17.3.2 Using #lang reader

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► | 17 | Creating Languages

17.1 | Module Languages
17.2 | Reader Extensions
17.3 | Defining new #lang Languages

▼ | 17.3 | Defining new #lang Languages

17.3.1 | Designating a #lang Language
17.3.2 | Using #lang reader
17.3.3 | Using #lang s-exp syntax/ module-reader
17.3.4 | Installing a Language
17.3.5 | Source-Handling Configuration
17.3.6 | Module-Handling Configuration

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#### 17.3.2 Using #lang reader🔗ℹ

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 object 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"

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