Records

A record type groups a fixed set of named fields, each with its own type. Record types are introduced as type aliases, with the struct keyword followed by the fields between curly braces, separated by semicolons:

type rat = struct { n: real; d: real };

A record value is constructed by giving a value to each field, using the type name followed by the field assignments (note that fields are assigned with =):

r = rat { n = 1.0; d = 2.0 };

The field f of a record r is accessed with the dot syntax r.f. For instance, r.n evaluates to 1.0 for the record r above.

Element update

A new record that is identical to an existing one except at selected fields can be constructed with the element update syntax r[f := v]. It denotes a copy of the record r in which field f is replaced by v; the original record r is not modified. For example,

type rat = struct { n: real; d: real };
node x (r: rat) returns (y: rat)
let
  y = r[n := r.n * 2.0];
tel

defines y to be equal to r except that its n field is doubled. Several fields can be updated at once by separating the individual updates with semicolons, as in r[n := 1.0; d := 2.0].