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record

`record`

fastn supports record types. These are also called struct in some languages.

Declaring a `record`

Before a record can be used it must be declared using the record syntax:

Declaring a Person record

-- record person:
caption name:
integer age:
optional body bio:

Lang:

ftd

Here we are creating a record. The name of the record is person. It has three fields: name, age and bio.

Declaring a Company record

-- record company:
caption name:
person list employees:

Lang:

ftd

In this case, the name of the record is company. It has two fields: caption name and list of employees of person type.

Declaring a `record` with default values

Sometimes, the programmer might want to provide default values to the record fields, in case if he/she doesn't specify those during initialization.

Declaring a Person record with default field values

-- record person:
caption name: Undefined
integer age:
optional body bio: Not specified

;; << Alternative way >>
-- record person:
caption name: Undefined
integer age:

-- optional body person.bio:

No bio is specified for this person.

Lang:

ftd

Declaring a Company record with default field values

-- record company:
string name: FifthTry

-- person list company.employees:

-- person:
name: Arpita
age: 22

-- person:
name: Abrar
age: 24

-- end: company.employees

Lang:

ftd

Field Types

Fields can be either one of the built-in types, another type like a record or or-type.

Record Variable

A variable can be created with type record:

-- person john-snow: John Snow
age: 14

Lang:

ftd

Here we have created a new variable of type person, called it amitu, and the value of name, since its declared as caption in the record definition, is read from the "caption" area, and age is read from the "header".

Note that we have not passed bio, since bio is declared as optional body, so it's not a problem. Had it been just body the above would not have been valid.

Record Field Update Syntax

The field which needs to be updated has to be mutable before updating its value. An individual field of a record can be updated using a syntax like this:

-- $john-snow.age: 15

-- person $john-snow: John Snow
$age: 14

Lang:

ftd

Here we have used -- $john-snow.age: 15 to update a single field of a record.

This also works if the field is a list:

-- record person:
caption name:
string list alias:

-- person $john-snow: John Snow

-- $john-snow.alias:

-- string: Aegon Targaryen
-- string: Lord Crow
-- string: The White Wolf
-- string: The Prince That Was Promised

-- end: $john-snow.alias

Lang:

ftd

Reading A Record From Rust

A record in fastn is equivalent of a struct in Rust.

Rust Type

To read the above .ftd file from Rust you will have to first create a struct in Rust that is compatible with our person definition:

#[derive(serde::Deserialize)]
struct Person {
    name: String,
    age: i32,
    bio: Option<String>,
}

Lang:

For each field in person record, we have a corresponding field in our Person struct.

Note that we used age as i32, but it could have been any type that can be deserialised from JSON Number since fastn integer is converted to JSON Number.