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Update Rewrites.hs #1

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17 changes: 4 additions & 13 deletions ch10/Rewrites.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -65,21 +65,12 @@ squishAgain = squishMap id
-- It takes a comparison function and a list and returns the greatest element of
-- the list based on the last value that the comparison returned GT for
myMaximumBy :: (a -> a -> Ordering) -> [a] -> a
myMaximumBy comp l@(x:xs) =
foldr (\a b -> if comp a b == GT then a else b) x l
-- It doesn't produce the desired result for:
-- myMaximumBy (\_ _ -> LT) [1..10]
-- => 1 (should be 10)
myMaximumBy _ [] = undefined
myMaximumBy f xs = foldl (\b a -> if f a b == GT then a else b) (last xs) xs

-- 11
-- It takes a comparison function and a list and returns the least element of
-- the list based on the last value that the comparison returned LT for
myMinimumBy :: (a -> a -> Ordering) -> [a] -> a
myMinimumBy comp l@(x:xs) =
foldr (\a b -> if comp a b == LT then a else b) x l
-- It doesn't produce the desired result for:
-- myMinimumBy (\_ _ -> GT) [1..10]
-- => 1 (should be 10)

-- N.B. The only way I know how to get a value of type a is by taking the value
-- from the given list because we don't know anything about the type a.
myMinimumBy _ [] = undefined
myMinimumBy f xs = foldr (\a b -> if f a b == LT then a else b) (head xs) xs