Functional arrays with constant-time update.

Diff arrays have an immutable interface, but rely on internal updates in place to provide fast functional update operator //.

When the // operator is applied to a diff array, its contents are physically updated in place. The old array silently changes its representation without changing the visible behavior: it stores a link to the new current array along with the difference to be applied to get the old contents.

So if a diff array is used in a single-threaded style, i.e. after // application the old version is no longer used, a!i takes O(1) time and a // d takes O(length d). Accessing elements of older versions gradually becomes slower.

Updating an array which is not current makes a physical copy. The resulting array is unlinked from the old family. So you can obtain a version which is guaranteed to be current and thus have fast element access by a // [].

Type synonyms for the two most important IO array types.

Module Data.Array.IArray provides the interface of diff arrays. They are instances of class IArray.

These are really internal functions, but you will need them to make further IArray instances of various diff array types (for either more MArray types or more unboxed element types).