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://gitlab.com/jjba-projects/free-alacarte .
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"
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.