2006-11-19
On #haskell,
Tony Morris asked how he could improve the following function:
replaceAll :: Eq a => [a] -> [a] -> [a] -> [a]
replaceAll [] _ xs = xs
replaceAll _ [] xs = xs
replaceAll _ _ [] = []
replaceAll xs ys zs'@(z:zs) = if xs `isPrefixOf` zs' then ys ++ drop (length xs) zs' else z : replaceAll xs ys zs
A quick clean up to use guards instead, and add some lightweight
automated testing followed:
import Data.List
import Text.Printf
import Test.QuickCheck
replaceAll :: Eq a => [a] -> [a] -> [a] -> [a]
replaceAll [] _ xs = xs
replaceAll _ [] xs = xs
replaceAll _ _ [] = []
replaceAll xs ys zs'@(z:zs)
| xs `isPrefixOf` zs' = ys ++ drop (length xs) zs'
| otherwise = z : replaceAll xs ys zs
prop_length xs ys zs =
length xs == length ys ==>
length (replaceAll xs ys zs) == length zs
where _ = xs :: [Int]
run = mapM_ (\(s,a) -> printf "%-25s: " s >> a) [("length", test prop_length)]
It's great how you can so quickly roll together nice resuable (higher order + polymorphic = reusable), and well specified code in Haskell.
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