functional programming - How to find occurrences of char in an input string in Haskell

Question

I am trying to write a function that will take a String and a Char and output the indexes where the char occurs in the string.

stringCount str ch = 
Input : "haskell is hard" `h`
Output:[0,11]
Input : "haskell is hard" `a`
Output:[1,12]

Please help me I'm struggling to understand Haskell.

Answer 1

There are many ways to do this, but since you mention you're a Haskell beginner, a list comprehension may be easiest to understand (I'm assuming this is homework, so you have to implement it yourself, not use elemIndices):

stringCount str ch = [ y | (x, y) <- zip str [0..], x == ch ]
stringCount "haskell is hard" 'a'
-- [1,12]
stringCount "haskell is hard" 'h'
-- [0,11]

Here we zip, the string str with the infinite list starting from 0, producing the tuples ('h', 0), ('a', 1), ('s', 2), etc. We then only select the tuples where the character (bound to x) equals the argument ch and return the index (bound to y) for each of them.

If you wanted to keep your current argument order but use elementIndices you can use the following:

stringCount' = flip elemIndices 
stringCount' "haskell is hard" 'h'
-- [0,11]