4 Operator and Macro Transformers
Some contexts in a Rhombus language (likely including expression contexts) will support infix, prefix, and postfix operators. The Rhombus expander provides an enforestation framework for parsing forms that involve multiple operators, each with a declared precedence and associativity. The enforestation process also allows an operator transformer to completely take over parsing of terms that follow the operator within a shrubbery group. An “operator” in this sense can be named by either a shrubbery operator or a shrubbery identifier, so the prefix-operator protocol suffices for defining macros in a traditional sense.
For an infix operator, enforestation always parses the left-hand argument (i.e., the part before the operator) in the same context as the operator’s context. For example, the left-hand argument to an infix expression operator + or . is always parsed as an expression. For the right-hand (or only, in the case of prefix) argument, the operator’s mapping selects one of two protocols: automatic, where the right-hand argument is also parsed in the same context, or macro, where the operator’s transformer receives the full sequence of terms remaining in the enclosing group. An operator using the macro protocol parses remaining terms as it sees fit, and then it returns the still-remaining terms that it does not consume. For example, + for expressions is likely implemented as an automatic infix operator, since both of its arguments are also expressions, while . is likely implemented as a macro infix operator so that it’s right-hand “argument” is always parsed as a field identifier. In the earlier <> and -> examples, <> is implemented as an automatic infix operator for expressions, while <> for bindings and -> for expressions were implemented as macro infix operators.
Roughly, an operator that uses the macro protocol takes on some of the burden of dealing with precedence, at least for terms after the operator. For operators like . or ->, this is no problem, because the right-hand side has a fixed shape. Other operators may need to call back into the enforestation algorithm, and the Rhombus expander provides facilities to enable that.
A postfix operator is implemented as a macro infix operator that consumes no additional terms after the operator. For example, a postfix ! might be defined (shadowing the normal ! for “not”) as follows:
| factorial(0): 1
expr.macro '$a ! $tail ...':
Since the Rhombus expander provides a way for macro transformers to resume enforestation, all operators could be implemented with the macro protocol. The automatic protocol is just a convenient shortcut.
Some contexts might constrain the allowed forms of operators to prefix or infix, constrain the names used for operators, and/or eschew one of the operator protocols. For example, declaration and definition contexts might allow only macro prefix operators with identifier names. The Rhombus implementation makes that choice, and it also allows expression forms with operators to appear in the same places as declaration and definition forms.