IdrisDoc: Prelude.List

# Prelude.List

zipWith3 : (f : a -> b -> c -> d) -> (x : List a) -> (y : List b) -> (z : List c) -> List d

Combine three lists elementwise using some function. If they are different
lengths, the result is truncated to the length of the shortest list.

f

the function to combine elements with

x

the first list

y

the second list

z

the third list

zipWith : (f : a -> b -> c) -> (l : List a) -> (r : List b) -> List c

Combine two lists elementwise using some function. If they are different
lengths, the result is truncated to the length of the shorter list.

f

the function to combine elements with

l

the first list

r

the second list

zip3 : (x : List a) -> (y : List b) -> (z : List c) -> List (a, b, c)

Combine three lists elementwise into tuples

zip : (l : List a) -> (r : List b) -> List (a, b)

Combine two lists elementwise into pairs

unzip3 : List (a, b, c) -> (List a, List b, List c)

Split a list of triples into three lists

unzip : List (a, b) -> (List a, List b)

Split a list of pairs into two lists

unionBy : (a -> a -> Bool) -> List a -> List a -> List a

The unionBy function returns the union of two lists by user-supplied equality predicate.

union : Eq a => List a -> List a -> List a

Compute the union of two lists according to their `Eq` implementation.

``````union ['d', 'o', 'g'] ['c', 'o', 'w']
``````
transpose : List (List a) -> List (List a)

Transposes rows and columns of a list of lists.

``````with List transpose [[1, 2], [3, 4]]
``````

This also works for non square scenarios, thus
involution does not always hold:

transpose [[], [1, 2]] = [[1], [2]]
transpose (transpose [[], [1, 2]]) = [[1, 2]]

TODO: Solution which satisfies the totality checker?

toList : Foldable t => t a -> List a

Convert any Foldable structure to a list.

takeWhile : (p : a -> Bool) -> List a -> List a

Take the longest prefix of a list such that all elements satisfy some
Boolean predicate.

p

the predicate

take : (n : Nat) -> (xs : List a) -> List a

Take the first `n` elements of `xs`

If there are not enough elements, return the whole list.

n

how many elements to take

xs

the list to take them from

tails : List a -> List (List a)

The tails function returns all final segments of the argument, longest
first. For example,

``````tails [1,2,3] == [[1,2,3], [2,3], [3], []]
``````
tail' : (l : List a) -> Maybe (List a)

Attempt to get the tail of a list.

If the list is empty, return `Nothing`.

tail : (l : List a) -> {auto ok : NonEmpty l} -> List a

Get the tail of a non-empty list.

ok

proof that the list is non-empty

splitOn : Eq a => a -> List a -> List (List a)

Split on the given element.

``````splitOn 0 [1,0,2,0,0,3]
``````
splitAt : (n : Nat) -> (xs : List a) -> (List a, List a)

A tuple where the first element is a List of the n first elements and
the second element is a List of the remaining elements of the list
It is equivalent to (take n xs, drop n xs)

n

the index to split at

xs

the list to split in two

split : (a -> Bool) -> List a -> List (List a)

Split on any elements that satisfy the given predicate.

``````split (<2) [2,0,3,1,4]
``````
span : (a -> Bool) -> List a -> (List a, List a)

Given a list and a predicate, returns a pair consisting of the longest
prefix of the list that satisfies a predicate and the rest of the list.

``````span (<3) [1,2,3,2,1]
``````
sorted : Ord a => List a -> Bool

Check whether a list is sorted with respect to the default ordering for the type of its elements.

sortBy : (cmp : a -> a -> Ordering) -> (xs : List a) -> List a

Sort a list using some arbitrary comparison predicate.

cmp

how to compare elements

xs

the list to sort

sort : Ord a => List a -> List a

Sort a list using the default ordering for the type of its elements.

scanl1 : (a -> a -> a) -> List a -> List a

The scanl1 function is a variant of scanl that doesn't require an explicit
starting value.
It assumes the first element of the list to be the starting value and then
starts the fold with the element following it.

``````scanl1 (+) [1,2,3,4]
``````
scanl : (b -> a -> b) -> b -> List a -> List b

The scanl function is similar to foldl, but returns all the intermediate
accumulator states in the form of a list.

``````scanl (+) 0 [1,2,3,4]
``````
reverse : List a -> List a

Return the elements of a list in reverse order.

replicate : (n : Nat) -> (x : a) -> List a

Construct a list with `n` copies of `x`

n

how many copies

x

the element to replicate

replaceOn : Eq a => a -> a -> List a -> List a

Replaces all occurences of the first argument with the second argument in a list.

``````replaceOn '-' ',' ['1', '-', '2', '-', '3']
``````
partition : (a -> Bool) -> List a -> (List a, List a)

The partition function takes a predicate a list and returns the pair of
lists of elements which do and do not satisfy the predicate, respectively;
e.g.,

``````partition (<3) [0, 1, 2, 3, 4, 5]
``````
nubBy : (a -> a -> Bool) -> List a -> List a

The nubBy function behaves just like nub, except it uses a user-supplied

nub : Eq a => List a -> List a

O(n^2). The nub function removes duplicate elements from a list. In
particular, it keeps only the first occurrence of each element. It is a
special case of nubBy, which allows the programmer to supply their own
equality test.

``````nub (the (List _) [1,2,1,3])
``````
nonEmpty : (xs : List a) -> Dec (NonEmpty xs)

Decide whether a list is non-empty

mergeBy : (a -> a -> Ordering) -> List a -> List a -> List a

Merge two sorted lists using an arbitrary comparison
predicate. Note that the lists must have been sorted using this

merge : Ord a => List a -> List a -> List a

Merge two sorted lists using the default ordering for the type of their elements.

mapPreservesLength : (f : a -> b) -> (l : List a) -> length (map f l) = length l

Mapping a function over a list doesn't change its length.

mapMaybe : (a -> Maybe b) -> List a -> List b

Apply a partial function to the elements of a list, keeping the ones at which
it is defined.

mapFusion : (f : b -> c) -> (g : a -> b) -> (l : List a) -> map f (map g l) = map (f . g) l

Mapping two functions is the same as mapping their composition.

mapDistributesOverAppend : (f : a -> b) -> (l : List a) -> (r : List a) -> map f (l ++ r) = map f l ++ map f r

Mapping a function over two lists and appending them is equivalent
to appending them and then mapping the function.

lookupBy : (a -> a -> Bool) -> a -> List (a, b) -> Maybe b

Find associated information in a list using a custom comparison.

lookup : Eq a => a -> List (a, b) -> Maybe b

Find associated information in a list using Boolean equality.

listToMaybe : List a -> Maybe a

Either return the head of a list, or `Nothing` if it is empty.

list : (nil : Lazy b) -> (cons : Lazy (a -> List a -> b)) -> (xs : List a) -> b

Simply-typed recursion operator for lists.

nil

what to return at the end of the list

cons

what to do at each step of recursion

xs

the list to recurse over

lengthAppend : (left : List a) -> (right : List a) -> length (left ++ right) = length left + length right

The length of two lists that are appended is the sum of the lengths
of the input lists.

length : List a -> Nat

Compute the length of a list.

Runs in linear time

last' : (l : List a) -> Maybe a

Attempt to retrieve the last element of a non-empty list.

If the list is empty, return `Nothing`.

last : (l : List a) -> {auto ok : NonEmpty l} -> a

Retrieve the last element of a non-empty list.

ok

proof that the list is non-empty

isSuffixOfBy : (a -> a -> Bool) -> List a -> List a -> Bool
isSuffixOf : Eq a => List a -> List a -> Bool

The isSuffixOf function takes two lists and returns True iff the first list is a suffix of the second.

isPrefixOfBy : (eq : a -> a -> Bool) -> (left : List a) -> (right : List a) -> Bool

Check whether a list is a prefix of another according to a user-supplied equality test.

eq

the equality comparison

left

the potential prefix

right

the list that may have `left` as its prefix

isPrefixOf : Eq a => List a -> List a -> Bool

The isPrefixOf function takes two lists and returns True iff the first list is a prefix of the second.

isNil : List a -> Bool

Returns `True` iff the argument is empty

isInfixOf : Eq a => List a -> List a -> Bool

The isInfixOf function takes two lists and returns True iff the first list
is contained, wholly and intact, anywhere within the second.

``````isInfixOf ['b','c'] ['a', 'b', 'c', 'd']
``````
``````isInfixOf ['b','d'] ['a', 'b', 'c', 'd']
``````
isCons : List a -> Bool

Returns `True` iff the argument is not empty

intersperse : a -> List a -> List a

Insert some separator between the elements of a list.

``````with List (intersperse ',' ['a', 'b', 'c', 'd', 'e'])
``````
intercalate : (sep : List a) -> (xss : List (List a)) -> List a

Given a separator list and some more lists, produce a new list by
placing the separator between each of the lists.

sep

the separator

xss

the lists between which the separator will be placed

inits : List a -> List (List a)

The inits function returns all initial segments of the argument, shortest
first. For example,

``````inits [1,2,3]
``````
init' : (l : List a) -> Maybe (List a)

Attempt to Return all but the last element of a list.

If the list is empty, return `Nothing`.

init : (l : List a) -> {auto ok : NonEmpty l} -> List a

Return all but the last element of a non-empty list.

ok

proof that the list is non-empty

index' : (n : Nat) -> (l : List a) -> Maybe a

Attempt to find a particular element of a list.

If the provided index is out of bounds, return Nothing.

index : (n : Nat) -> (xs : List a) -> {auto ok : InBounds n xs} -> a

Find a particular element of a list.

ok

a proof that the index is within bounds

inBounds : (k : Nat) -> (xs : List a) -> Dec (InBounds k xs)

Decide whether `k` is a valid index into `xs`

head' : (l : List a) -> Maybe a

Attempt to get the first element of a list. If the list is empty, return
`Nothing`.

head : (l : List a) -> {auto ok : NonEmpty l} -> a

Get the first element of a non-empty list

ok

proof that the list is non-empty

hasAnyNilFalse : Eq a => (l : List a) -> hasAny [] l = False

No list contains an element of the empty list.

hasAnyByNilFalse : (p : a -> a -> Bool) -> (l : List a) -> hasAnyBy p [] l = False

No list contains an element of the empty list by any predicate.

hasAnyBy : (a -> a -> Bool) -> List a -> List a -> Bool

Check if any elements of the first list are found in the second, using
a custom comparison.

hasAny : Eq a => List a -> List a -> Bool

Check if any elements of the first list are found in the second, using
Boolean equality.

foldlMatchesFoldr : (f : b -> a -> b) -> (q : b) -> (xs : List a) -> foldl f q xs = foldlAsFoldr f q xs

The definition of foldl works the same as the default definition
in terms of foldr

foldlAsFoldr : (b -> a -> b) -> b -> List a -> b

Implement foldl using foldr, for a later equality proof.

findIndices : (a -> Bool) -> List a -> List Nat

Find the indices of all elements that satisfy some predicate.

findIndex : (a -> Bool) -> List a -> Maybe Nat

Find the index of the first element of a list that satisfies a predicate, or
`Nothing` if none do.

find : (a -> Bool) -> List a -> Maybe a

Find the first element of a list that satisfies a predicate, or `Nothing` if none do.

filterSmaller : (xs : List a) -> LTE (length (filter p xs)) (length xs)

A filtered list is no longer than its input

filter : (a -> Bool) -> List a -> List a

filter, applied to a predicate and a list, returns the list of those
elements that satisfy the predicate; e.g.,

``````filter (< 3) [Z, S Z, S (S Z), S (S (S Z)), S (S (S (S Z)))]
``````
elemIndicesBy : (a -> a -> Bool) -> a -> List a -> List Nat

Find all indices for an element in a list, using a custom equality test.

elemIndices : Eq a => a -> List a -> List Nat

Find all indices for an element in a list, using the default equality test for the type of list elements.

elemIndexBy : (a -> a -> Bool) -> a -> List a -> Maybe Nat

Find the index of the first occurrence of an element in a list, using a custom equality test.

elemIndex : Eq a => a -> List a -> Maybe Nat

Find the index of the first occurrence of an element in a list,
using the default equality test for the type of list elements.

elemBy : (a -> a -> Bool) -> a -> List a -> Bool

Check if something is a member of a list using a custom comparison.

elem : Eq a => a -> List a -> Bool

Check if something is a member of a list using the default Boolean equality.

dropWhile : (p : a -> Bool) -> List a -> List a

Remove the longest prefix of a list such that all removed elements satisfy some
Boolean predicate.

p

the predicate

drop : (n : Nat) -> (xs : List a) -> List a

Drop the first `n` elements of `xs`

If there are not enough elements, return the empty list.

n

how many elements to drop

xs

the list to drop them from

deleteBy : (a -> a -> Bool) -> a -> List a -> List a

The deleteBy function behaves like delete, but takes a user-supplied equality predicate.

delete : Eq a => a -> List a -> List a

`delete x` removes the first occurrence of `x` from its list argument. For
example,

``````delete 'a' ['b', 'a', 'n', 'a', 'n', 'a']
``````

It is a special case of deleteBy, which allows the programmer to supply
their own equality test.

consInjective : (x :: xs = y :: ys) -> (x = y, xs = ys)

(::) is injective

catMaybes : List (Maybe a) -> List a

Keep the `Just` elements in a list, discarding the `Nothing` elements.

break : (a -> Bool) -> List a -> (List a, List a)

Given a list and a predicate, returns a pair consisting of the longest
prefix of the list that does not satisfy a predicate and the rest of the
list.

``````break (>=3) [1,2,3,2,1]
``````
appendNilRightNeutral : (l : List a) -> l ++ [] = l

The empty list is a right identity for append.

appendAssociative : (l : List a) -> (c : List a) -> (r : List a) -> l ++ c ++ r = (l ++ c) ++ r

Appending lists is associative.

(\\) : Eq a => List a -> List a -> List a

The `\\` function is list difference (non-associative). In the result of
`xs \\ ys`, the first occurrence of each element of ys in turn (if any) has
been removed from `xs`, e.g.,

``````(([1,2] ++ [2,3]) \\ [1,2])
``````
Fixity
Non-associative, precedence 5
data NonEmpty : (xs : List a) -> Type

Satisfiable if `xs` is non-empty (e.g., if `xs` is a cons).

IsNonEmpty : NonEmpty (x :: xs)

The proof that a cons cell is non-empty

data List : (elem : Type) -> Type

Generic lists

Nil : List elem

Empty list

(::) : (x : elem) -> (xs : List elem) -> List elem

A non-empty list, consisting of a head element and the rest of
the list.

Fixity
Left associative, precedence 7
data InBounds : (k : Nat) -> (xs : List a) -> Type

Satisfiable if `k` is a valid index into `xs`

k

the potential index

xs

the list into which k may be an index

InFirst : InBounds 0 (x :: xs)

Z is a valid index into any cons cell

InLater : InBounds k xs -> InBounds (S k) (x :: xs)

Valid indices can be extended

(++) : List a -> List a -> List a

Append two lists

Fixity
Left associative, precedence 7