sepisoad / micro-lisp

A very small Lisp programming language, in under 200 lines of C

Micro Lisp

Objective: implement a small Lisp/Scheme language in as little C code as possible.

The interpreter supports lambda, e.g.

  ((lambda (x) (cons x (quote 1))) (quote 7))
  (7 1)

Note that lambda does not capture free variables (variables that are not passed as arguments and refer to an outer scope). Free variables will resolve to their assigned values in the environment when the body of the lambda is evaluated.

The special forms if and quote behave in a typical way:

  (if (quote t) (quote 7) (quote 0))
  7

The only types are symbols and pairs.

Non-quoted symbols are looked up in the environment. If they have no associated value the result is null in fact zero. Because there is no numeric type a number for example 7 will be treated as any other symbol and looked up in the environment. Note in the examples above numbers are quoted to prevent that.

The built-in primitives in the environment are: car, cdr, cons, eq?, pair?, read, write.

Also provided is apply which takes a function and a single list argument:

  (apply write (quote ((hello world))))
  (hello world)
  (quote t)

Lists can be built up by consing:

  (apply write (cons (cons (quote hello) (cons (quote world) null)) null))
  (hello world)
  (quote t)

Read Eval Print Loop

A REPL is implemented in micro-lisp itself. To try it out in a terminal:

  cat repl.lisp - | ./micro-lisp

To exit, press 'control c' to terminate the process.

Note the - argument to cat to pipe stdin through, otherwise micro-lisp will receive end-of-file.

The source code for the REPL is in repl.lisp. It implements eval and provides an environment which resolves symbols to the primitive functions in the underlying micro-lisp interpreter.

Debugging with GDB

A .gdbinit file sets the target, breakpoints and runs the executable. Simply run gdb.