From 6a0d7f5c434c3564d0119befb6799fd77581050a Mon Sep 17 00:00:00 2001 From: Paul Oliver Date: Fri, 3 Jan 2025 11:01:20 -0800 Subject: Initial --- ch10_10.2-ii.hs | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) create mode 100644 ch10_10.2-ii.hs (limited to 'ch10_10.2-ii.hs') diff --git a/ch10_10.2-ii.hs b/ch10_10.2-ii.hs new file mode 100644 index 0000000..d8b1275 --- /dev/null +++ b/ch10_10.2-ii.hs @@ -0,0 +1,18 @@ +{-# LANGUAGE DataKinds #-} +{-# LANGUAGE TypeFamilies #-} +{-# LANGUAGE UndecidableInstances #-} + +-- Exercise 10.2-ii +-- Defunctionalize `foldr :: (a -> b -> b) -> b -> [a] -> b`. + +import Data.Kind (Type) + +type Exp a = a -> Type + +type family Eval (e :: Exp a) :: a + +data Foldr :: (a -> b -> Exp b) -> b -> [a] -> Exp b + +type instance Eval (Foldr f z '[]) = z + +type instance Eval (Foldr f z (x ': xs)) = Eval (f x (Eval (Foldr f z xs))) -- cgit v1.2.1