Portability | portable |
---|---|

Stability | experimental |

Maintainer | leon@melding-monads.com |

Safe Haskell | Safe-Inferred |

This module implements bag and set operations on ordered lists. For the purposes of this module, a "bag" (or "multiset") is a non-decreasing list, whereas a "set" is a strictly ascending list. Bags are sorted lists that may contain duplicates, whereas sets are sorted lists that do not contain duplicates.

Except for the `nub`

, `sort`

, `nubSort`

, and `isSorted`

families of
functions, every function assumes that any list arguments are sorted
lists. Assuming this precondition is met, every resulting list is also
sorted.

Because `isect`

handles multisets correctly, it does not return results
comparable to `Data.List.`

on them. Thus `intersect`

`isect`

is more than just a more efficient `intersect`

on ordered lists. Similar
statements apply to other associations between functions this module and
functions in `Data.List`

, such as `union`

and `Data.List.`

.
`union`

All functions in this module are left biased. Elements that appear in earlier arguments have priority over equal elements that appear in later arguments, and elements that appear earlier in a single list have priority over equal elements that appear later in that list.

- member :: Ord a => a -> [a] -> Bool
- memberBy :: (a -> a -> Ordering) -> a -> [a] -> Bool
- has :: Ord a => [a] -> a -> Bool
- hasBy :: (a -> a -> Ordering) -> [a] -> a -> Bool
- subset :: Ord a => [a] -> [a] -> Bool
- subsetBy :: (a -> a -> Ordering) -> [a] -> [a] -> Bool
- isSorted :: Ord a => [a] -> Bool
- isSortedBy :: (a -> a -> Bool) -> [a] -> Bool
- insertBag :: Ord a => a -> [a] -> [a]
- insertBagBy :: (a -> a -> Ordering) -> a -> [a] -> [a]
- insertSet :: Ord a => a -> [a] -> [a]
- insertSetBy :: (a -> a -> Ordering) -> a -> [a] -> [a]
- isect :: Ord a => [a] -> [a] -> [a]
- isectBy :: (a -> b -> Ordering) -> [a] -> [b] -> [a]
- union :: Ord a => [a] -> [a] -> [a]
- unionBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
- minus :: Ord a => [a] -> [a] -> [a]
- minusBy :: (a -> b -> Ordering) -> [a] -> [b] -> [a]
- minus' :: Ord a => [a] -> [a] -> [a]
- minusBy' :: (a -> b -> Ordering) -> [a] -> [b] -> [a]
- xunion :: Ord a => [a] -> [a] -> [a]
- xunionBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
- merge :: Ord a => [a] -> [a] -> [a]
- mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
- mergeAll :: Ord a => [[a]] -> [a]
- mergeAllBy :: (a -> a -> Ordering) -> [[a]] -> [a]
- unionAll :: Ord a => [[a]] -> [a]
- unionAllBy :: (a -> a -> Ordering) -> [[a]] -> [a]
- nub :: Ord a => [a] -> [a]
- nubBy :: (a -> a -> Bool) -> [a] -> [a]
- sort :: Ord a => [a] -> [a]
- sortBy :: (a -> a -> Ordering) -> [a] -> [a]
- sortOn :: Ord b => (a -> b) -> [a] -> [a]
- sortOn' :: Ord b => (a -> b) -> [a] -> [a]
- nubSort :: Ord a => [a] -> [a]
- nubSortBy :: (a -> a -> Ordering) -> [a] -> [a]
- nubSortOn :: Ord b => (a -> b) -> [a] -> [a]
- nubSortOn' :: Ord b => (a -> b) -> [a] -> [a]
- foldt :: (a -> a -> a) -> a -> [a] -> a
- foldt' :: (a -> a -> a) -> a -> [a] -> a

# Predicates

subset :: Ord a => [a] -> [a] -> BoolSource

The `subset`

function returns true if the first list is a sub-list
of the second.

isSorted :: Ord a => [a] -> BoolSource

The `isSorted`

predicate returns `True`

if the elements of a list occur
in non-descending order, equivalent to

.
`isSortedBy`

(`<=`

)

isSortedBy :: (a -> a -> Bool) -> [a] -> BoolSource

The `isSortedBy`

function returns `True`

iff the predicate returns true
for all adjacent pairs of elements in the list.

# Insertion Functions

insertBag :: Ord a => a -> [a] -> [a]Source

The `insertBag`

function inserts an element into a list. If the element
is already there, then another copy of the element is inserted.

insertBagBy :: (a -> a -> Ordering) -> a -> [a] -> [a]Source

The `insertBagBy`

function is the non-overloaded version of `insertBag`

.

insertSet :: Ord a => a -> [a] -> [a]Source

The `insertSet`

function inserts an element into an ordered list.
If the element is already there, then the element replaces the existing
element.

insertSetBy :: (a -> a -> Ordering) -> a -> [a] -> [a]Source

The `insertSetBy`

function is the non-overloaded version of `insertSet`

.

# Set-like operations

isect :: Ord a => [a] -> [a] -> [a]Source

The `isect`

function computes the intersection of two ordered lists.
An element occurs in the output as many times as the minimum number of
occurrences in either input. If either input is a set, then the output
is a set.

isect [ 1,2, 3,4 ] [ 3,4, 5,6 ] == [ 3,4 ] isect [ 1, 2,2,2 ] [ 1,1,1, 2,2 ] == [ 1, 2,2 ]

union :: Ord a => [a] -> [a] -> [a]Source

The `union`

function computes the union of two ordered lists.
An element occurs in the output as many times as the maximum number
of occurrences in either input. The output is a set if and only if
both inputs are sets.

union [ 1,2, 3,4 ] [ 3,4, 5,6 ] == [ 1,2, 3,4, 5,6 ] union [ 1, 2,2,2 ] [ 1,1,1, 2,2 ] == [ 1,1,1, 2,2,2 ]

minus :: Ord a => [a] -> [a] -> [a]Source

The `minus`

function computes the difference of two ordered lists.
An element occurs in the output as many times as it occurs in
the first input, minus the number of occurrences in the second input.
If the first input is a set, then the output is a set.

minus [ 1,2, 3,4 ] [ 3,4, 5,6 ] == [ 1,2 ] minus [ 1, 2,2,2 ] [ 1,1,1, 2,2 ] == [ 2 ]

minus' :: Ord a => [a] -> [a] -> [a]Source

The `minus'`

function computes the difference of two ordered lists.
The result consists of elements from the first list that do not appear
in the second list. If the first input is a set, then the output is
a set.

minus' [ 1,2, 3,4 ] [ 3,4, 5,6 ] == [ 1,2 ] minus' [ 1, 2,2,2 ] [ 1,1,1, 2,2 ] == [] minus' [ 1,1, 2,2 ] [ 2 ] == [ 1,1 ]

xunion :: Ord a => [a] -> [a] -> [a]Source

The `xunion`

function computes the exclusive union of two ordered lists.
An element occurs in the output as many times as the absolute difference
between the number of occurrences in the inputs. If both inputs
are sets, then the output is a set.

xunion [ 1,2, 3,4 ] [ 3,4, 5,6 ] == [ 1,2, 5,6 ] xunion [ 1, 2,2,2 ] [ 1,1,1, 2,2 ] == [ 1,1, 2 ]

merge :: Ord a => [a] -> [a] -> [a]Source

The `merge`

function combines all elements of two ordered lists.
An element occurs in the output as many times as the sum of the
occurrences in both lists. The output is a set if and only if
the inputs are disjoint sets.

merge [ 1,2, 3,4 ] [ 3,4, 5,6 ] == [ 1,2, 3,3,4,4, 5,6 ] merge [ 1, 2,2,2 ] [ 1,1,1, 2,2 ] == [ 1,1,1,1, 2,2,2,2,2 ]

mergeAll :: Ord a => [[a]] -> [a]Source

The `mergeAll`

function merges a (potentially) infinite number of
ordered lists, under the assumption that the heads of the inner lists
are sorted. An element is duplicated in the result as many times as
the total number of occurrences in all inner lists.

The `mergeAll`

function is closely related to

.
The former does not assume that the outer list is finite, whereas
the latter does not assume that the heads of the inner lists are sorted.
When both sets of assumptions are met, these two functions are
equivalent.
`foldr`

`merge`

[]

This implementation of `mergeAll`

uses a tree of comparisons, and is
based on input from Dave Bayer, Heinrich Apfelmus, Omar Antolin Camarena,
and Will Ness. See `CHANGES`

for details.

mergeAllBy :: (a -> a -> Ordering) -> [[a]] -> [a]Source

The `mergeAllBy`

function is the non-overloaded variant of the `mergeAll`

function.

unionAll :: Ord a => [[a]] -> [a]Source

The `unionAll`

computes the union of a (potentially) infinite number
of lists, under the assumption that the heads of the inner lists
are sorted. The result will duplicate an element as many times as
the maximum number of occurrences in any single list. Thus, the result
is a set if and only if every inner list is a set.

The `unionAll`

function is closely related to

.
The former does not assume that the outer list is finite, whereas
the latter does not assume that the heads of the inner lists are sorted.
When both sets of assumptions are met, these two functions are
equivalent.
`foldr`

`union`

[]

Note that there is no simple way to express `unionAll`

in terms of
`mergeAll`

or vice versa on arbitrary valid inputs. They are related
via `nub`

however, as

.
If every list is a set, then `nub`

. `mergeAll`

== `unionAll`

. `map`

`nub`

`map nub == id`

, and in this special case
(and only in this special case) does `nub . mergeAll == unionAll`

.

This implementation of `unionAll`

uses a tree of comparisons, and is
based on input from Dave Bayer, Heinrich Apfelmus, Omar Antolin Camarena,
and Will Ness. See `CHANGES`

for details.

unionAllBy :: (a -> a -> Ordering) -> [[a]] -> [a]Source

The `unionAllBy`

function is the non-overloaded variant of the `unionAll`

function.

# Lists to Ordered Lists

nubBy :: (a -> a -> Bool) -> [a] -> [a]Source

The `nubBy`

function is the greedy algorithm that returns a
sublist of its input such that:

isSortedBy pred (nubBy pred xs) == True

This is true for all lists, not just ordered lists, and all binary predicates, not just total orders. On infinite lists, this statement is true in a certain mathematical sense, but not a computational one.

sortOn :: Ord b => (a -> b) -> [a] -> [a]Source

The `sortOn`

function provides the decorate-sort-undecorate idiom,
also known as the "Schwartzian transform".

sortOn' :: Ord b => (a -> b) -> [a] -> [a]Source

This variant of `sortOn`

recomputes the sorting key every comparison.
This can be better for functions that are cheap to compute.
This is definitely better for projections, as the decorate-sort-undecorate
saves nothing and adds two traversals of the list and extra memory
allocation.

nubSort :: Ord a => [a] -> [a]Source

The `nubSort`

function is equivalent to

, except
that duplicates are removed as it sorts. It is essentially the same
implementation as `nub`

`.`

`sort`

`Data.List.sort`

, with `merge`

replaced by `union`

.
Thus the performance of `nubSort`

should better than or nearly equal
to `sort`

alone. It is faster than

when the
input contains significant quantities of duplicated elements.
`nub`

`.`

`sort`

nubSortOn' :: Ord b => (a -> b) -> [a] -> [a]Source

This variant of `nubSortOn`

recomputes the sorting key for each comparison