monads

Monads for Crystal.

Inspired from https://github.com/dry-rb/dry-monads

Installation

Add this to your application's shard.yml:

dependencies:
  monads:
    github: alex-lairan/monads

Usage

require "monads"

Many monads exists.

Maybe(T)

The Maybe monad help to avoid nil and chain instructions.

There is two kind of Maybe, Just and Nothing.

Just(T)

This is just a value.

Monads::Just.new(5)

Nothing(T)

This a absance of value.

Monads::Nothing(Int32).new

Either(E, T)

The Either monad help to manage errors at the end of the chain instruction.

There is two kind of Either, Right and Left.

Right(T)

This is just a value.

Monads::Right.new("Hello world")

Left(E)

This is an error.

Monads::Left.new("User password is incorrect")

List(T)

The List monad help to manipulate an Array like a monad.

Monads::List[1, 6, 4, 2]

head

head returns the first element wrapped with a Maybe.

tail

tail returns the list without the first element.

Try(T)

The Try monad is a layer between Object Oriented Exception and Fuctional Programming Monads. It can be transformed into a Maybe or an Either.

Task(T)

The Task monad is a parallelized Try monad. Its goal is to use the power of fibers with monads.

How to use a monad ?

Monads have some methods who help to chain instructions.

ps: Try and Task monads should be translated into a Maybe(T) of an Either(Exception, T) one.

fmap

The fmap procedure modify internal value of a monad.

This doesn't affect Nothing and Left monads.

Example :

value = Monads::Just.new(5)
  .fmap(->(x : Int32) { x.to_s })
  .fmap(->(x : String) { x + "12" })
  .fmap(->(x : String) { x.to_i })
  .value!
value.should eq(512)

bind

The bind procedure allow to create a whole new monads from the internal data of another.

This doesn't affect Nothing and Left monads.

Example :

value = Monads::Just.new(5)
  .bind(->(x : Int32) { Monads::Try(Int32).new(-> { x / 0}).to_maybe })
value.should eq(Monads::Nothing(Int32).new)

Development

Clone then let's go, no special requirements.

Contributing

1. Fork it (https://github.com/alex-lairan/monads/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Contributors

• alex-lairan Alexandre Lairan - creator, maintainer
• moba1 moba - maintainer