Generic Classes and Records¶

This primer covers generic classes and records.

A class is generic if it contains a type alias, contains a field that is a parameter, or contains a field with no type and no initialization expression. The following three classes are each generic in one of these ways.

class TypeAliasField {
  type t;
  var a, b: t;
}

class ParamField {
  param p: int;
  var tup: p*int;
}

record UntypedField {
  var a;
}

To create objects from generic classes, the classes must be instantiated. This is accomplished by passing the type or parameter value of the generic fields as an argument to the default constructor. In the class with the untyped field, the class is instantiated using the types of the arguments representing the generic fields in the default constructor.

var taf  = new TypeAliasField(real, 1.0, 2.0);
var taf2 = new TypeAliasField(int, 3, 4);
writeln("taf = ", taf, ", taf2 = ", taf2);

var pf  = new ParamField(3);
var pf2 = new ParamField(2);
writeln("pf = ", pf, ", pf2 = ", pf2);

var uf  = new UntypedField(3.14 + 2.72i);
var uf2 = new UntypedField(new ParamField(2));
writeln("uf = ", uf, ", uf2 = ", uf2);

To specify a generic class type (without creating an instance), don’t use the new keyword and just specify the generic arguments. For fields that have no types, specify a type for that field, instead of a value.

var taf3: borrowed TypeAliasField(real)?;
var pf3: borrowed ParamField(3)?;
var uf3: UntypedField(complex);

taf3 = taf;
pf3 = pf;
uf3 = uf;

writeln("taf3 = ", taf3);
writeln("pf3 = ", pf3);
writeln("uf3 = ", uf3);