3 Type Model🔗ℹ

Rhombus is not a statically typed language, but it has a type model in the sense of facilities for connecting expressions with the values those expressions will produce. This connection allows a field access to be resolved at compile time to a field of a particular class, for example, instead of waiting until run time to discover the target object’s class. The main distinction between this capability and a statically typed language, at least in the usual sense, is that Rhombus offers only limited guarantees that static predictions about values will be correct. Rhombus is a safe language,Although Rhombus is safe by default, unsafe facilities or the use of an use_unsafe declaration can opt into unsafe mode. because run-time checks will enforce predictions where the compiler cannot prove that they will hold, but limited guarantees mean that the checks can fail.

The connection between run-time values and syntactic forms and run-time values is implemented in Rhombus by a set of protocols layered on the syntax model:

• static information as a key–value mapping associated with an expression or a binding, where expression information is communicated to enclosing forms, and binding information is communicated to uses of bound names;

• macro-extensible binding forms, where the parsing protocol for bindings communicates static information about an input expression (such as the right-hand side of a def form) and static information about identifiers that are bound from those inputs (such as identifiers on the left-hand side of a def form); and

• annotations, which pair a run-time predicate or converter with static information in a way that can be plugged into both expression and binding forms.