Typed tables

A table associates keys with values. Write its type as {[K]: V} or Table<K, V>, where K is the key type and V is the value type:

local scores: {[String]: Int} = {
    ada = 10,
    grace = 20,
}

In the implemented literal form, a field name such as ada becomes a string key when the expected table type uses String keys. As with arrays and record aggregates, the literal needs an expected type.

Looking up values

Indexing requires the exact key type and returns an optional value because the key may be missing:

local adaScore: Int? = scores["ada"]
local missing: Int? = scores["linus"]

A present key produces its Int; a missing key produces nil. Pop does not manufacture a default value.

Inserting and replacing

Indexed assignment inserts a missing key or replaces an existing value:

scores["grace"] = 21
scores["linus"] = 15

The table, key, and value expressions each run exactly once in that order. New entries preserve deterministic insertion order, while replacing a value does not move its key. The table can grow without changing its identity or invariant key/value types.

Table assignment never treats nil as deletion. When V is optional, a value that already has the complete optional type can be stored. The rc.3 checker does not widen a standalone nil literal to V?, so direct table[key] = nil is currently rejected. rc.3 has no deletion, key-inspection, or generalized iteration API. Table equality is not defined.

For data with a known set of named fields, use a record. For a fixed ordered collection, use an array. Use a table for a mutable association whose exact key and value types are known.

See Table lookup and mutation for focused examples of missing keys, insertion, replacement, and optional values.