Lispy (Build Your Own Lisp in C)

a small Lisp interpreter written in C, built following: Build Your Own Lisp.

Current implementation includes:

• numbers, symbols, strings
• S-expressions (...) and Q-expressions {...}
• variables (def, =)
• lambdas (\) with partial application
• variable arguments with &
• conditionals and comparisons (if, ==, !=, >, <, >=, <=)
• file loading (load)
• printing and user errors (print, error)
• line comments starting with ;

Files

• main.c - interpreter implementation
• mpc.c, mpc.h - parser combinator library used by the interpreter

Build

On macOS/Linux:

cc -std=c99 -Wall -Wextra main.c mpc.c -ledit -lm -o a.out

If -ledit is missing on your machine, install/editline first, then rebuild.

Language Notes

• Expressions are separated by whitespace/newlines, not commas.
• Anything non-zero is true in if.
• ; starts a comment until end of line.

Valid

"hello"
"hello\n"
(print "Hello" "world")

Invalid

"hello", "world"

The comma is not a token in this grammar.

Builtins

Core list/eval:

• list, head, tail, join, eval

Math:

• +, -, *, /

Variables/functions:

• def, =, \

Conditionals/comparison:

• if, ==, !=, >, <, >=, <=

Strings/IO:

• load, print, error

Example Session

def {add-mul} (\ {x y} {+ x (* x y)})
add-mul 10 20
; => 210

def {add-mul-ten} (add-mul 10)
add-mul-ten 50
; => 510

if (== 1 1) {print "yes"} {print "no"}

Example Script (hello.lspy)

; demo script
(print "Hello from file")
(def {x y} 100 200)
(print x y)
(if (== x y) {print "equal"} {print "not equal"})

Run it:

./a.out hello.lspy