Language Oriented Programming: Building a #lang | Schema Programming Part 15

Racket's manifesto is Language Oriented Programming (LOP). The idea is: don't just solve the problem; design a language that makes the problem easy to solve.

What is a #lang?

When you write #lang racket, you are telling the Racket system to use the "racket" reader and expander. You can make your own!

A Simple Language: #lang stacker

Let's imagine a language that just pushes numbers onto a stack.

stacker.rkt (The implementation)

#lang racket
(provide (all-defined-out))

(define stack empty)

(define (push x)
  (set! stack (cons x stack)))

(define (pop)
  (define top (first stack))
  (set! stack (rest stack))
  top)

; Reader hook
(module reader syntax/module-reader
  stacker)

my-program.rkt (Using the language)

#lang stacker
(push 10)
(push 20)

Use syntax/module-reader to define how your language parses text. You can define entirely new syntaxes (like Python-style indentation or datalog logic) that compile down to Racket macros.

The Power of LOP

People have built:

• Scribble: A language for documentation (which these docs are often written in).
• Video: A language for video editing.
• Typed Racket: A statically typed dialect.

Summary

Building a language in Racket isn't a PhD thesis project; it's a weekend afternoon project. This capability makes Racket a unique tool in the programmer's arsenal.