CS-581/Homework
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Re-use a fold because Parisa may not have my HW1 file.

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This commit is contained in:
Danila Fedorin 2020-10-06 21:41:27 -07:00
parent 93604a3c07
commit 26d7927c35
1 changed files with 4 additions and 4 deletions
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8
HW2.fedorind.hs
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@ -5,8 +5,8 @@ module HW2 where
import HW1 import HW1
-- Copied from my HW1 for your convenience! -- Copied from my HW1 for your convenience!
fold :: (Int -> a) -> (a -> a -> a) -> (a -> a -> a) -> Expr -> a fold' :: (Int -> a) -> (a -> a -> a) -> (a -> a -> a) -> Expr -> a
fold f1 f2 f3 = rec fold' f1 f2 f3 = rec
where where
rec (Lit i) = f1 i rec (Lit i) = f1 i
rec (Add l r) = f2 (rec l) (rec r) rec (Add l r) = f2 (rec l) (rec r)
@ -35,7 +35,7 @@ fold f1 f2 f3 = rec
-- True -- True
-- --
toRPN :: Expr -> String toRPN :: Expr -> String
toRPN = fold show (rpn "+") (rpn "*") toRPN = fold' show (rpn "+") (rpn "*")
where rpn op l r = concat [l, " ", r, " ", op] where rpn op l r = concat [l, " ", r, " ", op]
@ -82,7 +82,7 @@ fromRPN = head . foldl step [] . words
-- >>> eval (neg e2) -- >>> eval (neg e2)
-- -65 -- -65
-- --
neg = fold (Lit . negate) Add (Mul . neg) -- Not efficient, but short :^) neg = fold' (Lit . negate) Add (Mul . neg) -- Not efficient, but short :^)
-- | Takes two expressions and returns an expression that evalautes to the -- | Takes two expressions and returns an expression that evalautes to the