Indexing

The Vec type allows access to values by (0-based) index. An example will be more explicit:

>>> vec <- fromFoldable @_ @_ @Int [0, 2, 4, 6]
>>> two <- unsafeRead vec 1
>>> unsafeWrite vec 1 7
>>> seven <- unsafeRead vec 1
>>> two + seven
9

However, be careful: if you try to access an index which isn't in the Vec, your program will exhibit undefined behaviour ("UB"), either returning garbage or segfaulting. You cannot do this:

  v <- fromFoldable [0, 2, 4, 6]
  unsafeRead v 6 -- this is UB
  

If you want safe access, use read:

>>> vec <- fromFoldable @_ @_ @Int [0, 2, 4, 6]
>>> read vec 1
Just 2
>>> read vec 6
Nothing

Capacity and reallocation

The capacity of a vector is the amount of space allocated for any future elements that will be added onto the vector. This is not to be confused with the length of a vector, which specifies the number of actual elements within the vector. If a vector's length exceeds its capacity, its capacity will automatically be increased, but its elements will have to be reallocated.

For example, a vector with capacity 10 and length 0 would be an empty vector with space for 10 more elements. Pushing 10 or fewer elements onto the vector will not change its capacity or cause reallocation to occur. However, if the vector's length is increased to 11, it will have to reallocate, which can be slow. For this reason, it is recommended to use withCapacity whenever possible to specify how big the vector is expected to get.

Guarantees

Vec is a (pointer, capacity, length) triplet. The pointer will never be null.

Because of the semantics of the GHC runtime, creating a new vector will always allocate. In particular, if you construct a MutableArray-backed Vec with capacity 0 via new 0, fromFoldable [], or shrinkToFit on an empty Vec, the 16-byte header to GHC 'ByteArray#' will be allocated, but nothing else (for the curious: this is needed for 'sameMutableByteArray#' to work). Similarly, if you store zero-sized types inside such a Vec, it will not allocate space for them. Note that in this case the Vec may not report a capacity of 0. Vec will allocate if and only if 'sizeOf (undefined :: a) * capacity > 0.

If a Vec has allocated memory, then the memory it points to is on the heap (as defined by GHC's allocator), and its pointer points to length initialised, contiguous elements in order (i.e. what you would see if you turned it into a list), followed by capacity - length logically uninitialised, contiguous elements.

Vec will never automatically shrink itself, even if completely empty. This ensures no unnecessary allocations or deallocations occur. Emptying a Vec and then filling it back up to the same length should incur no calls to the allocator. If you wish to free up unused memory, use shrinkToFit.

push will never (re)allocate if the reported capacity is sufficient. push will (re)allocate if length == capacity. That is, the reported capacity is completely accurate, and can be relied on. Bulk insertion methods may reallocate, even when not necessary.

Vec does not guarantee any particular growth strategy when reallocating when full, nor when reserve is called. The strategy is basic and may prove desirable to use a non-constant growth factor. Whatever strategy is used will of course guarantee O(1) amortised push.

fromFoldable and withCapacity will produce a Vec with exactly the requested capacity.

Vec will not specifically overwrite any data that is removed from it, but also won't specifically preserve it. Its uninitialised memory is scratch space that it may use however it wants. It will generally just do whatever is most efficient or otherwise easy to implement. Do not rely on removed data to be erased for security purposes. Even if a Vec drops out of scope, its buffer may simply be reused by another Vec. Even if you zero a Vecs memory first, this may not actually happen when you think it does because of Garbage Collection.

\(O(1)\). Constructs a new, empty Vec s a.

>>> vec <- new @_ @_ @Int
>>> assertM (== 0) (length vec)
>>> assertM (== 0) (capacity vec)

\(O(1)\). Constructs a new, empty Vec arr s a.

The vector will be able to hold exactly capacity elements without reallocating. It is important to note that althrough the returned vector has the capacity specified, with vector will have a zero length.

>>> vec <- withCapacity @_ @_ @Int 10
-- The vector contains no items, even though it has capacity for more
>>> assertM (== 0) (length vec)
>>> assertM (== 10) (capacity vec)
-- These are all done without reallocating...
>>> forM_ [1..10] $ \i -> push vec i
>>> assertM (== 10) (length vec)
>>> assertM (== 10) (capacity vec)
-- ..but this may make the vector reallocate
>>> push vec 11
>>> assertM (== 11) (length vec)
>>> assertM (>= 11) (capacity vec)

\(O(1)\). Returns the number of elements in the vector.

\(O(1)\). Returns the maximum number of elements the vector can hold without reallocating.

>>> vec <- withCapacity @_ @_ @Word 10
>>> push vec 42
>>> assertM (== 10) (capacity vec)

\(O(n)\). Reserves capacity for at least additional more elements to be inserted in the given Vec s a. The collection may reserve more space to avoid frequent reallocations. After calling reserve, capacity will be greater than or equal to length + additional. Does nothing if capacity is already sufficient.

>>> vec <- fromFoldable @_ @_ @Int @_ [1]
>>> reserve vec 10
>>> assertM (>= 11) (capacity vec)

\(O(n)\). Reserves the minimum capacity for exactly additional more elements to be inserted in the given Vec s a. After calling reserveExact, capacity will be greater than or equal to length + additional. Does nothing if the capacity is already sufficient.

Capacity cannot be relied upon to be precisely minimal. Prefer reserve if future insertions are expected.

>>> vec <- fromFoldable @_ @_ @Int @_ [1]
>>> reserveExact vec 10
>>> assertM (>= 11) (capacity vec)

\(O(n)\). Shrinks the capacity of the vector as much as possible.

>>> vec <- withCapacity @_ @_ @Int 10
>>> extend vec [1, 2, 3]
>>> assertM (== 10) (capacity vec)
>>> shrinkToFit vec
>>> assertM (== 3) (capacity vec)

\(O(n)\). Shrinks the capacity of the vector with a lower bound.

The capacity will remain at least as large as both the length and the supplied value.

If the current capacity is less than the lower limit, this is a no-op.

>>> vec <- withCapacity @_ @_ @Int 10
>>> extend vec [1, 2, 3]
>>> assertM (== 10) (capacity vec)
>>> shrinkTo vec 4
>>> assertM (>= 4) (capacity vec)
>>> shrinkTo vec 0
>>> assertM (>= 3) (capacity vec)

\(O(1)\). Return the element at the given position.

If the element index is out of bounds, this function will segfault.

The bounds are defined as [0, length).

This function is intended to be used in situations where you have manually proved that your indices are within bounds, and you don't want to repeatedly pay for bounds checking.

Consider using read instead.

\(O(1)\). Return the element at the given position, or Nothing if the index is out of bounds.

The bounds are defined as [0, length).

\(O(1)\). Write a value to the vector at the given position.

If the index is out of bounds, this function will segfault.

The bounds are defined as [0, length). If you want to add an element past length - 1, use push or extend, which can intelligently handle resizes.

This function is intended to be used in situations where you have manually proved that your indices are within bounds, and you don't want to repeatedly pay for bounds checking.

Consider using write instead.

\(O(1)\). Write a value to the vector at the given position.

If the index is in bounds, this returns Just (). If the index is out of bounds, this returns Nothing.

The bounds are defined as [0, length). If you want to add an element past length - 1, use push or extend, which can intelligently handle resizes.

Amortised \(O(1)\). Appends an element to the vector.

>>> vec <- fromFoldable @_ @_ @Int [1, 2]
>>> push vec 3
>>> assertM (== [1, 2, 3]) (toList vec)

\(O(1)\). Removes the last element from a vector and returns it, or Nothing if it is empty.

>>> vec <- fromFoldable @_ @_ @Int [1, 2, 3]
>>> assertM (== Just 3) (pop vec)
>>> assertM (== [1, 2]) (toList vec)

\(O(m)\). Extend the vector with the elements of some Foldable structure.

>>> vec <- fromFoldable @_ @_ @Char ['a', 'b', 'c']
>>> extend vec ['d', 'e', 'f']
>>> assertM (== "abcdef") (toList vec)

\(O(n)\). Create a vector with the elements of some Foldable structure.

>>> vec <- fromFoldable @_ @_ @Int [1..10]
>>> assertM (== [1..10]) (toList vec)

\(O(n)\). Create a list with the elements of the vector.

>>> vec <- new @_ @_ @Int
>>> extend vec [1..5]
>>> toList vec
[1,2,3,4,5]

\(O(n)\). Create a lifted Vector copy of a vector.

\(O(n)\). Create a lifted Vector copy of a vector by applying a function to each element and its corresponding index.

\(O(n)\). Create a Primitive Vector copy of a vector.

\(O(n)\). Create a Primitive Vector copy of a vector by applying a function to each element and its corresponding index.

\(O(n)\). Create a Storable Vector copy of a vector.

\(O(n)\). Create a Storable Vector copy of a vector by applying a function to each element and its corresponding index.

\(O(n)\). Map over an array, modifying the elements in place.

>>> vec <- fromFoldable @_ @_ @Int [1..10]
>>> map (+ 1) vec
>>> assertM (== [2, 3 .. 11]) (toList vec)

\(O(n)\). Map strictly over an array, modifying the elements in place.

>>> vec <- fromFoldable @_ @_ @Int [1..10]
>>> map' (* 2) vec
>>> assertM (== [2, 4 .. 20]) (toList vec)

\(O(n)\). Map over an array with a function that takes the index and its corresponding element as input, modifying the elements in place.

>>> vec <- fromFoldable @_ @_ @Int [1..10]
>>> imap (\ix el -> ix + 1) vec
>>> assertM (== zipWith (+) [0..] [1..10]) (toList vec)

\(O(n)\). Map strictly over an array with a function that takes the index and its corresponding element as input, modifying the elements in place.

>>> vec <- fromFoldable @_ @_ @Int [1..10]
>>> imap' (\ix el -> ix + 1) vec
>>> assertM (== zipWith (+) [0..] [1..10]) (toList vec)