How to find the frequency of characters in a string in Haskell?

Question

How can I count the frequency of characters in a string and then output them in sort of a table?

For example, if I input the word \"happy\" the result would be

Answer 1

I'll scetch a solution step by step. A shorter solution is possible using standard functions.

You want a sorted result, therefore

result = sort cs
    where

cs would be a list of tuples, where the first element is the character and the second element is the number of times it appears.

        cs = counts "happy"
        counts [] = []
        counts (c:cs) = (c, length otherc + 1) : counts nonc where
             (otherc, nonc) = partition (c==) cs

That's all.

Interestingly, counts works on any list of items that support the == operator.