17.3.2 Using #lang reader

The reader language for #lang is similar to s-exp, in that it acts as a kind of meta-language. Whereas s-exp lets a programmer specify a module language at the expander layer of parsing, reader lets a programmer specify a language at the reader level.

A #lang reader must be followed by a module path, and the specified module must provide two functions: read and read-syntax. The protocol is the same as for a #reader implementation, but for #lang, the read and read-syntax functions must produce a module form that is based on the rest of the input file for the module.

The following "literal.rkt" module implements a language that treats its entire body as literal text and exports the text as a data string:

"literal.rkt"

#lang racket

(require syntax/strip-context)

(provide (rename-out [literal-read read]

[literal-read-syntax read-syntax]))

(define (literal-read in)

(syntax->datum

(literal-read-syntax #f in)))

(define (literal-read-syntax src in)

(with-syntax ([str (port->string in)])

(strip-context

#'(module anything racket

(provide data)

(define data 'str)))))

The "literal.rkt" language uses strip-context on the generated module expression, because a read-syntax function should return a syntax obejct with no lexical context. Also, the "literal.rkt" language creates a module named anything, which is an arbitrary choice; the language is intended to be used in a file, and the longhand module name is ignored when it appears in a required file.

The "literal.rkt" language can be used in a module "tuvalu.rkt":

"tuvalu.rkt"

#lang reader "literal.rkt"

Technology!

System!

Perfect!

Importing "tuvalu.rkt" binds data to a string version of the module content:

> (require "tuvalu.rkt")

> data

"\nTechnology!\nSystem!\nPerfect!\n"