Pair

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Pair is an immutable, fixed-size collection with exactly two elements (possibly of different types)

HHVM provides a native implementation for this class. The Hack class definition below is not actually used at run time; it is simply provided for the typechecker and for developer reference.

Like all objects in PHP, Pairs have reference-like semantics. The elements of a Pair cannot be mutated (i.e. you can't assign to the elements of a Pair) though Pairs may contain mutable objects.

Pairs only support integer keys. If a non-integer key is used, an exception will be thrown.

Pairs support $m[$k] style syntax for getting values by key. Pairs also support isset($m[$k]) and empty($m[$k]) syntax, and they provide similar semantics as arrays.

Pair keys are always 0 and 1, respectively.

You may notice that many methods affecting the instace of Pair return an ImmVector -- Pairs are essentially backed by 2-element ImmVectors.

Guides

• Introduction
• Classes

Interface Synopsis

namespace HH;

final class Pair implements \ConstVector<mixed> {...}

Public Methods

• ->__toString(): string
  Returns the string version of the current Pair, which is "Pair"
• ->at(int $key): mixed
  Returns the value at the specified key in the current Pair
• ->concat<Tu super mixed>(Traversable<mixed, Tu> $traversable): ImmVector<mixed, Tu>
  Returns an ImmVector that is the concatenation of the values of the current Pair and the values of the provided Traversable
• ->containsKey<Tu super int>(Tu $key): bool
  Checks whether a provided key exists in the current Pair
• ->count(): int
  Returns 2; a Pair always has two values
• ->filter((function(mixed): bool) $callback): ImmVector<mixed>
  Returns a ImmVector containing the values of the current Pair that meet a supplied condition
• ->filterWithKey((function(int, mixed): bool) $callback): ImmVector<mixed>
  Returns an ImmVector containing the values of the current Pair that meet a supplied condition applied to its keys and values
• ->firstKey(): int
  Returns the first key in the current Pair
• ->firstValue(): Tv1
  Returns the first value in the current Pair
• ->get(int $key): mixed
  Returns the value at the specified key in the current Pair
• ->getIterator(): KeyedIterator<int, mixed>
  Returns an iterator that points to beginning of the current Pair
• ->immutable(): this
  Returns an immutable version of this collection
• ->isEmpty(): bool
  Returns false; a Pair cannot be empty
• ->items(): Iterable<mixed>
  Returns an Iterable view of the current Pair
• ->keys(): ImmVector<int>
  Returns an ImmVector with the values being the keys of the current Pair
• ->lastKey(): int
  Returns the last key in the current Pair
• ->lastValue(): Tv2
  Returns the last value in the current Pair
• ->lazy(): KeyedIterable<int, mixed>
  Returns a lazy, access elements only when needed view of the current Pair
• ->linearSearch<Tu super mixed>(mixed $search_value): int
  Returns the index of the first element that matches the search value
• ->map<Tu>((function(mixed): Tu) $callback): ImmVector<Tu>
  Returns an ImmVector containing the values after an operation has been applied to each value in the current Pair
• ->mapWithKey<Tu>((function(int, mixed): Tu) $callback): ImmVector<Tu>
  Returns an ImmVector containing the values after an operation has been applied to each key and value in the current Pair
• ->skip(int $n): ImmVector<mixed>
  Returns an ImmVector containing the values after the n-th element of the current Pair
• ->skipWhile((function(mixed): bool) $callback): ImmVector<mixed>
  Returns an ImmVector containing the values of the current Pair starting after and including the first value that produces true when passed to the specified callback
• ->slice(int $start, int $len): ImmVector<mixed>
  Returns a subset of the current Pair starting from a given key up to, but not including, the element at the provided length from the starting key
• ->take(int $n): ImmVector<mixed>
  Returns an ImmVector containing the first n values of the current Pair
• ->takeWhile((function(mixed): bool) $callback): ImmVector<mixed>
  Returns an ImmVector containing the values of the current Pair up to but not including the first value that produces false when passed to the specified callback
• ->toDArray(): darray<int, mixed>
• ->toImmMap(): ImmMap<int, mixed>
  Returns an immutable, integer-keyed map (ImmMap) based on the elements of the current Pair
• ->toImmSet(): ImmSet<arraykey, mixed>
  Returns an immutable set (ImmSet) with the values of the current Pair
• ->toImmVector(): ImmVector<mixed>
  Returns an immutable vector (ImmVector) containing the elements of the current Pair
• ->toKeysArray(): varray<int>
  Returns a varray whose values are the keys from the current Pair
• ->toMap(): Map<int, mixed>
  Returns an integer-keyed Map based on the elements of the current Pair
• ->toSet(): Set<arraykey, mixed>
  Returns a Set with the values of the current Pair
• ->toVArray(): varray
• ->toValuesArray<Tu>(): varray<Tu>
  Returns an varray containing the values from the current Pair
• ->toVector(): Vector<mixed>
  Returns a Vector containing the elements of the current Pair
• ->values(): ImmVector<mixed>
  Returns an ImmVector containing the values of the current Pair
• ->zip<Tu>(Traversable<Tu> $traversable): ImmVector<Pair<mixed, Tu>>
  Returns an ImmVector where each element is a Pair that combines each element of the current Pair and the provided Traversable