Monads in Racket, A Dynamically-Typed Language

This library demonstrates an approach to monads in a dynamically-typed language, including a way of achieving return-type polymorphism. It started off as an experiment, documented in this blog post, which see for motivation and comparison to other approaches etc.

The code uses Racket's generic interfaces feature in place of Haskell's type classes.

The approach taken here employs two techniques: (1) coercions between monad representations, and (2) "undecided" quasi-monadic placeholder values. Taken together, these give a Haskell-like feel for monads in a dynamically-typed language.

The techniques are illustrated using Racket, but aren't specific to Racket.

Example

Oleg's article on monads in Scheme gave a kind of challenge problem, near the end. It uses many different types of monad together, using the monomorphic technique he developed:

(IO::run
 (IO::>> (put-line "Enter a number: ")
         (IO::>>= (read-int)
                  (λ (n)
                    (IO::>>= (IO::return (for/list [(i n)] i))
                             (λ (all-n)
                               (IO::>>=
                                (IO::return
                                 (List:>>= all-n
                                           (λ (i)
                                             (if (even? i)
                                                 (List::return i)
                                                 (List::fail "odd")))))
                                (λ (evens) (IO::return evens)))))))))

With the polymorphic monads provided by this library, we are able to use our generic do-notation macro to write instead

(run-io (do (mdisplay "Enter a number: ")
            n <- mread
            all-n <- (return (for/list [(i n)] i))
            evens <- (return (do i <- all-n
                                 #:guard (even? i)
                                 (return i)))
            (return evens)))

This compares favourably to the Haskell

main = do putStr "Enter a number: "
          n <- getInt
          allN <- return [0 .. n-1]
          evens <- return $ do i <- allN
                               guard $ i `mod` 2 == 0
                               return i
          return evens

Copyright and License

Copyright © 2015 Tony Garnock-Jones

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

See the files lgpl.txt and gpl.txt for the relevant license terms.