https://github.com/robrix/freer-cofreer

freer monads and cofreer comonads.
https://github.com/robrix/freer-cofreer

comonads haskell monads

Last synced: 5 months ago
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freer monads and cofreer comonads.

• Host: GitHub
• URL: https://github.com/robrix/freer-cofreer
• Owner: robrix
• License: bsd-3-clause
• Created: 2017-01-29T15:53:38.000Z (almost 9 years ago)
• Default Branch: master
• Last Pushed: 2018-06-26T16:12:45.000Z (over 7 years ago)
• Last Synced: 2025-08-23T22:49:24.871Z (5 months ago)
• Topics: comonads, haskell, monads
• Language: Haskell
• Size: 86.9 KB
• Stars: 23
• Watchers: 5
• Forks: 1
• Open Issues: 6
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

          
# freer-cofreer: freer monads and cofreer comonads.

This is an implementation of the Freer monad described by Oleg Kiselykov in _[Free and Freer Monads: Putting Monads Back in the Closet][Free and Freer]_, and the Cofreer comonad, defined by analogy with Freer.

`Freer` and `Cofreer` differentiate themselves from `Free` and `Cofree` by admitting `Functor` & `Monad`/`Comonad` instances even when the signature is not a `Functor`. This makes them particularly suitable for working with certain GADTs. For example:

```Haskell

data PromptF a where

  Write :: String -> PromptF ()

  Read :: PromptF String

type Prompt = Freer PromptF

write :: String -> Prompt ()

write s = Write s `Then` Return

read :: Prompt String

read = Read `Then` Return

greeting :: Prompt ()

greeting = do

  write "Hi! What’s your name?\n"

  name <- read

  write $ "Pleased to meet you, " ++ name ++ "!"

runPrompt :: Prompt a -> IO a

runPrompt = iterFreer (\ yield instruction -> case instruction of

  Write s -> putStrLn >>= yield

  Read -> getLine >>= yield)

```

The constraints placed on the constructors of `PromptF` mean that it doesn’t admit a `Functor` instance, and thus is not very useful with `Free`. With `Freer`, you get `Functor`, `Applicative`, and `Monad` instances with `PromptF` “for free,” complete with the majority of the API defined in `Control.Monad.Free.Freer`.

[Free and Freer]: http://okmij.org/ftp/Computation/free-monad.html