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Free à la Carte

april 20, 2024

Free monads based on from intuitions from the Data types à la Carte paper. Combine functors and make embedded DSLs in Haskell.

See the original paper, by Wouter Swierstra: https://webspace.science.uu.nl/~swier004/publications/2008-jfp.pdf .

See the Haskell library I built around it: https://github.com/jjba23/free-alacarte .

# How do you use this ?

This section gives a brief demonstration of using free monads to model effects.

Four effectful functions are defined, categorized into two separate data types. 

data Teletype a
  = GetChar (Char -> a)
  | PutChar Char a
  deriving (Functor)


data FileSystem a
  = ReadFile FilePath (String -> a)
  | WriteFile FilePath String a
  deriving (Functor)

If you are into it, you can also write the Functor instances by hand, for your free monads, e.g.: 

instance Functor Teletype where
  fmap :: (a -> b) -> Teletype a -> Teletype b
  fmap f = \case
    GetChar g   -> GetChar (f . g)
    PutChar c g -> PutChar c (f g)

An exec function can execute values of these data types using the Free free monad. This uses intuitions of category theory to describe imperative sequence of computations as a fold over a functor. NOTE: the exec function is provided by this library and you don’t need to implement it yourself. 

exec :: Exec f => Free f a -> IO a
exec = foldFree return execAlgebra

You should then write the Exec instances, in other words, the concrete implementations. NOTE: the typeclass Exec, and Exec (f :+: g) instance are also provided by this library, and you don’t need to implement it yourself. 

class Functor f => Exec f where
  execAlgebra :: f (IO a) -> IO a

instance (Exec f, Exec g) => Exec (f :+: g) where
  execAlgebra = \case
    Left' e -> execAlgebra e
    Right' e -> execAlgebra e

Then you can write the actual implementations of those effects: 

instance Exec Teletype where
  execAlgebra = \case
    GetChar f    -> Prelude.getChar >>= f
    PutChar c io -> Prelude.putChar c >> io


instance Exec FileSystem where
  execAlgebra (ReadFile path f) = Prelude.readFile path >>= f
  execAlgebra (WriteFile path s f) = Prelude.writeFile path s >> f

Then we can define some smart constructors to create our embedded DSL and save us some boilerplate, while adding syntactic sugar. 

getChar :: (Teletype :<: f) => Free f Char
getChar = injectFree (GetChar Pure)

putChar :: (Teletype :<: f) => Char -> Free f ()
putChar c = injectFree (PutChar c (Pure ()))

readFile :: (FileSystem :<: f) => FilePath -> Free f String
readFile path = injectFree (ReadFile path Pure)

writeFile :: (FileSystem :<: f) => FilePath -> String -> Free f ()
writeFile path s = injectFree (WriteFile path s (Pure ()))

The cat function serves as an example of composition. In the following, I use a more general type than that used in the paper. Here we use mapM_ instead of mapM to discard the resulting list of unit. 

cat :: (FileSystem :<: f, Teletype :<: f) => FilePath -> Free f ()
cat path = mapM_ putChar =<< readFile path

The following example uses the cat function to print the content of the README.md file in this directory. 

main :: IO ()
main = exec @(FileSystem :+: Teletype) $ cat "README.md"

# More on the topic

I can only extremely recommend the following resources to gain more understanding about the ideas and intuitions behind this library, and behind Data types à la Carte.