Mostify

Like promisify but for most.js

npm install @partially-applied/mostify

Quick Guide in Code

JavaScript (ES5)

// mostify returns an object 
// with two functions .withError and .default

var mostify = (require ('@partially-applied/mostify')).withError 

// since we are dealing with fs in this example 
// we will be using .withError option

var fsRaw = require ('fs')


var fs = mostify(fsRaw)

fs.readFile ('hello.txt')

.map(function (input){  
 response = input[0] // returns an array   
 console.log (response.toString()) // text file string
})
.drain()

Babel User

Using preset es2015, the only issue you need to worry about is imports:

import mostify from "@partially-applied/mostify" // for .default

import {withError as mostify} from '@partially-applied/mostify' 
// for .withError

LiveScript (ES5)

# mostify returns an object 
# with two functions .withError and .default

mostify = (require '@partially-applied/mostify').withError 

# since we are dealing with fs in this example 
# we will be using .withError option

fs-raw = require 'fs'


fs = mostify fs-raw

fs.readFile 'hello.txt'
.map ([response]) -> # returns an array
    console.log response.toString! # text file string
.drain!

Features

1. Input Tracking

The return value as you can observe is an array, the array has two elements

fs.readFile 'hello.txt'
.map ([response,user-input]) -> 
    console.log user-input[0] # 'hello.txt'
.drain()

or with livescript pattern matching to make it nicer

fs.readFile 'hello.txt'
.map ([response,[text-file]]) -> # important! returns an array
    console.log text-file # 'hello.txt'
.drain()

Application of Input Tracking

Input tracking is useful if you want to match requests and responses:

 request stream : --a--b--c--->
response stream : --b--c--a--->

The letters are paired - --a-- in request stream corresponds to --a-- in respose stream. Due to the nature of async computation its not possible to guarantee order, this is why sometimes you want to pass some variable id from request stream into the response stream as a way to track and pair them.

most = require 'most'

fs = (require '@partially-applied/mostify').withError (require 'fs')

# a list of sample files to read
list-of-files = ['hello.txt','foo.txt','bar.txt'] 

responses = [] # an array that will store a list of streams

for file in list-of-files
    response.push (fs.readFile file)


most.mergeArray  responses # merge all the streams in the array
.map ([value,[filename]]) ->
    console.log value # how will you match which value is the output of which file ?
    # good thing the secound array element has filenames.


.drain()

Example with passing extra arguments

# we can even pass an unique index in case file names are not unique

files = ['hello.txt','foo.txt','bar.txt'] # make sure the files exist !

responses = []

for I from 0 til files.length
    response.push (fs.readFile files[I],'utf8',I)


most.mergeArray  responses
.map (value) ->
    
    [output,[filename,encoding,index]] = value

    console.log index # => 0 then 1 then 2 
    # essentially all input arguments get passed
.drain()

2. Single Functions

Sometimes you do not want to mostify the entire module but singleton functions. The entry function type checks and if the parameter is a function then it only mostifies the function.

Type Signature

:: [response,userinput]

• staying close to all stream API specs, returns a single argument in this case a Array.
• where response is the argument object from the callback that we are intercepting.
• where userinput is the argument object which the user passed to the orignal function call
• If you pass 'with error' flag then the assumption is made that the first argument of the callback we are intercepting is the error object:
  • if error object is defined then the error stream is activated
  • response argument would be shifted by one element to the left.

Why use mostify rather than just use promises ?

If you are using most.js , creating a promise object seem like a extra unwanted step. Callbacks are the lowest level of abstraction you can find and rather than wrapping callbacks using promises and then rewrapping it using most streams, I think its more elegant to use most streams directly. It also helps that most streams are also more general while providing all the error handling goodiness that promise provides.

#Before
Node.js style Callback -> Promise -> Most Stream

#After
Node.js style Callback  -> Most Stream