Writing a Lisp, Part 1: Booleans

Last time we wrote a simple interpreter that could read in numbers. That’s cool and all, but we’ll need to read in more kinds of input to actually call this a Lisp interpreter. Like booleans, for example! And while we’re at it, we might as well fix our situation with negative numbers.

We’re going to want to input booleans like #t and #f for true and false, respectively. This addition of the # does two things:

1. It makes clear to the reader that this is not a variable, but a fundamental constant
2. It makes writing our boolean reader easier

But I suppose we’ll need to modify our reader to be able to handle booleans, now, won’t we. Why not just add a condition to check if the character c is a #?

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 if c = '#' then
      match (read_char stm) with
      | 't' -> Boolean(true)
      | 'f' -> Boolean(false)
      | x -> raise (SyntaxError ("Invalid boolean literal " ^ (Char.escaped x)))
  else raise (SyntaxError ("Unexpected char " ^ (Char.escaped c)));;

Seems reasonable. Let’s add the type constructor to our lobject so that this all actually compiles:

type lobject =
  | Fixnum of int
  | Boolean of bool

The last thing we’ll want to fix is the repl function. Right now it assumes that read_sexp will always return a Fixnum, but that’s not the case:

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

I propose we make a separate function entirely for printing any type of lobject:

let rec print_sexp e =
    match e with
    | Fixnum(v) -> print_int v
    | Boolean(b) -> print_string (if b then "#t" else "#f")

let rec repl stm =
  print_string "> ";
  flush stdout;
  let sexp = read_sexp stm in
  print_sexp sexp;
  print_newline ();
  repl stm;;

This also ends up making repl easier to read. Read. Print. Loop.

And now because I promised, I suppose we should make negative numbers work. To do this we’ll have to make sure that we check for a negative sign or a digit:

  if (is_digit c) || (c = '~') then read_fixnum (Char.escaped (if c='~' then '-' else c))
  else if c = '#' then [...]

So we’re not actually using the usual minus sign, -, for negative numbers. We’re using the tilde, SML-style, because it makes parsing symbols (next) just a little bit easier. Here’s what an interaction with our new REPL looks like:

$ ocaml 01_booleans.ml
> 14
14
> ~123
-123
> #t
#t
> #f
#f
> -
Exception: Failure "int_of_string".
$

At some point we should probably handle the int_of_string parse failure. But it works!

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

Next up, symbols.