monad-transformers

About this library

It is inspired by the mtl library in Haskell, but contains some JS-specific goodies.

About monad transformers

Monad transformers are composable monads. They are cool. See this article.

Maybe you need to understand monads first, before you start with transformers. Read this book and/or see my other library.

Features

Highly composable

We test every monad transformer type against every other to make sure everything works together.

Functions all the way down

The monad transformers are defined by just writing functions transforming one type to another. The stack may contain any value, including values coming from third party libs and the build-ins.

Chaining API

The library features an chaining API, inspired by underscore.js , familiar and easy-to-use for JS developers.

Handles wrapping and unwrapping of Values.

This package contains a stack component which abstracts away the process of wrapping and unwrapping of values, thus making the types easy to write and understand.

Quick tutorial

Installing

From CommonJS environment

Import the module and start playing:

      var mtl = require('monad-transformers')

From browser

import one of the files from the "target" directory and use window.mtl to access the lib.

Using

Call mtl.make to compose two or several types in a new type:

 var mtl = require('monad-transformers')
 var listMaybe = mtl.make(mtl.data.list, mtl.data.maybe)

Create an instance of the new type and use it.

  listMaybe.fromArray([{name: 'foo'}, {name: 'bar'}, {noname: 'baz'}])

Use the methods coming from the types that you composed:

  listMaybe.fromArray([{name: 'foo'}, {name: 'bar'}, {noname: 'baz'}])

    //Calling a 'maybe' method
    .maybeGet('name') // [maybe('foo'), maybe('bar'), maybe(undefined)]
    
    //Calling a 'list' method
    .filter(a => a.name !== 'bar') //[maybe('foo'), maybe(undefined)]
    
    //Calling a generic monad method
    .map((val)=>console.log(val)) //prints 'foo'

Slow tutorial

1. Using the monad stack. Using the Task monad, the Maybe monad and the Writer monad.
2. Using the monad stack. Using the Reader monad.
3. Creating and using custom monads.

Docs

• Overview
• An example
• Object wrapper API
• Types API
• Implementing a monad transformer