Module DA.List¶
List
Functions¶
 sort
: Ord a => [a] > [a]
The
sort
function implements a stable sorting algorithm. It is a special case ofsortBy
, 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 (a stable sort).
 sortBy
: (a > a > Ordering) > [a] > [a]
The
sortBy
function is the nonoverloaded version ofsort
.
 sortOn
: Ord k => (a > k) > [a] > [a]
Sort a list by comparing the results of a key function applied to each element.
sortOn f
is equivalent tosortBy (comparing f)
, but has the performance advantage of only evaluatingf
once for each element in the input list. This is sometimes called the decoratesortundecorate paradigm.Elements are arranged from from lowest to highest, keeping duplicates in the order they appeared in the input.
 mergeBy
: (a > a > Ordering) > [a] > [a] > [a]
Merge two sorted lists using into a single, sorted whole, allowing the programmer to specify the comparison function.
 combinePairs
: (a > a > a) > [a] > [a]
Combine elements pairwise by means of a programmer supplied function from two list inputs into a single list.
 foldBalanced1
: (a > a > a) > [a] > a
Fold a nonempty list in a balanced way. Balanced means that each element has approximately the same depth in the operator tree. Approximately the same depth means that the difference between maximum and minimum depth is at most 1. The accumulation operation must be associative and commutative in order to get the same result as
foldl1
orfoldr1
.
 group
: Eq a => [a] > [[a]]
The ‘group’ function groups equal elements into sublists such that the concatenation of the result is equal to the argument.
 groupBy
: (a > a > Bool) > [a] > [[a]]
The ‘groupBy’ function is the nonoverloaded version of ‘group’.
 groupOn
: Eq k => (a > k) > [a] > [[a]]
Similar to ‘group’, except that the equality is done on an extracted value.
 dedup
: Ord a => [a] > [a]
dedup l
removes duplicate elements from a list. In particular, it keeps only the first occurrence of each element. It is a special case ofdedupBy
, which allows the programmer to supply their own equality test.dedup
is callednub
in Haskell.
 dedupOn
: Ord k => (a > k) > [a] > [a]
A version of
dedup
where deduplication is done after applyng function. Example use:dedupOn (.employeeNo) employees
 dedupSort
: Ord a => [a] > [a]
The
dedupSort
function sorts and removes duplicate elements from a list. In particular, it keeps only the first occurrence of each element.
 dedupSortBy
: (a > a > Ordering) > [a] > [a]
A version of
dedupSort
with a custom predicate.
 unique

Returns True if and only if there are no duplicate elements in the given list.
 uniqueOn
: Ord k => (a > k) > [a] > Bool
Returns True if and only if there are no duplicate elements in the given list after applyng function. Example use:
assert $ uniqueOn (.employeeNo) employees
 replace
: Eq a => [a] > [a] > [a] > [a]
Given a list and a replacement list, replaces each occurance of the search list with the replacement list in the operation list.
 dropPrefix
: Eq a => [a] > [a] > [a]
Drops the given prefix from a list. It returns the original sequence if the sequence doesn’t start with the given prefix.
 dropSuffix
: Eq a => [a] > [a] > [a]
Drops the given suffix from a list. It returns the original sequence if the sequence doesn’t end with the given suffix.
 stripPrefix
: Eq a => [a] > [a] > Optional [a]
The
stripPrefix
function drops the given prefix from a list. It returnsNone
if the list did not start with the prefix given, orSome
the list after the prefix, if it does.
 stripSuffix
: Eq a => [a] > [a] > Optional [a]
Return the prefix of the second list if its suffix matches the entire first list.
 stripInfix
: Eq a => [a] > [a] > Optional ([a], [a])
Return the string before and after the search string or
None
if the search string is not found.>>> stripInfix [0,0] [1,0,0,2,0,0,3] Some ([1], [2,0,0,3]) >>> stripInfix [0,0] [1,2,0,4,5] None
 isPrefixOf

The
isPrefixOf
function takes two lists and returnsTrue
if and only if the first is a prefix of the second.
 isSuffixOf

The
isSuffixOf
function takes two lists and returnsTrue
if and only if the first list is a suffix of the second.
 isInfixOf

The
isInfixOf
function takes two lists and returnsTrue
if and only if the first list is contained anywhere within the second.
 mapAccumL
: (acc > x > (acc, y)) > acc > [x] > (acc, [y])
The
mapAccumL
function combines the behaviours ofmap
andfoldl
; it applies a function to each element of a list, passing an accumulating parameter from left to right, and returning a final value of this accumulator together with the new list.
 inits
: [a] > [[a]]
The
inits
function returns all initial segments of the argument, shortest first.
 intersperse
: a > [a] > [a]
The
intersperse
function takes an element and a list and “intersperses” that element between the elements of the list.
 intercalate
: [a] > [[a]] > [a]
intercalate
inserts the listxs
in between the lists inxss
and concatenates the result.
 tails
: [a] > [[a]]
The
tails
function returns all final segments of the argument, longest first.
 dropWhileEnd
: (a > Bool) > [a] > [a]
A version of
dropWhile
operating from the end.
 takeWhileEnd
: (a > Bool) > [a] > [a]
A version of
takeWhile
operating from the end.
 transpose
: [[a]] > [[a]]
The
transpose
function transposes the rows and columns of its argument.
 breakOn
: Eq a => [a] > [a] > ([a], [a])
Find the first instance of
needle
inhaystack
. The first element of the returned tuple is the prefix ofhaystack
beforeneedle
is matched. The second is the remainder ofhaystack
, starting with the match. If you want the remainder without the match, usestripInfix
.
 breakOnEnd
: Eq a => [a] > [a] > ([a], [a])
Similar to
breakOn
, but searches from the end of the string.The first element of the returned tuple is the prefix of
haystack
up to and including the last match ofneedle
. The second is the remainder ofhaystack
, following the match.
 linesBy
: (a > Bool) > [a] > [[a]]
A variant of
lines
with a custom test. In particular, if there is a trailing separator it will be discarded.
 wordsBy
: (a > Bool) > [a] > [[a]]
A variant of
words
with a custom test. In particular, adjacent separators are discarded, as are leading or trailing separators.
 head
: [a] > a
Extract the first element of a list, which must be nonempty.
 tail
: [a] > [a]
Extract the elements after the head of a list, which must be nonempty.
 last
: [a] > a
Extract the last element of a list, which must be finite and nonempty.
 init
: [a] > [a]
Return all the elements of a list except the last one. The list must be nonempty.
 foldl1
: (a > a > a) > [a] > a
Left associative fold of a list that must be nonempty.
 foldr1
: (a > a > a) > [a] > a
Right associative fold of a list that must be nonempty.
 repeatedly
: ([a] > (b, [a])) > [a] > [b]
Apply some operation repeatedly, producing an element of output and the remainder of the list.
 delete
: Eq a => a > [a] > [a]
delete x
removes the first occurrence ofx
from its list argument. For example,> delete "a" ["b","a","n","a","n","a"] ["b","n","a","n","a"]
It is a special case of ‘deleteBy’, which allows the programmer to supply their own equality test.
 deleteBy
: (a > a > Bool) > a > [a] > [a]
The ‘deleteBy’ function behaves like ‘delete’, but takes a usersupplied equality predicate.
> deleteBy (<=) 4 [1..10] [1,2,3,5,6,7,8,9,10]
 (\\)
: Eq a => [a] > [a] > [a]
The
\\
function is list difference (nonassociative). In the result ofxs \\ ys
, the first occurrence of each element ofys
in turn (if any) has been removed fromxs
. Thus(xs ++ ys) \\ xs == ys
Note this function is O(n*m) given lists of size n and m.
 singleton
: a > [a]
Produce a singleton list.
>>> singleton True [True]
 (!!)
: [a] > Int > a
List index (subscript) operator, starting from 0. For example,
xs !! 2
returns the third element inxs
. Raises an error if the index is not suitable for the given list. The function has complexity O(n) where n is the index given, unlike in languages such as Java where array indexing is O(1).