Records, pre-defining

Elsewhere declarations and recursive record types

A record type may be declared elsewhere.

  declare record point elsewhere

The elsewhere declaration does not create a record type. The full record declaration must be given before variables of the record type are declared. However, the elsewhere declaration makes the type name available for use in other record type declarations. This allows you to create recursive record types:

  declare record bar elsewhere
  declare record foo
   field integer a
   field bar x variable
  declare record bar
   field integer b
   field foo y

In this example, foo and bar are both record types. Foo has a field of type bar and bar has a field of type foo. This is perfectly legal and potentially useful. Notice however, that at least one of the fields of the mutually referenced types need to be declared variable. If not, each new foo creates a new bar and each new bar creates a new foo, to infinity. Circular data structures are permitted. However, it is your responsibility to see that they are properly bounded.

You can also create a record type which has a field of is own type. This is particularly useful if you want to create a tree:

  declare record node
   field stream name
   field node children variable

Records which have been declared elsewhere can be extended, but the extended types must also be declared elsewhere:

  declare record point elsewhere
  declare record pixel extends point elsewhere

This exposes the relationship between pixels and points without defining their fields. (The fields will need to be defined before either type can be used. The fields for the point record must be defined before those of pixel.)