How to return every position of letters from a string of letters every time they occur using the zip-CodePudding

I want to write a function search :: String -> Char -> [Int] that returns the positions of all occurrences of the second argument in the first. For example:

search "Bookshop" 'o' == [1,2,6]
search "senselessness" 's' == [0,3,7,8,11,12]

How can one use the function zip :: [a] -> [b] -> [(a,b)], the function length :: [a] -> Int, and the term forms [m..n] and [m..] in a list comprehension to do this?

I am writing a code for this, and (correct me if I'm wrong) since the output is not binary as a zip one should, the string one should be omitted from the list comprehension and a parallel list comprehension should be used?

CodePudding user response：

No need to call length. There rarely is, if ever.

search :: (Num b, Eq a, Enum b) => [a] -> a -> [b]
search s char = [ i | (i,c) <- zip [0..] s, c==char]

Testing:

> search "senselessness" 's'
[0,3,7,8,11,12]

As you can see, we draw both i and c from the zipped list, use c in comparison and output i if the test passes.