record

Syntax

Records are written using curly braces {} with named fields. Each field is written as name: value and fields are separated by commas.

{ x: 1, y: true }
{ name: "Alice", age: 42, active: true }

When parsed and rendered, this code will appear as:

{x:1, y:true}{name:"Alice", age:42, active:true}

Records may be written inline or across multiple lines for readability:

{
  x: 1.2,
  y: true,
  label: "pt1"
}

Kind

In general, a record has the kind:

<{field1<kind1>,field2<kind2>}>

The kind of a record is determined by the names and kinds of its fields. The kind of a filed is inferred from its value, but can also be specified explicitly. For example, the following record has two fields, x of kind f64 and y of kind bool. Therefore the kind is {x y}.

The kind of a field can be specified with a type annotation:

Custom record kinds can be defined:

Construction

There are three ways to create a record:

• Record literals

• Selecting a row from a table

• Comprehensions

Record Literals

Record literals are written using curly braces {} with named fields. Each field is written as name: value and fields are separated by commas.

Field kinds can be specified explicitly:

Nested Records

Records can be nested:

Nested records can also be written across multiple lines for readability:

r := {
  pos: {
    x: 1
    y: 2
  }
  label: "A"
}

From Tables

Selecting a single row from a table produces a record whose fields correspond to the table columns.

Record Comprehensions

Set comprehensions can be used to create sets of records. Each element in the resulting set is a record constructed from the comprehension's output expression.

Likewise, list comprehensions can create lists of records:

my-list := [1 2 3]
[{x: i, y: i * i } | i <- my-list]

Accessing Fields

Fields are accessed using dot indexing ..

Dot Indexing

Likewise, y can be accessed to return a bool value:

Nested Records

By chaining dot indexing, nested fields can be accessed:

Assigning Fields

Records are immutable unless defined with the ~ operator.

If you try to assign to a field of an immutable record, an error is raised:

r:={x:1, y:2}
        r.x
        =
        42
      --
 Error: cannot assign to field of immutable record

When records originate from tables, updates are performed at the table level, not on the record value itself.

In this example, updating r.x also updates the corresponding value in table T.

Records and Tables

Tables are conceptually sets of records. Each row in a table corresponds to one record with the same field names as the table columns.

Here the type of T is a table with two columns, x of kind u8 and y of kind bool. A single row of the table can be selected to produce a record of a corresponding kind:

Selecting multiple rows produces a table, not a set of records: