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A highly efficient immutable list implementing the Data.Array API.


purescript-rrb-list is PureScript bindings for the TypeScript library List which is a highly optimized implementation of the data-structure RRB-trees.

purescript-rrb-list implements the Data.Array API and can serve as a drop-in replacement for Data.Array.

Why use purescript-rrb-list

JavaScript's native arrays are designed for imperative programming. Due to the nature of arrays they cannot benefit from structural sharing and any operation that changes an array must copy the entire array.

RRB-trees, which purescript-rrb-list implements, offers good time complexity and low constants for a wide range of operations. This makes it very well suited as a general purporse immutable list.

The table below compares the running-time of key operations.

Operation purescript-rrb-list purescript-array purescript-list
cons O(1) O(n) O(1)
snoc O(1) O(n) O(n)
append O(log(max(n, m)) O(n + m) O(n + m)
slice O(log(n)) O(n) O(n)
insertAt O(log(n)) O(n) O(n)
head O(1) O(1) O(1)
last O(1) O(1) O(n)

When not to use purescript-rrb-list

purescript-rrb-list is very fast for all operations which makes it suitable as a go-to general purpose immutable list. However, this means that for use cases that only require few operations it is often possible to find a specialized data-structure that is faster.

For instance, even though purescript-rrb-list has O(1) running-time for both cons and tail a single-linked list has lower constants for these two operations. Thus for use cases that only require a stack a single-linked list is faster.

Use purescript-rrb-list if:

  • You need a wide range of operations. In this case RRB-trees cannot be beat.
  • If performance isn't a major concern. In this case you will benefit from the generality of RRB-trees and get good performance without having to think about it.

Do not use purescript-rrb-list if:

  • You have more specialized use-case that only require few operations and performance is critical. In this case it is likely that you can find a specialized data-structure that performs better.


First install the TypeScript library from npm.

npm install list

And then the PureScript bindings from Bower.

bower install --save purescript-rrb-list


The TypeScript library on which purescript-rrb-list is based provides an extensive benchmark suite that compares the implementation to JavaScript arrays and other immutable lists.

purescript-rrb-list has also been inserted into the benchmark suite by purescript-sequences. The results can be seen in this comment.

API Docs

API documentation is published on Pursuit.