listasIguais :: (Eq a) => [a] -> [a] -> Bool
listasIguais [] p = False
listasIguais p [] = False
listasIguais (x: xs) (y:ys) = x == y && (listasIguais xs ys)
listasIguais [(2,4),(4,6),(8,9)] [(1,4),(2,6),(4,9)]
False
This result should be true. I think I was wrong in the case of a stop. Can you tell me what my mistake is?
If I understand you correctly, you mean that the lists are the same if the second element of each item is the same. That is, the comparison should be made only by the second element of the tuple.
Actually, the current code is comparing tuples, not just the second element. Since the tuple (2, 4) is different from (1, 4) the comparison already fails there.
To compare by the second element can be done as follows:
module Main where
listasIguais :: (Eq b) => [(a, b)] -> [(a, b)] -> Bool
listasIguais [] [] = True
listasIguais [] p = False
listasIguais p [] = False
listasIguais (x:xs) (y:ys) = (snd x) == (snd y) && (listasIguais xs ys)
emptyList = []::[(Int, Int)]
main = do
print $ listasIguais emptyList emptyList -- True
print $ listasIguais [] [(1,4),(2,6),(4,9)] -- False
print $ listasIguais [(2,4),(4,6),(8,9)] [(1,4),(2,6),(4,9)] -- True
Note that it was necessary to change the statement from listasIguais to assume that it is a list of tuples.
Also note the use of snd to get the second element of each tuple.
The declaration of emptyList is required to help the compiler understand the type when the list is empty.
And there was no need to include the case of two empty lists. Without this equation, the function would always return False because at the end it would fall into one of the two empty list equations that return False .