continuable

Idea for callbacks as values

Example

var readFile = function (uri) {
    return function (cb) {
        fs.readFile(uri, cb)
    }
}

function map(source, lambda) {
    return function continuable(callback) {
        source(function (err, value) {
            callback(err, err ? null : lambda(value))
        })
    }
}

var asString = map(readFile("/tmp/foo.json"), String)
var asJSON = map(asString, function (x) { return JSON.parse(x) })

asJSON(function (err, value) {
    /* do stuff with JSON */
})

Docs

Continuable(callback)

type Continuable := (callback:(Error, Value) => void) => void

A continuable is simply a function that takes a single argument, a callback. The callback get's called with the normal node error and value pattern.

// readFile := (String) => Continuable<Buffer>
var readFile = function (uri) {
    return function continuable(callback) {
        fs.readFile(uri, callback)
    }
}

The reason to have a continuable instead of passing a callback directly into another value is that a continuable is a concrete value that can be returned.

Which means you can call useful functions on this value like map and join

to(asyncFunction)

take an async function and return a function that works as async function or continuable.

var readFile = continuable.to(fs.readFile)

readFile (path, 'utf8') (function (err, text) {
  //there you go
})

map(source, lambda)

map := (source:Continuable<A>, lambda:(A) => B) => Continuable<B>

map takes a transformation function and a continuable and returns a new continuable. The new continuable is the value of the first continuable transformed by your mapping function.

var asString = map(readFile("/tmp/foo.json"), String)
var asJSON = map(asString, function (x) { return JSON.parse(x) })

asJSON(function (err, json) {
    /* do stuff */
})

mapAsync(source, lambda)

mapAsync := (source: Continuable<A>, lambda: (A, Callback<B>))
    => Continuable<B>

mapAsync takes an asynchronous transformation function and a source continuable. The new continuable is the value of the first continuable passed through the async transformation.

var asString = map(readFile("/tmp/foo.json"), String)
var asJSON = map(asString, function (x) { return JSON.parse(x) })

var written = mapAsync(asJSON, function (json, cb) {
    fs.writeFile("/tmp/bar.json", JSON.stringify(json), cb)
})

written(function (err, writeResult) {
    /* stuff */
})

join(continuable)

join := (source:Continuable<Continuable<T>>) => Continuable<T>

join takes a continuable that contains another continuable and flattens it by one layer. This is useful if you return another asynchronous operation from map

var asString = map(readFile("/tmp/foo.json"), String)
var asJSON = map(asString, function (x) { return JSON.parse(x) })

var write = map(asJSON, function (json) {
    return function continuable(cb) {
        fs.writeFile("/tmp/bar.json", JSON.stringify(json), cb)
    }
})

join(write)(function (err, writeResult) {
    /* stuff */
})

both(source)

continuable := (Continuable<A>) => Continuable<[Error, A]>

both takes a continuable and returns a continuable containing a tuple of the error and the value. The returned continuable will never contain an error.

This is useful for handling errors using if statements

var fileOrNull = function (uri) {
    var source = fs.readFile.bind(null, uri)
    var maybeFile = both(source)

    return map(maybeFile, function (err, tuple) {
        if (tuple[0]) {
            return null
        }

        return tuple[1]
    })
}

of(value)

of := (Value) => Continuable<Value>

of takes any value and returns a Continuable for this value. This is useful if you want to implement a function that either returns a value or a continuable.

function getThing() {
    var thing = localStorage.getItem("thing")

    if (thing) return of(thing)

    return ajax("/thing")
}

error(err)

error := (Error) => Continuable<void>

error takes any error and returns a Continuable that will return said error. This is useful if you want to transform a normal continuable into an error state one.

var body = getBody(req, res)

var dbWrite = map(body, function (body) {
    if (!body) {
        return error(new Error("Need body"))
    }

    return db.write(body)
})

join(dbWrite)(function (err, writeResult) {
    /* do stuff */
})

chain(continuable, lambda)

chain := (Continuable<A>, (A) => Continuable<B>) => Continuable<B>

chain takes a lambda function that is given the value and returns another continuables. The result will be a continuable given the value of the returned continuable.

In combination with of this makes Continuable a monad.

Alternatively this can be seen as sugar for map followed by join

var body = getBody(req, res)

var dbWrite = chain(body, function (body) {
    if (!body) {
        return error(new Error("Need body"))
    }

    return db.write(body)
})

dbWrite(function (err, writeResult) {
    /* do stuff */
})

either(continuable, left, right?)

either := (source: Continuable<A>,
           left: (Error, cb: Callback<B>) => Continuable<B>,
          right?: (A) => Continuable<B>)
    => Continuable<B>

either takes a source continuable and a left and right function. It will either call the left function with the error in source or call the right function with the value in the source.

The returned continuable will contain the value returned from either left or right. Note that left and right return continuables themself.

var fs = require("fs")
var either = require("continuable/either")

var fileStat = fs.stat.bind(null, "./package.json")
var fileExists = either(fileStat, function left(err) {
    return fs.writeFile.bind(null, "./package.json", "{}")
}) // note the right function is optional

var file = chain(fileExists, function () {
    return fs.readFile.bind(null, "./package.json")
})

file(function (err, body) {
    // There is no error because we create an empty file if the
    // stat failed. Body is either body or {}
})

The left function can either return a Continuable or call the passed callback directly. For example:

var item = fs.stat.bind(null, "./file")
var maybeItem = either(item, function left(err, cb) {
    if (err.code === "ENOENT") {
        return cb(null, null)
    }

    cb(err)
})

maybeItem(function (err, item) {
    // if stat returns a file not found then item is null
    // if stat returns a random disk error then error!
    // if stat returns the stat then item!
})

Using the callback form is convenient and avoids the usage of return return of(null) and return error(err)

series([continuables])

See continuable-series

Given an array of continuables return a continuable that invokes them in order, or until one errors.

para([continuables])

See continuable-para

Given an array on continuables return a continuable

Installation

npm install continuable

Contributors

• Raynos

MIT Licenced