Getting Started

Installation

Add the shard to your shard.yml:

dependencies:
  monads:
    github: alex-lairan/monads

Then install it:

shards install

Requiring the library

require "monads"

Everything lives under the Monads namespace.

Your first monad

Wrap a value in Just and transform it with fmap. Each step runs only when there is a value to operate on:

require "monads"

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

value # => 512

If a value is absent, the same chain simply produces Nothing and never calls your functions:

result = Monads::Nothing(Int32).new
  .fmap(->(x : Int32) { x * 2 })
  .value_or(0)

result # => 0

Handling errors

Use Either when an operation can fail with a meaningful error value rather than just "nothing":

def divide(a : Int32, b : Int32) : Monads::Either(String, Int32)
  if b == 0
    Monads::Left(String, Int32).new("Division by zero")
  else
    Monads::Right(String, Int32).new(a // b)
  end
end

divide(10, 2)
  .fmap(->(x : Int32) { x * 2 })
  .value_or(->(err : String) { 0 }) # => 10

Where to next?

• Learn the shared chaining interface (fmap, bind, fold).
• Explore each monad: Maybe, Either, List, Try, Task.
• Browse worked Examples.