1 files changed, 48 insertions, 0 deletions
diff --git a/src/Control/CacheArrow.hs b/src/Control/CacheArrow.hs
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+++ b/src/Control/CacheArrow.hs
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+{-# LANGUAGE ExistentialQuantification, FlexibleInstances, MultiParamTypeClasses, Arrows #-}
+
+module Control.CacheArrow ( CacheArrow
+                          , runCache'
+                          , runCache
+                          ) where
+
+import Prelude hiding ((.), id)
+import Control.Arrow
+import Control.Arrow.Transformer
+import Control.Category
+
+
+data CacheArrow a b c = CacheArrow (Maybe (b, c)) (a b c)
+
+instance Arrow a => Category (CacheArrow a) where
+  (CacheArrow _ g) . (CacheArrow _ f) = CacheArrow Nothing (g . f)
+
+  id = arr id
+
+instance Arrow a => Arrow (CacheArrow a) where
+  arr = lift . arr
+  first (CacheArrow _ f) = lift . first $ f
+  second (CacheArrow _ f) = lift . second $ f
+
+instance ArrowChoice a => ArrowChoice (CacheArrow a) where
+  left (CacheArrow _ f) = lift . left $ f
+  right (CacheArrow _ f) = lift . right $ f
+
+instance Arrow a => ArrowTransformer CacheArrow a where
+  lift = CacheArrow Nothing
+
+
+runCacheAlways :: (ArrowChoice a, Eq b) => CacheArrow a b c -> a b (c, CacheArrow a b c)
+runCacheAlways (CacheArrow _ f) = proc b -> do
+  c <- f -< b
+  returnA -< (c, CacheArrow (Just (b, c)) f)
+
+runCache' :: (ArrowChoice a, Eq b) => CacheArrow a b c -> a b (c, Bool, CacheArrow a b c)
+runCache' a@(CacheArrow cache _) = proc b ->
+  case cache of
+    (Just (b', c')) | b == b' -> returnA -< (c', False, a)
+    _ -> do
+      (c, cache') <- runCacheAlways a -< b
+      returnA -< (c, True, cache')
+
+runCache :: (ArrowChoice a, Eq b) => CacheArrow a b c -> a b (c, CacheArrow a b c)
+runCache a = runCache' a >>^ (\(c, _, cache) -> (c, cache))