
| var a = delay(500, R.always(1)); | |
| var b = delay(1000, R.always(2)); | |
| var c = delay(1500, R.always(3)); | |
| // A plain JS function to apply | |
| var add3 = (a, b, c) => a + b + c; | |
| // Lift the plain function to now accept Promises | |
| var promiseAdd3 = promiseLift(add3); | |
| // Should print `6` to console immediately | |
| promiseAdd3(1, 2, 3).then(console.log.bind(console)); | |
| // Should print `6` to console after 1500 ms | |
| promiseAdd3(a, b, c).then(console.log.bind(console)); | |
| // Alternatively pass all arguments at once | |
| promiseLift(add3, a, b, c).then(console.log.bind(console)); |
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