thp-lang.org/md/learn/data-structures/maps.md

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# Maps
Also known as Associative Arrays, or Objects in other languages.
All maps must have a definition, which define their fields and datatypes.
There can also be anonymous maps, which may contain any key.
## Named Maps
```thp
// Here we define a map, called Person
map Person {
String name,
String surname,
Int age,
}
// Here we declare an instance of a Person.
val john_doe = Person {
name: "John",
surname: "Doe",
age: 33,
}
// If the compiler can infer the type of a Map,
// we can omit its type
var Person mary_jane = .{
name: "Mary",
surname: "Jane",
age: 27,
}
```
To access the fields of a map we use square braces `[]`.
```thp
mary_jane["age"] += 1
print(mary_jane["name"]) // Mary
```
Or dot access `.` if the field's name is a valid identifier.
```thp
mary_jane.age += 1
print(mary_jane.name)
```
## Anonymous maps
An anonymous map allows us to store and retrieve any key of any datatype.
They are declared as `Map`.
```thp
val car = Map {
brand: "Toyota",
model: "Corolla",
year: 2012,
}
```
Anonymous maps can also can have their type omitted.
```thp
var car = .{
brand: "Toyota",
model: "Corolla",
year: 2012,
}
```
If the compiler encounters a map without a type (that is, `.{}`)
and doesn't expect a specific type, it will assume it is an
anonymous map.
We can freely assign fields to an anonymous map:
```thp
// Modify an existing field
car["year"] = 2015
// Create a new field
car["status"] = "used"
```
However, if we try to access a field of an anonymous map we'll get
a nullable type, and we must annotate it.
```thp
// This is ok, we are declaring what datatype we expect
String? car_status = car["status"]
// This won't work, the compiler doesn't know what datatype to use
var car_status = car["status"]
```
Instead, we can use the `get` function of the map, which expects a
datatype and returns that type as nullable
```thp
val car_status = car.get[String]("status")
```
Both ways to get a value will check that the key exists in the map,
and that it has the correct datatype. If either the key doesn't exist
or it has a different datatype, it will return `null`.
We can also use dynamic keys, following the same rules:
```thp
val generated_value = "key"
String? v = map[generated_value]
// or
val v = map[String](generated_value)
```