Implementing `split`

Question

I implemented the split function in Haskell:

split :: (Eq a) => a -> [a] -> [[a]]
split _ [] = []
split x ys = f : split x rest 
 where (f, rest) = break (== x) (dropWhile (== x) ys) 

Note that I'm calling dropWhile (== x) since break's second-tuple value will include the "broken on" value.

example:

*Main> break (== 'a') "dogacactus"
("dog","acactus")

Testing

*Main> split2 '3' "123aaaBBB3"
["12","aaaBBB",""]

Answer 1

Generally it looks good, at least to an Haskell beginner like me. I find the use of dropWhile to be a bit confusing. I admit I had to read your comment to understand why you used it.

I think it could also have introduced a bug. What is the expected behaviour for split ',' ",,"? Your code returns [""] but I would expect ["","",""]. Was your behaviour intended?

This solution fixed the issue.

split :: (Eq a) => a -> [a] -> [[a]]
split _ [] = []
split separator ys = f : (split separator (dropSeparator separator rest))
 where (f, rest) = break (== separator) ys
dropSeparator :: Eq a => a -> [a] -> [a]
dropSeparator _ [] = []
dropSeparator separator (x:xs) = if x == separator then xs else x:xs

I prefer using meaningful variable names, such as separator, even if I'm not sure if it is idiomatic Haskell. Ditto for the parentheses and the introduction of the helper function.

• Kevin Meredith: thank you for letting me know that my usage of dropWhile was both confusing and incorrect! Initially I wanted to do something like where (f, _:rest) = ... , but that would fail to match for [].