Haskell: surprising behavior of “groupBy”

Question

I\'m trying to figure out the behavior of the library function groupBy (from Data.List), which purports to group elements of a list by an \"equality test\" function passed i

Answer 1

I'd just like to point out that the groupBy function also requires your list to be sorted before being applied.

For example:

equalityOp :: (a, b1) -> (a, b2) -> Bool
equalityOp x y = fst x == fst y

testData = [(1, 2), (1, 4), (2, 3)]

correctAnswer = groupBy equalityOp testData == [[(1, 2), (1, 4)], [(2, 3)]]

otherTestData = [(1, 2), (2, 3), (1, 4)]

incorrectAnswer = groupBy equalityOp otherTestData == [[(1, 2)], [(2, 3)], [(1, 4)]]

This behaviour comes about because groupBy is using span in its definition. To get reasonable behaviour which doesn't rely on us having the underlying list in any particular order we can define a function:

groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]
groupBy' eq []     = []
groupBy' eq (x:xs) = (x:similarResults) : (groupBy' eq differentResults)
    where similarResults   = filter (eq x) xs
          differentResults = filter (not . eq x) xs