Comparing elements at the same index

Question

I'm trying to compare element at the same index of two lists in haskell. If the two elements are equal an at the same index then I have to return that index. i know how to compare two function using filter and zip, but my function return the value instead of the index.

 compare l1 l2 = map fst . filter (\(x,y) -> x == y) $ zip l1 l2

When I run it with ghci i have

compare [1,2,3,4,5] [1,2,3,7,8,9,11,5]
[1,2,3]

I want to have [0,1,2] instead. Can anybody give me hints how to go about this?

Answer 1

You could use zip3 and [0..] to include an index as the first item of a tuple of size three:

compare l1 l2 = map (\(i, _, _) -> i) . filter (\(_, x,y) -> x == y) $ zip3 [0..] l1 l2

Answer 2

A list comprehension makes Chad Gilbert's answer a little cleaner.

compare l1 l2 = [i | (i, x, y) <- zip3 [0..] l1 l2, x == y]