There are special forms called constructors that enable users to create and initialize "record", [tex2html_wrap2909]struct, "oneof", and "maybe" objects. A constructor has the form
<tagged_type_desig> "{" <field_inits> "}"
where
<field_inits> -> <field_init> ["," <field_init>]* <field_init> -> <idn> := <expr>The tagged_type_desig is the type of the constructed object. If a "struct" or "record" is being constructed, the component names in the field list must be exactly the field names in the tagged_type_desig, although the names may appear in any order in the constructor; the type of the initialization expression for a field must be a subtype of the declared type of that field. The expressions are evaluated in an unspecified order; the results form the components of the newly constructed object, which is the value of the constructor expression. For example,
rt = record[x: int, c: char]
x: rt := rt{c := 'A', x := 7} % legal
x := rt{x := 7} % compile-time error -- not enough fields
x := rt{x := 7, d: 'A'} % compile-time error -- misnamed field
If a "oneof" or "maybe"
is being constructed, just one field name of the type can
be present, and the type of the expression must be a subtype of the
declared type of that field; the result is a new "oneof" or "maybe"
object with
the given tag whose value is the object resulting from evaluating
the expression. For example,
ot = oneof[none: null, some: int]
x: ot := ot{none := nil} % x's object has tag none and value nil
x := ot{some: 7} % now x's object has tag some and value 7
x := ot{some: 3.1} % compile-time error -- expression has wrong type
Decomposition of "oneof" objects is usually done via the tagcase
statement (8.11).