Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Use deforestation to simplify Core related to iter #187

Open
wants to merge 1 commit into
base: master
Choose a base branch
from
Open

Use deforestation to simplify Core related to iter #187

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
30 changes: 25 additions & 5 deletions src/Control/Monad/Free.hs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -39,7 +39,6 @@ module Control.Monad.Free
) where

import Control.Applicative
import Control.Arrow ((>>>))
import Control.Monad (liftM, MonadPlus(..), (>=>))
import Control.Monad.Fix
import Control.Monad.Trans.Class
Expand Down Expand Up @@ -340,11 +339,24 @@ retract :: Monad f => Free f a -> f a
retract (Pure a) = return a
retract (Free as) = as >>= retract

type FCata f a = (forall r. (f r -> r) -> (a -> r) -> r)

buildF :: FCata f a -> Free f a
buildF f = f Free Pure

{-# INLINE [1] buildF #-}

-- | Tear down a 'Free' 'Monad' using iteration.
iter :: Functor f => (f a -> a) -> Free f a -> a
iter _ (Pure a) = a
iter phi (Free m) = phi (iter phi <$> m)

{-# INLINE [0] iter #-}

{-# RULES "iter/buildF" [~1]
forall (phi :: f a -> a) (g :: FCata f a).
iter phi (buildF g) = g phi id #-}

-- | Like 'iter' for applicative values.
iterA :: (Applicative p, Functor f) => (f (p a) -> p a) -> Free f a -> p a
iterA _ (Pure x) = pure x
Expand Down Expand Up @@ -385,13 +397,21 @@ toFreeT (Free f) = FreeT.FreeT (return (FreeT.Free (fmap toFreeT f)))
-- Calling 'retract . cutoff n' is always terminating, provided each of the
-- steps in the iteration is terminating.
cutoff :: (Functor f) => Integer -> Free f a -> Free f (Maybe a)
cutoff n _ | n <= 0 = return Nothing
cutoff n (Free f) = Free $ fmap (cutoff (n - 1)) f
cutoff _ m = Just <$> m
cutoff n b = buildF $ \u p ->
let c n' x
| n <= 0 = p Nothing
| otherwise = case x of
Pure v -> p (Just v)
Free f -> u (fmap (c (n' - 1)) f)
in c n b

-- | Unfold a free monad from a seed.
unfold :: Functor f => (b -> Either a (f b)) -> b -> Free f a
unfold f = f >>> either Pure (Free . fmap (unfold f))
unfold f b = buildF $ \u p ->
let c x = case f x of
Left a -> p a
Right g -> u (fmap c g)
in c b

-- | Unfold a free monad from a seed, monadically.
unfoldM :: (Traversable f, Applicative m, Monad m) => (b -> m (Either a (f b))) -> b -> m (Free f a)
Expand Down