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.

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]]

groupBy operates like group, but uses the provided equality predicate instead of ==.

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

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

O(n) Group characters in a string according to a predicate.

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

Grouping input according to an equality function. Subject to fusion Since 0.3.0

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])].

Perform a Fold while grouping the data according to a specified group projection function. Returns the folded result grouped as a map keyed by the group.

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.

Groups elements using results of the given function as keys.

>>> groupBy even [1..6] :: HashMap Bool (NonEmpty Int)
fromList [(False,5 :| [3,1]),(True,6 :| [4,2])]

Group elements of a stream in accordance with the supplied comparison.

>>> S.print $ mapped S.toList $ S.groupBy (>=) $ each [1,2,3,1,2,3,4,3,2,4,5,6,7,6,5]
[1]
[2]
[3,1,2,3]
[4,3,2,4]
[5]
[6]
[7,6,5]

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

group takes a sequence and returns a list of sequences such that the concatenation of the result is equal to the argument. Each subsequence in the result contains only equal elements, using the supplied equality test.

> groupBy (==) Mississippi
[M,"i","ss","i","ss","i","pp","i"]

Improper lens that splits after the first group of matching bytes, as defined by the given equality predicate

The groupBy function is a generalized version of group.

The groupBy function is a generalized version of group.

Generic version of group.

O(n) Group characters in a string according to a predicate.

GROUP BY clause. You can enclose multiple columns in a tuple.

select $ from \(foo `InnerJoin` bar) -> do
on (foo ^. FooBarId ==. bar ^. BarId)
groupBy (bar ^. BarId, bar ^. BarName)
return (bar ^. BarId, bar ^. BarName, countRows)

With groupBy you can sort by aggregate functions, like so (we used let to restrict the more general countRows to SqlSqlExpr (Value Int) to avoid ambiguity---the second use of countRows has its type restricted by the :: Int below):

r <- select $ from \(foo `InnerJoin` bar) -> do
on (foo ^. FooBarId ==. bar ^. BarId)
groupBy $ bar ^. BarName
let countRows' = countRows
orderBy [asc countRows']
return (bar ^. BarName, countRows')
forM_ r $ \(Value name, Value count) -> do
print name
print (count :: Int)

Need more columns?

The ToSomeValues class is defined for SqlExpr and tuples of SqlExprs. We only have definitions for up to 8 elements in a tuple right now, so it's possible that you may need to have more than 8 elements. For example, consider a query with a groupBy call like this:

groupBy (e0, e1, e2, e3, e4, e5, e6, e7)

This is the biggest you can get with a single tuple. However, you can easily nest the tuples to add more:

groupBy ((e0, e1, e2, e3, e4, e5, e6, e7), e8, e9)

groupBy splits a Producer into two Producers after the first group of elements that are equal according to the equality predicate

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

O(n log n). Group using a user supplied function.