This is a small extension that allows for inline macro operations.
The source for this extension is available here.
- (begin-syntax EXPR ...)syntax
Evaluates its body and immediately inserts the final result into the program, as though by macro expansion.
If the result is a macro transformer, then this is immediately invoked with no arguments and the result is inserted into the program. Note that in this case the final expression of the begin-syntax body will be evaluated twice.
This form can be thought of as a macro definition followed by its immediate expansion, for example:
(begin-syntax '(+ 1 2)) ; => (let-syntax ((a (er-macro-transformer ; (lambda (_ _ _) '(+ 1 2))))) ; (a))