What is monads?

monads is a small, dependency-free library that brings monads to Crystal. It is inspired by Ruby's dry-monads.

A monad is a container for a value together with a consistent way to chain operations over that value. Instead of writing nested nil checks or scattering begin/rescue blocks across your code, you wrap a value once and then compose transformations that automatically short-circuit when something is absent or fails.

Why use it?

• Avoid nil. Maybe makes the absence of a value explicit with Just and Nothing, so you can never forget to handle it.
• Handle errors without exceptions. Either carries either a success (Right) or an error (Left) through a whole chain, letting you deal with the failure at the end.
• Bridge exceptions and FP. Try runs code that may raise and turns the outcome into a Maybe or an Either. Task does the same on a fiber for concurrency.
• Treat arrays as monads. List wraps an Array and gives it fmap/bind plus helpers like head, tail, and subsequences.

The shared interface

Every monad implements the same core operations, so once you learn them for one type they apply everywhere:

• fmap — transform the value inside the monad.
• bind — transform the value into a whole new monad.
• fold — pattern-match on the two variants.

See Chaining Monads for the full tour, or jump straight to Getting Started.

A taste

require "monads"

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