The
elemIndex function returns the index of the first element
in the given list which is equal (by
==) to the query element,
or
Nothing if there is no such element. For the result to be
Nothing, the list must be finite.
Examples
>>> elemIndex 4 [0..]
Just 4
>>> elemIndex 'o' "haskell"
Nothing
>>> elemIndex 0 [1..]
* hangs forever *