CODEWITHSUNDEEP
haskell
LK4
LK4

Reputation: 21

Function that removes the first letter of a word and attaches it at the end (Haskell)

I am stuck with the following question: Write a function mangle :: String -> String which removes the first letter of a word and attaches it at the end.

What I have so far:

mangle :: [a] -> [a] -> [a]       (I suspect that this is possibly wrong)
mangle x
  = a ++ b
  where
    a = tail x
    b = head x

Can anyone help me and tell/explain what I should put on the top?

Thanks!

Upvotes: 1

Views: 79

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476624

There are two problems here. First of all, the signature of mangle should be mangle :: [a] -> [a]. Indeed, it takes as parameter a [a], a list of as, and it returns a list of as.

Another problem is that head :: [a] -> a returns a single element. So you can not use (++) :: [a] -> [a] -> [a] since that expects two lists with the same type of items.

You thus need to wrap the head x in a singleton list:

mangle :: [a] -> [a]
mangle x = tail x ++ [head x]

There is still another problem: head and tail can not work with empty lists. You thus probably should write a base-case for an empty list as well.

We can make the function more elegant by performing pattern matching instead of using head and tail, like:

mangle :: [a] -> [a]
mangle [] = …
mangle (x:xs) = xs ++ [x]

Upvotes: 2

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