travitch / persistent-vector

Persistent vectors for Haskell based on array mapped tries
BSD 3-Clause "New" or "Revised" License
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Persistent Vector

A library providing persistent (purely functional) vectors for Haskell based on array mapped tries.

Description

These persistent vectors are modeled on the persistent vector used by clojure, with an API modeled after Data.Sequence from the containers library. This data structure is spine strict and is not useful for incremental consumption. If you need that, stick to lists. It is still lazy in the elements.

While per-element operations are O(log(n)), the internal tree can never be more than 7 or 8 deep. Thus, they are effectively constant time.

This implementation adds O(1) slicing support for vectors that I do not believe clojure supports. The implementation cheats, though, and slices can retain references to objects that cannot be indexed.

Performance

Performance is an important consideration for a data structure like this. The package contains a criterion benchmark suite that attempts to compare the performance of persistent vectors against a variety of existing persistent data structures. As an overview of the results I have observed:

Overall, it seems like persistent vectors are efficient at most tasks. If you only need a (strict) left fold, they are efficient for traversal. Indexing and construction are very fast, but Sequences are superior for element-wise updates.

Implementation

TODO