:: (a -> a -> Bool) -> [a] -> [[a]] -package:Diff

The groupBy function is the non-overloaded version of group. When a supplied relation is not transitive, it is important to remember that equality is checked against the first element in the group, not against the nearest neighbour:

>>> groupBy (\a b -> b - a < 5) [0..19]
[[0,1,2,3,4],[5,6,7,8,9],[10,11,12,13,14],[15,16,17,18,19]]

It's often preferable to use Data.List.NonEmpty.groupBy, which provides type-level guarantees of non-emptiness of inner lists.

Examples

>>> groupBy (/=) [1, 1, 1, 2, 3, 1, 4, 4, 5]
[[1],[1],[1,2,3],[1,4,4,5]]

>>> groupBy (>) [1, 3, 5, 1, 4, 2, 6, 5, 4]
[[1],[3],[5,1,4,2],[6,5,4]]

>>> groupBy (const not) [True, False, True, False, False, False, True]
[[True,False],[True,False,False,False],[True]]

This function compares adjacent elements of a list. If two adjacent elements satisfy a relation then they are put into the same sublist. Example:

>>> groupBy (<) "abcdebcdef"
["abcde","bcdef"]

In contrast to that groupBy compares the head of each sublist with each candidate for this sublist. This yields

>>> List.groupBy (<) "abcdebcdef"
["abcdebcdef"]

The second b is compared with the leading a. Thus it is put into the same sublist as a. The sublists are never empty. Thus the more precise result type would be [(a,[a])].

The groupBy function is the non-overloaded version of group.

Classify values by a given comparison function.

> classifyBy (\(x,_) (y,_) -> x == y) [(1,1),(1,2),(2,1),(2,2)]
[[(1,1),(1,2)],[(2,1),(2,2)]]

(cf. classify, classifyOn)

Alternative to groupBy.

groupBy' (\a b -> abs (a - b) <= 1) [1,2,3] = [[1,2,3]]

whereas

groupBy (\a b -> abs (a - b) <= 1) [1,2,3] = [[1,2],[3]]

TODO: Check in ghc 6.12 release if groupBy == groupBy'.

The groupBy function is the non-overloaded version of group. When a supplied relation is not transitive, it is important to remember that equality is checked against the first element in the group, not against the nearest neighbour:

>>> groupBy (\a b -> b - a < 5) [0..19]
[[0,1,2,3,4],[5,6,7,8,9],[10,11,12,13,14],[15,16,17,18,19]]

It's often preferable to use Data.List.NonEmpty.groupBy, which provides type-level guarantees of non-emptiness of inner lists.

Given a list of nodes, and a function that determines whether there is an edge between any two nodes, yields a list of maximal cliques -- sets of nodes such that every node is connected to every other, and such that no other node may be added while maintaining this property.