Maps

A map links keys to values. Its type is map[K, V], and a literal lists key: value pairs in braces:

map[str, int] ages = {"ana": 31, "luis": 25}
ages["sofia"] = 40          // adds a key
ages["luis"] += 1           // changes a value
print(ages["luis"], len(ages), "ana" in ages)  // 26 3 true

Keys can be integers, bool, char, str, or plain enums, which are compared by their contents, or values of a type with hash and equals methods, such as one that derives Hash and Equal. Values can be any type. A map keeps its keys in the order they were first added, so printing or looping over it is predictable, and setting an existing key keeps its place.

Reading a key that is missing stops the program with a runtime error, as does a compound assignment such as += on a missing key, since it reads the old value; a plain = adds the key. These built-ins work on maps:

Built-in	Result	Behavior
`len(m)`	`i64`	the number of keys
`get(m, key, default)`	value type	the value for `key`, or `default` when it is missing
`remove(m, key)`	`bool`	removes `key` and says whether it was there
`keys(m)`	key array	the keys, in order
`values(m)`	value array	the values, in the same order

map[str, int] counts = {}
for word in split("a b a", " ") do
    counts[word] = get(counts, word, 0) + 1
end
print(counts["a"], counts["b"])  // 2 1

An empty literal {} needs a declared type, like []. Without one, the key and value types come from the entries. Maps are references, like arrays: passing a map to a function shares it, and it may be null. Maps cannot be printed directly.