iter

Overview ¶

Package iter provides basic definitions and operations related to iterators over sequences.

Iterators ¶

An iterator is a function that passes successive elements of a sequence to a callback function, conventionally named yield. The function stops either when the sequence is finished or when yield returns false, indicating to stop the iteration early. This package defines Seq and Seq2 (pronounced like seek—the first syllable of sequence) as shorthands for iterators that pass 1 or 2 values per sequence element to yield:

type (
	Seq[V any]     func(yield func(V) bool)
	Seq2[K, V any] func(yield func(K, V) bool)
)

Seq2 represents a sequence of paired values, conventionally key-value or index-value pairs.

Yield returns true if the iterator should continue with the next element in the sequence, false if it should stop.

For instance, maps.Keys returns an iterator that produces the sequence of keys of the map m, implemented as follows:

func Keys[Map ~map[K]V, K comparable, V any](m Map) iter.Seq[K] {
	return func(yield func(K) bool) {
		for k := range m {
			if !yield(k) {
				return
			}
		}
	}
}

Further examples can be found in The Go Blog: Range Over Function Types.

Iterator functions are most often called by a range loop, as in:

func PrintAll[V any](seq iter.Seq[V]) {
	for v := range seq {
		fmt.Println(v)
	}
}

Naming Conventions ¶

Iterator functions and methods are named for the sequence being walked:

// All returns an iterator over all elements in s.
func (s *Set[V]) All() iter.Seq[V]

The iterator method on a collection type is conventionally named All, because it iterates a sequence of all the values in the collection.

For a type containing multiple possible sequences, the iterator's name can indicate which sequence is being provided:

// Cities returns an iterator over the major cities in the country.
func (c *Country) Cities() iter.Seq[*City]

// Languages returns an iterator over the official spoken languages of the country.
func (c *Country) Languages() iter.Seq[string]

If an iterator requires additional configuration, the constructor function can take additional configuration arguments:

// Scan returns an iterator over key-value pairs with min ≤ key ≤ max.
func (m *Map[K, V]) Scan(min, max K) iter.Seq2[K, V]

// Split returns an iterator over the (possibly-empty) substrings of s
// separated by sep.
func Split(s, sep string) iter.Seq[string]

When there are multiple possible iteration orders, the method name may indicate that order:

// All returns an iterator over the list from head to tail.
func (l *List[V]) All() iter.Seq[V]

// Backward returns an iterator over the list from tail to head.
func (l *List[V]) Backward() iter.Seq[V]

// Preorder returns an iterator over all nodes of the syntax tree
// beneath (and including) the specified root, in depth-first preorder,
// visiting a parent node before its children.
func Preorder(root Node) iter.Seq[Node]

Single-Use Iterators ¶

Most iterators provide the ability to walk an entire sequence: when called, the iterator does any setup necessary to start the sequence, then calls yield on successive elements of the sequence, and then cleans up before returning. Calling the iterator again walks the sequence again.

Some iterators break that convention, providing the ability to walk a sequence only once. These “single-use iterators” typically report values from a data stream that cannot be rewound to start over. Calling the iterator again after stopping early may continue the stream, but calling it again after the sequence is finished will yield no values at all. Doc comments for functions or methods that return single-use iterators should document this fact:

// Lines returns an iterator over lines read from r.
// It returns a single-use iterator.
func (r *Reader) Lines() iter.Seq[string]

Pulling Values ¶

Functions and methods that accept or return iterators should use the standard Seq or Seq2 types, to ensure compatibility with range loops and other iterator adapters. The standard iterators can be thought of as “push iterators”, which push values to the yield function.

Sometimes a range loop is not the most natural way to consume values of the sequence. In this case, Pull converts a standard push iterator to a “pull iterator”, which can be called to pull one value at a time from the sequence. Pull starts an iterator and returns a pair of functions—next and stop—which return the next value from the iterator and stop it, respectively.

For example:

// Pairs returns an iterator over successive pairs of values from seq.
func Pairs[V any](seq iter.Seq[V]) iter.Seq2[V, V] {
	return func(yield func(V, V) bool) {
		next, stop := iter.Pull(seq)
		defer stop()
		for {
			v1, ok1 := next()
			if !ok1 {
				return
			}
			v2, ok2 := next()
			// If ok2 is false, v2 should be the
			// zero value; yield one last pair.
			if !yield(v1, v2) {
				return
			}
			if !ok2 {
				return
			}
		}
	}
}

If clients do not consume the sequence to completion, they must call stop, which allows the iterator function to finish and return. As shown in the example, the conventional way to ensure this is to use defer.

Standard Library Usage ¶

A few packages in the standard library provide iterator-based APIs, most notably the maps and slices packages. For example, maps.Keys returns an iterator over the keys of a map, while slices.Sorted collects the values of an iterator into a slice, sorts them, and returns the slice, so to iterate over the sorted keys of a map:

for _, key := range slices.Sorted(maps.Keys(m)) {
	...
}

Mutation ¶

Iterators provide only the values of the sequence, not any direct way to modify it. If an iterator wishes to provide a mechanism for modifying a sequence during iteration, the usual approach is to define a position type with the extra operations and then provide an iterator over positions.

For example, a tree implementation might provide:

// Positions returns an iterator over positions in the sequence.
func (t *Tree[V]) Positions() iter.Seq[*Pos[V]]

// A Pos represents a position in the sequence.
// It is only valid during the yield call it is passed to.
type Pos[V any] struct { ... }

// Pos returns the value at the cursor.
func (p *Pos[V]) Value() V

// Delete deletes the value at this point in the iteration.
func (p *Pos[V]) Delete()

// Set changes the value v at the cursor.
func (p *Pos[V]) Set(v V)

And then a client could delete boring values from the tree using:

for p := range t.Positions() {
	if boring(p.Value()) {
		p.Delete()
	}
}