Immutable

This repository contains generic immutable collection types for Go. It includes List, Map, and SortedMap implementations. Immutable collections can provide efficient, lock free sharing of data by requiring that edits to the collections return new collections.

The collection types in this library are meant to mimic Go built-in collections such asslice and map. The primary usage difference between Go collections and immutable collections is that immutable collections always return a new collection on mutation so you will need to save the new reference.

Immutable collections are not for every situation, however, as they can incur additional CPU and memory overhead. Please evaluate the cost/benefit for your particular project.

Special thanks to the Immutable.js team as the List & Map implementations are loose ports from that project.

List

The List type represents a sorted, indexed collection of values and operates similarly to a Go slice. It supports efficient append, prepend, update, and slice operations.

Adding list elements

Elements can be added to the end of the list with the Append() method or added to the beginning of the list with the Prepend() method. Unlike Go slices, prepending is as efficient as appending.

// Create a list with 3 elements.
l := immutable.NewList[string]()
l = l.Append("foo")
l = l.Append("bar")
l = l.Prepend("baz")

fmt.Println(l.Len())  // 3
fmt.Println(l.Get(0)) // "baz"
fmt.Println(l.Get(1)) // "foo"
fmt.Println(l.Get(2)) // "bar"

Note that each change to the list results in a new list being created. These lists are all snapshots at that point in time and cannot be changed so they are safe to share between multiple goroutines.

Updating list elements

You can also overwrite existing elements by using the Set() method. In the following example, we'll update the third element in our list and return the new list to a new variable. You can see that our old l variable retains a snapshot of the original value.

l := immutable.NewList[string]()
l = l.Append("foo")
l = l.Append("bar")
newList := l.Set(2, "baz")

fmt.Println(l.Get(1))       // "bar"
fmt.Println(newList.Get(1)) // "baz"

Deriving sublists

You can create a sublist by using the Slice() method. This method works with the same rules as subslicing a Go slice:

l = l.Slice(0, 2)

fmt.Println(l.Len())  // 2
fmt.Println(l.Get(0)) // "baz"
fmt.Println(l.Get(1)) // "foo"

Please note that since List follows the same rules as slices, it will panic if you try to Get(), Set(), or Slice() with indexes that are outside of the range of the List.

Iterating lists

Iterators provide a clean, simple way to iterate over the elements of the list in order. This is more efficient than simply calling Get() for each index.

Below is an example of iterating over all elements of our list from above:

itr := l.Iterator()
for !itr.Done() {
	index, value, _ := itr.Next()
	fmt.Printf("Index %d equals %v\n", index, value)
}

// Index 0 equals baz
// Index 1 equals foo

By default iterators start from index zero, however, the Seek() method can be used to jump to a given index.

Efficiently building lists

If you are building large lists, it is significantly more efficient to use the ListBuilder. It uses nearly the same API as List except that it updates a list in-place until you are ready to use it. This can improve bulk list building by 10x or more.

b := immutable.NewListBuilder[string]()
b.Append("foo")
b.Append("bar")
b.Set(2, "baz")

l := b.List()
fmt.Println(l.Get(0)) // "foo"
fmt.Println(l.Get(1)) // "baz"

Builders are invalid after the call to List().

Map

The Map represents an associative array that maps unique keys to values. It is implemented to act similarly to the built-in Go map type. It is implemented as a Hash-Array Mapped Trie.

Maps require a Hasher to hash keys and check for equality. There are built-in hasher implementations for most primitive types such as int, uint, and string keys. You may pass in a nil hasher to NewMap() if you are using one of these key types.

Currently []byte keys are not supported since Go doesn't included them in constraints.Ordered unline strings. For now Ordered constraint it used even for Map where key order is unimportant but this could be relaxed in the future.

Setting map key/value pairs

You can add a key/value pair to the map by using the Set() method. It will add the key if it does not exist or it will overwrite the value for the key if it does exist.

Values may be fetched for a key using the Get() method. This method returns the value as well as a flag indicating if the key existed. The flag is useful to check if a nil value was set for a key versus a key did not exist.

m := immutable.NewMap[string,int](nil)
m = m.Set("jane", 100)
m = m.Set("susy", 200)
m = m.Set("jane", 300) // overwrite

fmt.Println(m.Len())   // 2

v, ok := m.Get("jane")
fmt.Println(v, ok)     // 300 true

v, ok = m.Get("susy")
fmt.Println(v, ok)     // 200, true

v, ok = m.Get("john")
fmt.Println(v, ok)     // nil, false

Removing map keys

Keys may be removed from the map by using the Delete() method. If the key does not exist then the original map is returned instead of a new one.

m := immutable.NewMap[string,int](nil)
m = m.Set("jane", 100)
m = m.Delete("jane")

fmt.Println(m.Len())   // 0

v, ok := m.Get("jane")
fmt.Println(v, ok)     // nil false

Iterating maps

Maps are unsorted, however, iterators can be used to loop over all key/value pairs in the collection. Unlike Go maps, iterators are deterministic when iterating over key/value pairs.

m := immutable.NewMap[string,int](nil)
m = m.Set("jane", 100)
m = m.Set("susy", 200)

itr := m.Iterator()
for !itr.Done() {
	k, v := itr.Next()
	fmt.Println(k, v)
}

// susy 200
// jane 100

Note that you should not rely on two maps with the same key/value pairs to iterate in the same order. Ordering can be insertion order dependent when two keys generate the same hash.

Efficiently building maps

If you are executing multiple mutations on a map, it can be much more efficient to use the MapBuilder. It uses nearly the same API as Map except that it updates a map in-place until you are ready to use it.

b := immutable.NewMapBuilder[string,int](nil)
b.Set("foo", 100)
b.Set("bar", 200)
b.Set("foo", 300)

m := b.Map()
fmt.Println(m.Get("foo")) // "300"
fmt.Println(m.Get("bar")) // "200"

Builders are invalid after the call to Map().

Implementing a custom Hasher

If you need to use a key type besides int, uint, or string then you'll need to create a custom Hasher implementation and pass it to NewMap() on creation.

Hashers are fairly simple. They only need to generate hashes for a given key and check equality given two keys.

type Hasher[K constraints.Ordered] interface {
	Hash(key K) uint32
	Equal(a, b K) bool
}

Please see the internal intHasher, uintHasher, stringHasher, and byteSliceHasher for examples.

Sorted Map

The SortedMap represents an associative array that maps unique keys to values. Unlike the Map, however, keys can be iterated over in-order. It is implemented as a B+tree.

Sorted maps require a Comparer to sort keys and check for equality. There are built-in comparer implementations for int, uint, and string keys. You may pass a nil comparer to NewSortedMap() if you are using one of these key types.

Currently []byte keys are not supported since Go doesn't included them in constraints.Ordered unline strings.

The API is identical to the Map implementation. The sorted map also has a companion SortedMapBuilder for more efficiently building maps.

Implementing a custom Comparer

If you need to use a key type besides int, uint, or string then you'll need to create a custom Comparer implementation and pass it to NewSortedMap() on creation.

Comparers on have one method—Compare(). It works the same as the strings.Compare() function. It returns -1 if a is less than b, returns 1 if a is greater than b, and returns 0 if a is equal to b.

type Comparer[K constraints.Ordered] interface {
	Compare(a, b K) int
}

Please see the internal intComparer, uintComparer, and stringComparer for examples.

Contributing

The goal of immutable is to provide stable, reasonably performant, immutable collections library for Go that has a simple, idiomatic API. As such, additional features and minor performance improvements will generally not be accepted. If you have a suggestion for a clearer API or substantial performance improvement, please open an issue first to discuss. All pull requests without a related issue will be closed immediately.

Please submit issues relating to bugs & documentation improvements.