Question

I’m trying to make a function in Haskell to split a string at a certain char and a list at a certain number.

For doing this the splitAt function is exactly what I need for numbers, but I can’t give a char with this function.

E.g.

splitAt 5 [1,2,3,4,5,6,7,8,9,10]

gives

([1,2,3,4,5],[6,7,8,9,10])

that is exactly what I needed with the 5 in the left side of the tuple.
But now I want to do this with a char and a string. But splitAt only takes and int for the second argument. I want

splitAt 'c' "abcde"

resulting in

("abc", "de")

I looking for something in the direction of

splitAt (findIndex 'c' "abcde") "abcde"

but the function findIndex returns something of the type Maybe Int and splitAt needs an Int. Then I tried the following

splitAt (head (findIndices (== 'c') "abcde")) "abcde"

This is a possible solution but it returns the following

("ab","cde")

with the c on the wrong side of the tupple. You can add succ to c but what will the result be if the char is a Z.

Is there an easy way to modify to make

splitAt (findIndex 'c' "abcde") "abcde"

work?

Answer 1

You can use findIndex, just unwrap the Maybe and add one:

import Data.List

splitAfter :: (a-> Bool) -> [a] -> ([a],[a])
splitAfter this xs = case findIndex this xs of
    Nothing -> (xs,[])
    Just n -> splitAt (n+1) xs

giving, for example

*Main> splitAfter (=='c') "abcde"
("abc","de")

Maybe is a handy datatype for encoding failure in a way that’s easy to recover. There’s even a function maybe :: b -> (a -> b) -> Maybe a -> b to use a default value and a function to handle the two cases separately:

splitAfter' :: (a-> Bool) -> [a] -> ([a],[a])
splitAfter' this xs = maybe (xs,[]) 
                  (\n -> splitAt (n+1) xs) 
                  (findIndex this xs)

which also works. For example

*Main> splitAfter' (==5) [1..10]
([1,2,3,4,5],[6,7,8,9,10])