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Object-parameterized value classes can provide expressions
constituting default values for their parameters. For example:
value class A|B b=someBExpression| {
In particular, the type of an object parameter of a value class A may
have a value class as its type which is or contains (by
equality-relevant value-to-value associations only) an A value.
Now, in order to construct this value of type A that is used somewhere
in the object parameters for the outermost A value's type, that A has
to have its object parameters set in turn. This, again, requires to
construct another A value that goes into its type's parameters, and so
This recursion can only end if at one point the same A value is used
as object parameter that occurs somewhere up in the object/parameter
tree, creating a cycle.
For example:
value class Number|Number|1| precision| {
Here, the precision argument to the Number class is again of type
Number, with precision 1 which in turn is a Number value which has a
precision which in this case happens to be 1 which closes the loop.
Another example:
value class A|A|value A|someAExpression|()|| {
Here, A defines an object parameter that is again of value class
A. This means that the parameter's A-type needs to be parameterized as
this is not a definition but a use of class A. In the example above,
I've used "value A|someAExpression|()" as the parameter value. I've
cheated by writing "someAExpression" because this again somehow (and
be it through a call to some function or method) has to constitute a
value of an A-parametrization. This recursion can only end if at some
point the value descriptor that is used as object parameter is denoted
by identifying an A value "up the tree."
Mapping this to the Number example above, this works only because 1
has as precision parameter the value 1 (itself), and moreover, the
literal "1" implies this precision value and it therefore doesn't
explicitly have to be specified. For such cases, I may make this work
because parser and runtime may treat number literals special and
establish the cyclic parametrization correctly.
For user-defined and not interpreter-supprted object-parameterized
value classes this may be more tricky. To establish the cycle in
parameter values, an inner parameter value would have to be able to
refer to one of the enclosing parameter values. This would require
specific syntax support, such as using "this" inside the parameter
definition. This all sounds too complicated and hard to
understand. Besides, the only use case we currently have to this type
of recursion is the special case "Number."
It may be more useful to support the recursive Number declaration by
special case handling and otherwise forbid recursive object parameter