So you want to write a Lisp interpreter. I did too, and then I wanted to write about it, so here we are with this post series.

I initially wanted to write a Lisp interpreter as an exercise. I decided to write V1 in C (based on these two series of blog posts).¹ After much pointer shenanigans and an unholy amount of curly braces later, it worked — but was significantly longer and harder to read than a Lisp interpreter should be. So I decided to write V2 in OCaml, a functional programming language descended from SML. The SML family of languages has some features like pattern matching and an extensive type system that make writing interpreters and compilers an absolute dream.

I’m going to walk through writing a fully-functional Lisp interpreter in OCaml. If you’re following along with the OCaml, great. If you want to try and build it in SML or Haskell or whatever, it shouldn’t be too hard to translate. If you want to follow along in C… well, good luck. It’ll be a lot more code.

I’m going to build up the interpreter in stages, starting with symbols and other literals, then moving to simple math, then finally writing the features to support a metacircular evaluator (meaning that you can write another Lisp interpreter in the Lisp you just wrote!).

Let’s begin!

We probably want our interaction with the interpreter to look something like this:

$ ./repl
> 4
4
> ^D
$

So it seems reasonable to have our main file consist of a function that can read an expression, evaluate it, print it, and continue. A Read-Eval-Print-Loop (REPL).

I found OCaml rather lacking in stream features, so I looked around the internet and found this lovely blog post that has a file stream implementation. I decided to improve on that.

Our file stream feature will carry a line number, have a char buffer, and also have an OCaml in_channel, which is more or less a glorified FILE *:

type stream =
  { mutable line_num: int; mutable chr: char list; chan: in_channel };;

Let’s start with a function that can read in a character:

let read_char stm =
    match stm.chr with
      | [] ->
              let c = input_char stm.chan in
              if c = '\n' then let _ = stm.line_num <- stm.line_num + 1 in c
              else c
      | c::rest ->
              let _ = stm.chr <- rest in c

Let’s take a look at that. There are two initial states that stm could be in when we want to read a character — either it has characters in the buffer, or it doesn’t.

If there are no characters in the buffer, we should read one character from the file stream. If it’s a newline, we should increment the line number. If there are characters in the buffer, we can pattern match against the list to get the first character, then remove it from the buffer.

Neat! But now we should probably have our corresponding unread_char function that we can use later for backtracking:

let unread_char stm c =
  stm.chr <- c :: stm.chr;;

This one will just concatenate the given character to the front of our char buffer.

Next up, we should have a function that trims all the leading whitespace before the text that we care about. We need this because Lisp is not whitespace-sensitive.

let is_white c =
  c = ' ' || c = '\t' || c = '\n';;

let rec eat_whitespace stm =
  let c = read_char stm in
  if is_white c then
    eat_whitespace stm
  else
    unread_char stm c;
    ();;

This will read whitespace characters and ignore them until it hits a non-whitespace character, at which point it will push it back on the buffer. See, told you it would be useful.

One last thing: we’ll want a type system to keep all of our different types of expressions straight. Right now we only have numbers, but we’ll add more later.

type lobject =
  | Fixnum of int

I think we’re ready to get started! Let’s read in a whole number:

let rec read_sexp stm =
  let is_digit c =
    let code = Char.code c in
    code >= Char.code('0') && code <= Char.code('9')
  in
  let rec read_fixnum acc =
    let nc = read_char stm in
    if is_digit nc
    then read_fixnum (acc ^ (Char.escaped nc))
    else
      let _ = unread_char stm nc in
      Fixnum(int_of_string acc)
  in
  eat_whitespace stm;
  let c = read_char stm in
  if is_digit c
  then read_fixnum (Char.escaped c)
  else raise (SyntaxError ("Unexpected char " ^ (Char.escaped c)));;

where Char.escaped turns a character into a string consisting of only that character.

That was a lot at once. Let’s break it down.

I defined several helper functions (is_digit, read_fixnum) inside of read_sexp because they’ll only really be useful there.

After the definitions we eat whitespace, then try and read in a number. If it fails, we just raise a SyntaxError. Oh well!

Speaking of SyntaxErrors, it’s really easy to define your own exceptions:

exception SyntaxError of string;;

For those unfamiliar with exceptions, they are a construct that changes the flow of execution in a program. When raised, they halt the program flow and go “up the chain” in function calls until they find a place that specifically looks out for them. If they don’t find one (as in this case), OCaml just stops.

Let’s give our program a whirl! Write a main function to be called on program start, and have it call read_sexp:

let main =
  let stm = { chr=[]; line_num=1; chan=stdin } in
  print_string "> ";
  flush stdout;
  let Fixnum(v) = read_sexp stm in
  print_string "Your int: ";
  print_int v;
  print_newline ();;

You have to do this annoying thing flush stdout because otherwise the > prompt will appear after you type your number in.

And let’s run it!

$ ocaml 00_fundamentals.ml
> 4
Your int: 4
$

Cool! Looks like it works. But what happens if we try and enter a non-number?

$ ocaml 00_fundamentals.ml
> k
Exception: SyntaxError "Unexpected char k".
$

And what happens if we try and enter a negative number?

$ ocaml 00_fundamentals.ml
> -123
Exception: SyntaxError "Unexpected char -".
$

Well, we don’t actually handle negative numbers right now. That’s definitely something that can be improved in the future.

Now let’s make it a REPL! How do we loop in OCaml? Recursion, of course!

let rec repl stm =
  print_string "> ";
  flush stdout;
  let Fixnum(v) = read_sexp stm in
  print_int v;
  print_newline ();
  repl stm;;

let main =
  let stm = { chr=[]; line_num=1; chan=stdin } in
  repl stm;;

And that seems to work just fine:

$ ocaml 00_fundamentals.ml
> 4
4
> 5
5
> Exception: End_of_file.
$

It looks like OCaml’s I/O functions raise the End_of_file exception if it encounters EOF (aka me hitting ^D or trying to pipe in a file), which we could handle if we wanted. But I think this is fine for now.

Download the code here if you want to mess with it.

Next up, booleans.