17.3.3 Using #lang s-exp syntax/module-reader

Parsing a module body is usually not as trivial as in "literal.rkt". A more typical module parser must iterate to parse multiple forms for a module body. A language is also more likely to extend Racket syntax—perhaps through a readtableinstead of replacing Racket syntax completely.

The syntax/module-reader module language abstracts over common parts of a language implementation to simplify the creation of new languages. In its most basic form, a language implemented with syntax/module-reader simply specifies the module language to be used for the language, in which case the reader layer of the language is the same as Racket. For example, with


#lang racket
(provide (except-out (all-from-out racket) lambda)
         (rename-out [lambda function]))



#lang s-exp syntax/module-reader


#lang reader "raquet.rkt"
(define identity (function (x) x))
(provide identity)

implements and exports the identity function, since "raquet-mlang.rkt" exports lambda as function.

The syntax/module-reader language accepts many optional specifications to adjust other features of the language. For example, an alternate read and read-syntax for parsing the language can be specified with #:read and #:read-syntax, respectively. The following "dollar-racket.rkt" language uses "dollar.rkt" (see Readtables) to build a language that is like racket but with a $ escape to simple infix arithmetic:


#lang s-exp syntax/module-reader
#:read $-read
#:read-syntax $-read-syntax
(require (prefix-in $- "dollar.rkt"))

The require form appears at the end of the module, because all of the keyword-tagged optional specifications for syntax/module-reader must appear before any helper imports or definitions.

The following module uses "dollar-racket.rkt" to implement a cost function using a $ escape:


#lang reader "dollar-racket.rkt"
(provide cost)
; Cost of n' $1 rackets with 7% sales
; tax and shipping-and-handling fee h':
(define (cost n h)