Binding syntactic keywords

Define-syntax, let-syntax, and letrec-syntax are analogous to define, let, and letrec, but they bind syntactic keywords to macro transformers instead of binding variables to locations that contain values. Furthermore, there is no define-syntax analogue of the internal definitions described in section [*].


\begin{rationale} As discussed below, the syntax and scope rules for definitions... ...}, with scope rules analogous to those for internal definitions. \end{rationale}

These new expression types and the pattern language described in section [*] are added to Scheme by augmenting the BNF in section [*] with the following new productions. Note that the identifier ... used in some of these productions is not a metasymbol.


\begin{grammar}% \meta{expression} \: \meta{macro use} \> \Vert \meta{macro bloc... ...nsformer spec}) \> \Vert (begin \arbno{\meta{syntax definition}})% \end{grammar}

Although macros may expand into definitions in any context that permits definitions, it is an error for a definition to shadow a syntactic keyword whose meaning is needed to determine whether some definition in the group of top-level or internal definitions that contains the shadowing definition is in fact a definition, or is needed to determine the boundary between the group and the expressions that follow the group. For example, the following are errors:


\begin{scheme} (define define 3) \par (begin (define begin list)) \par (let-synt... ...t ((x 3)) (foo (plus x y) (+ x y)) (define foo x) (plus foo x))) \end{scheme}


\begin{entry}{% \proto{let-syntax}{ \hyper{bindings} \hyper{body}}{\exprtype}} \... ...s () ((m) x)))) (let ((x 'inner)) (m)))) \ev outer\end{scheme}\par \end{entry}


\begin{entry}{% \proto{letrec-syntax}{ \hyper{bindings} \hyper{body}}{\exprtype}... ... (if even?)) (or x (let temp) (if y) y))) \ev 7\end{scheme}\par \end{entry}


\begin{entry}{% \proto{define-syntax}{ \hyper{keyword} \hyper{transformer spec}}... ...1)) (let* ((name2 val2) ...) body1 body2 ...))))) \end{scheme}\par \end{entry}