:: [a] -> Int -> Maybe a

List index (subscript) operator, starting from 0. Returns Nothing if the index is out of bounds This is the total variant of the partial !! operator. WARNING: This function takes linear time in the index.

Examples

>>> ['a', 'b', 'c'] !? 0
Just 'a'

>>> ['a', 'b', 'c'] !? 2
Just 'c'

>>> ['a', 'b', 'c'] !? 3
Nothing

>>> ['a', 'b', 'c'] !? (-1)
Nothing

A total variant of the list index function (!!).

[2,3,4] !? 1    == Just 3
[2,3,4] !? (-1) == Nothing
[]      !? 0    == Nothing

List index (subscript) operator, starting from 0. Returns Nothing if the index is out of bounds

>>> ['a', 'b', 'c'] !? 0
Just 'a'

>>> ['a', 'b', 'c'] !? 2
Just 'c'

>>> ['a', 'b', 'c'] !? 3
Nothing

>>> ['a', 'b', 'c'] !? (-1)
Nothing

This is the total variant of the partial !! operator. WARNING: This function takes linear time in the index.

Safer version of !!, returns a Maybe. Get element from list using index value starting from `0`.

>>> [] !!? 0
Nothing

>>> ["a", "b", "c"] !!? 3
Nothing

>>> [1, 2, 3] !!? (-1)
Nothing

>>> ["a", "b", "c"] !!? 2
Just "c"

Lookup function (safe). O(min n index).

Safe version of (!!).

O(1) Safe indexing

A (!!) that fails using mzero

O(1) Indexing in a monad. The monad allows operations to be strict in the vector when necessary. Suppose vector copying is implemented like this:

copy mv v = ... write mv i (v ! i) ...

For lazy vectors, v ! i would not be evaluated which means that mv would unnecessarily retain a reference to v in each element written. With indexM, copying can be implemented like this instead:

copy mv v = ... do
x <- indexM v i
write mv i x

Here, no references to v are retained because indexing (but not the elements) is evaluated eagerly.

Assumed complexity: O(1) Yield the element at the given position in a monad. No range checks are performed. The monad allows us to be strict in the vector if we want. Suppose we had

unsafeIndex :: v a -> Int -> a

instead. Now, if we wanted to copy a vector, we'd do something like

copy mv v ... = ... unsafeWrite mv i (unsafeIndex v i) ...

For lazy vectors, the indexing would not be evaluated which means that we would retain a reference to the original vector in each element we write. This is not what we want! With basicUnsafeIndexM, we can do

copy mv v ... = ... case basicUnsafeIndexM v i of
Box x -> unsafeWrite mv i x ...

which does not have this problem because indexing (but not the returned element!) is evaluated immediately.

O(1) Indexing in a monad without bounds checks. See indexM for an explanation of why this is useful.

!!? with its arguments flipped. Get element from list using index value starting from `0`.

>>> maybeAt 0 []
Nothing

>>> maybeAt 3 ["a", "b", "c"]
Nothing

>>> maybeAt (-1) [1, 2, 3]
Nothing

>>> maybeAt 2 ["a", "b", "c"]
Just "c"

Deprecated: Use XMonad.Prelude.(!?) instead.

Safer version of !! -

O(n) Convert the first n elements of a list to a vector

fromListN n xs = fromList (take n xs)

Examples

>>> import qualified Data.Vector as V

>>> V.fromListN 3 [1,2,3,4,5::Int]
[1,2,3]

>>> V.fromListN 3 [1::Int]
[1]

>>> fromReverseListN 3 [1,2,3] :: Data.Vector.Vector Int
[3,2,1]