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

Question

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!

Answer 1

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 a s, and it returns a list of a s.

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] -> 
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  =  ++ []