How to find the index of an element in a list?

Take a look here:

• Finding index of element in a list in Haskell?

i.e. use elemIndex from Data.List:

• http://hackage.haskell.org/packages/archive/base/4.2.0.0/doc/html/Data-List.html#v%3AelemIndex

elemIndex :: Eq a => a -> [a] -> Maybe Int