In Haskell,

What is the source code of FindIndex function ? (not using library functions)

It finds the index of the first argument of the function in the list, which is the second argument. And return its first index as a list.

FindIndex :: (Eq a) => a -> [a] -> [Integer]

FindIndex _ [] = []

FindIndex a (x:xs)

| a == x = return index of x ...

| otherwise = FindIndex a xs ...

ex. FindIndex 2 [4, 5, 10, 2] = [3]

, FindIndex "e" "agdfeg" = [4]