It's a pity that groupBy isn't defined a little differently: -- @'groupBy' rel xs@ returns the shortest list of lists such that -- -- * the concatenation of the lists is @xs@, and -- -- * @rel@ is 'True' for each consecutive pair of elements in a sublist. -- groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy rel [] = [] groupBy rel (x:xs) = (x:ys) : groupBy rel zs where (ys,zs) = groupByAux x xs groupByAux x0 (x:xs) | rel x0 x = (x:ys, zs) where (ys,zs) = groupByAux x xs groupByAux y xs = ([], xs) That's equivalent to the existing definition if rel is symmetric and transitive, but also useful in other cases, e.g. groupBy (<=) would generate a list of "runs", and the solution to the problem here would be longestInSequence :: (Enum a, Eq a) => [a] -> Int longestInSequence = maximum . map length . groupBy adjacent where adjacent x y = succ x == y