:: [a] -> Int -> Maybe a -package:errors package:rio

O(1) Safe indexing

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.

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]