:: Ord a => [a] -> [a] -package:html-entities is:exact -package:errors -package:containers -package:witherable -package:lens -package:base-prelude -package:mono-traversable -package:incipit-base -package:hedgehog -package:Cabal-syntax -package:Cabal -package:generic-deriving -package:ghc -package:lifted-base -package:vector

The sort function implements a stable sorting algorithm. It is a special case of sortBy, which allows the programmer to supply their own comparison function. Elements are arranged from lowest to highest, keeping duplicates in the order they appeared in the input.

>>> sort [1,6,4,3,2,5]
[1,2,3,4,5,6]

The argument must be finite.

O(n log n). The nubOrd function removes duplicate elements from a list. In particular, it keeps only the first occurrence of each element. Unlike the standard nub operator, this version requires an Ord instance and consequently runs asymptotically faster.

nubOrd "this is a test" == "this ae"
nubOrd (take 4 ("this" ++ undefined)) == "this"
\xs -> nubOrd xs == nub xs

O(n log n). The nubSort function sorts and removes duplicate elements from a list. In particular, it keeps only the first occurrence of each element.

nubSort "this is a test" == " aehist"
\xs -> nubSort xs == nub (sort xs)

The nub function which removes duplicate elements from a vector.

Sort a vector.

cycle ties a finite list into a circular one, or equivalently, the infinite repetition of the original list. It is the identity on infinite lists.

>>> cycle []
*** Exception: Prelude.cycle: empty list

>>> cycle [42]
[42,42,42,42,42,42,42,42,42,42...

>>> cycle [2, 5, 7]
[2,5,7,2,5,7,2,5,7,2,5,7...

Extract the elements after the head of a list, which must be non-empty.

>>> tail [1, 2, 3]
[2,3]

>>> tail [1]
[]

>>> tail []
*** Exception: Prelude.tail: empty list

WARNING: This function is partial. You can use case-matching or uncons instead.

Return all the elements of a list except the last one. The list must be non-empty.

>>> init [1, 2, 3]
[1,2]

>>> init [1]
[]

>>> init []
*** Exception: Prelude.init: empty list

WARNING: This function is partial. You can use reverse with case-matching or uncons instead.

reverse xs returns the elements of xs in reverse order. xs must be finite.

>>> reverse []
[]

>>> reverse [42]
[42]

>>> reverse [2,5,7]
[7,5,2]

>>> reverse [1..]
* Hangs forever *

Equivalent to drop 1, but likely to be faster and a single lexeme.

drop1 ""         == ""
drop1 "test"     == "est"
\xs -> drop 1 xs == drop1 xs

Equivalent to dropEnd 1, but likely to be faster and a single lexeme.

dropEnd1 ""         == ""
dropEnd1 "test"     == "tes"
\xs -> dropEnd 1 xs == dropEnd1 xs

Identical to tail, namely that fails on an empty list. Useful to avoid the x-partial warning introduced in GHC 9.8.

tailErr [] = error "Prelude.tail: empty list"
tailErr [1,2,3] = [2,3]

tailSafe [] = []
tailSafe [1,3,4] = [3,4]

List of elements of a structure, from left to right. If the entire list is intended to be reduced via a fold, just fold the structure directly bypassing the list.

Examples

Basic usage:

>>> toList Nothing
[]

>>> toList (Just 42)
[42]

>>> toList (Left "foo")
[]

>>> toList (Node (Leaf 5) 17 (Node Empty 12 (Leaf 8)))
[5,17,12,8]

For lists, toList is the identity:

>>> toList [1, 2, 3]
[1,2,3]

all nodes of a tree

Take a parser that may consume input, and on failure, go back to where we started and fail as if we didn't consume input.