monads.asd

Monads in Common Lisp

This package defines a fairly trivial implementation of monads in common lisp. It relies on the ‘seq’ moacro from vip-utils to implement something very much like Haskell’s ‘do’ notation (but with extra destructuring superpowers).

Silly Example

The following uses the :list monad to return pairs of numbers where the first is double the second from a list of numbers from 1 to 20.

(in-package :monads)

(let ((numbers (loop for i from 1 to 20 collect i)))
  (with-monad :list
    a <- numbers
    b <- numbers
    (when (= a (* 2 b))
      (unit (list a b)))))

Defining a Monad

To define a monad you should provide:-

1. #'bind method
2. #'fmap method
3. #'monad-return method
4. #'check-monad-return-type method (possibly).

Defining those are sufficient to get a fully functional monad which works as above. See list.lisp or continuation.lisp for trivial examples.

The monads are ‘named’ with a keyword symbol. This works around lack of return type polymorphism in Common Lisp. I think.