4.4 Fields and Properties🔗ℹ

A mutable declaration before a field enables assignment to the field using :=. An assignment can appear within methods using the field name directly, or it can appear on a . expression that accesses a field in an object.

Extra fields can be added to a class with field clauses. These fields are not represented the class’s constructor, and so a field has an expression to provide the field’s initial value. A field added with field is always mutatble, and := can be used to assign to the field.

Sometimes, you want a value associated to an object that is not stored in a field, but is still accessed and assigned with field-like notation. A property is like a field in that it is accessed without method-call parentheses, and it can also support assignment via :=. A property is different from a field, however, in that access and assignment can trigger arbitrary method-like comptation, and a property implementation can be overridden in a subclass.

A read-only property is written similar to a method, possibly with a result annotation, but without parentheses for arguments.

To define a property that supports assignment, use | similar to defining a function with fun and multiple cases. The first case should look like a simple property clause to implement property access, while the second case should resemble an assignment form. In the assignment-like form to define a property, the part after := is a binding position, just like a function or method argument.

A property can be a good choice for derived values like magnitude and angle in a Posn, because they require relatively little computation and are deterministically derived from the object’s fields. A property is probably not a good choice for a lookup action that involves visible side effects or that involves a lot of computation, because those features defeat the expectation of someone reading the code, because they see a field-like access.