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 readtable – instead 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
"raquet-mlang.rkt"
#lang racket (provide (except-out (all-from-out racket) lambda) (rename-out [lambda function]))
and
"raquet.rkt"
#lang s-exp syntax/module-reader "raquet-mlang.rkt"
then
| #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 spcified 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:
"dollar-racket.rkt"
#lang s-exp syntax/module-reader racket #: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:
"store.rkt"
#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) $n*107/100+h$)