69 lines
1.8 KiB
Haskell
69 lines
1.8 KiB
Haskell
{-# LANGUAGE CPP #-}
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{-# LANGUAGE ScopedTypeVariables #-}
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module Nix.Utils (module Nix.Utils, module X) where
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import Control.Monad
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import Control.Monad.Fix
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import Data.Fix
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import Data.Monoid (Endo)
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#define ENABLE_TRACING 1
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#if ENABLE_TRACING
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import Debug.Trace as X
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#else
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import Prelude as X
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trace :: String -> a -> a
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trace = const id
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traceM :: Monad m => String -> m ()
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traceM = const (return ())
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#endif
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type DList a = Endo [a]
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(&) :: a -> (a -> c) -> c
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(&) = flip ($)
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loeb :: Functor f => f (f a -> a) -> f a
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loeb x = go where go = fmap ($ go) x
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loebM :: (MonadFix m, Traversable t) => t (t a -> m a) -> m (t a)
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loebM f = mfix $ \a -> mapM ($ a) f
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para :: (a -> [a] -> b -> b) -> b -> [a] -> b
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para f base = h where
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h [] = base
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h (x:xs) = f x xs (h xs)
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-- | adi is Abstracting Definitional Interpreters:
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--
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-- https://arxiv.org/abs/1707.04755
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--
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-- Essentially, it does for evaluation what recursion schemes do for
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-- representation: allows threading layers through existing structure, only
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-- in this case through behavior.
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adi :: (Monoid b, Applicative s, Traversable t)
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=> (t a -> a)
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-> ((Fix t -> (b, s a)) -> Fix t -> (b, s a))
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-> Fix t -> (b, s a)
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adi f g = g (go . traverse (adi f g) . unFix)
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where
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go = fmap (fmap f . sequenceA)
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adiM :: (Monoid b, Applicative s, Traversable s, Traversable t, Monad m)
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=> (t a -> m a)
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-> ((Fix t -> m (b, s a)) -> Fix t -> m (b, s a))
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-> Fix t -> m (b, s a)
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adiM f g = g ((go <=< traverse (adiM f g)) . unFix)
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where
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go = traverse (traverse f . sequenceA) . sequenceA
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adiT :: forall s t m a. (Traversable t, Monad m, Monad s)
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=> (t a -> m a)
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-> ((Fix t -> s (m a)) -> Fix t -> s (m a))
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-> Fix t -> s (m a)
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adiT f g = g (go . fmap (adiT f g) . unFix)
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where
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go :: t (s (m a)) -> s (m a)
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go = fmap ((f =<<) . sequenceA) . sequenceA
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