alex-lairan / monads

Monads for Crystal

monads

Monads for Crystal.

Inspired by 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 exist.

Maybe(T)

The Maybe monad helps to avoid nil and chain instructions.

There are two kinds of Maybe, Just and Nothing.

Just(T)

This is just a value.

Monads::Just.new(5)

Nothing(T)

This is an absence of value.

Monads::Nothing(Int32).new

Either(E, T)

The Either monad helps to manage errors at the end of the chain of instructions.

There are two kinds 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 helps to manipulate an Array like a monad.

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

head

head returns the first element wrapped within 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 which help to chain instructions.

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

fmap

The fmap procedure modify the 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 allows to create a whole new monad 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

monads's Issues

Cannot use Abstract Module since Crystal 0.30.0

Since Crystal 0.30, it's impossible for the compiler to use Abstract Modules.

source: crystal-lang/crystal#8031

For this reason, we need to remove the module.
#56

Special thanks to @naqvis

Cannot use Block for manipulation methods

In Crystal 0.28, it's impossible for the compiler to defer a concrete type from a free variable declared using forall.
source: crystal-lang/crystal#7737

For this reason, we need to use Proc instead.
#25

While it's not resolve, the only way to manipulate the value of a monad is to provide a Proc.

Add CI for PR

A CI will automatically check if Tests and Syntax are ok.

In Crystal, Travis is commonly used.

Add Document

I would like to create documentation by using GitBook.

Change `Maybe' monad api

The Maybe monad module is named 'Maybe',
so I think it should be close to Haskell and Scala's API.