RacketProjectInterpreterMeta-circular
Project 1: Build a Lisp Interpreter | Racket Projects
3.555 min read
Md Nasim Sheikh
"If you don't know how compilers work, then you don't know how computers work." — Steve Yegge.
Racket makes writing interpreters easy because its own syntax (S-expressions) is processed using the same tools we will use to process our language.
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The Goal: "Mini-Scheme"
We will support:
- Arithmetic:
(+ 1 2) - Definition:
(define x 10) - Conditionals:
(if (> x 5) 1 0) - Functions:
(lambda (x) (+ x 1))
The Environment
We need a way to store variables.
#lang racket
(define env (make-hash))
(hash-set! env '+ +)
(hash-set! env '* *)
(hash-set! env '> >)
The Evaluator (eval)
This is the heart of the beast.
(define (my-eval exp)
(match exp
; 1. Symbols (Variables)
[(? symbol?) (hash-ref env exp)]
; 2. Numbers (Self-evaluating)
[(? number?) exp]
; 3. Define
[(list 'define var val)
(hash-set! env var (my-eval val))]
; 4. If
[(list 'if test conseq alt)
(if (my-eval test)
(my-eval conseq)
(my-eval alt))]
; 5. Lambda (Functions)
[(list 'lambda (list param) body)
(lambda (arg)
(hash-set! env param arg)
(my-eval body))]
; 6. Function Application
[(list f arg)
((my-eval f) (my-eval arg))]
[else (error "Unknown expression" exp)]))
Running It
(my-eval '(define x 10))
(my-eval '(if (> x 5) 100 0)) ; Returns 100
Note: This basic implementation has a bug with closures (scope). Can you spot it? We share one global env. Real interpreters create a new environment for each function call!
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Summary
In 30 lines, we built a calculator that processes logic. This is the foundation of structure and interpretation of computer programs.
Quick Quiz
What is the name of an interpreter written in the same language that it interprets?
Written by
Md Nasim Sheikh
Software Developer at softexForge
Verified Author150+ Projects
Published: