Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
1 contributor

Users who have contributed to this file

354 lines (323 sloc) 12.4 KB
//! In this example we build an [S-expression](https://en.wikipedia.org/wiki/S-expression)
//! parser and tiny [lisp](https://en.wikipedia.org/wiki/Lisp_(programming_language)) interpreter.
//! Lisp is a simple type of language made up of Atoms and Lists, forming easily parsable trees.
#![cfg(feature = "alloc")]
extern crate jemallocator;
extern crate nom;
#[global_allocator]
static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;
use nom::{
branch::alt,
bytes::complete::tag,
character::complete::{alpha1, char, digit1, multispace0, multispace1, one_of},
combinator::{cut, map, map_res, opt},
error::{context, VerboseError},
multi::many0,
sequence::{delimited, preceded, terminated, tuple},
IResult,
};
/// We start by defining the types that define the shape of data that we want.
/// In this case, we want something tree-like
/// Starting from the most basic, we define some built-in functions that our lisp has
#[derive(Debug, PartialEq, Clone, Copy)]
pub enum BuiltIn {
Plus,
Minus,
Times,
Divide,
Equal,
Not,
}
/// We now wrap this type and a few other primitives into our Atom type.
/// Remember from before that Atoms form one half of our language.
#[derive(Debug, PartialEq, Clone)]
pub enum Atom {
Num(i32),
Keyword(String),
Boolean(bool),
BuiltIn(BuiltIn),
}
/// The remaining half is Lists. We implement these as recursive Expressions.
/// For a list of numbers, we have `'(1 2 3)`, which we'll parse to:
/// ```
/// Expr::Quote(vec![Expr::Constant(Atom::Num(1)),
/// Expr::Constant(Atom::Num(2)),
/// Expr::Constant(Atom::Num(3))])
/// Quote takes an S-expression and prevents evaluation of it, making it a data
/// structure that we can deal with programmatically. Thus any valid expression
/// is also a valid data structure in Lisp itself.
#[derive(Debug, PartialEq, Clone)]
pub enum Expr {
Constant(Atom),
/// (func-name arg1 arg2)
Application(Box<Expr>, Vec<Expr>),
/// (if predicate do-this)
If(Box<Expr>, Box<Expr>),
/// (if predicate do-this otherwise-do-this)
IfElse(Box<Expr>, Box<Expr>, Box<Expr>),
/// '(3 (if (+ 3 3) 4 5) 7)
Quote(Vec<Expr>),
}
/// Continuing the trend of starting from the simplest piece and building up,
/// we start by creating a parser for the built-in operator functions.
fn parse_builtin_op<'a>(i: &'a str) -> IResult<&'a str, BuiltIn, VerboseError<&'a str>> {
// one_of matches one of the characters we give it
let (i, t) = one_of("+-*/=")(i)?;
// because we are matching single character tokens, we can do the matching logic
// on the returned value
Ok((
i,
match t {
'+' => BuiltIn::Plus,
'-' => BuiltIn::Minus,
'*' => BuiltIn::Times,
'/' => BuiltIn::Divide,
'=' => BuiltIn::Equal,
_ => unreachable!(),
},
))
}
fn parse_builtin<'a>(i: &'a str) -> IResult<&'a str, BuiltIn, VerboseError<&'a str>> {
// alt gives us the result of first parser that succeeds, of the series of
// parsers we give it
alt((
parse_builtin_op,
// map lets us process the parsed output, in this case we know what we parsed,
// so we ignore the input and return the BuiltIn directly
map(tag("not"), |_| BuiltIn::Not),
))(i)
}
/// Our boolean values are also constant, so we can do it the same way
fn parse_bool<'a>(i: &'a str) -> IResult<&'a str, Atom, VerboseError<&'a str>> {
alt((map(tag("#t"), |_| Atom::Boolean(true)), map(tag("#f"), |_| Atom::Boolean(false))))(i)
}
/// The next easiest thing to parse are keywords.
/// We introduce some error handling combinators: `context` for human readable errors
/// and `cut` to prevent back-tracking.
///
/// Put plainly: `preceded(tag(":"), cut(alpha1))` means that once we see the `:`
/// character, we have to see one or more alphabetic chararcters or the input is invalid.
fn parse_keyword<'a>(i: &'a str) -> IResult<&'a str, Atom, VerboseError<&'a str>> {
map(context("keyword", preceded(tag(":"), cut(alpha1))), |sym_str: &str| {
Atom::Keyword(sym_str.to_string())
})(i)
}
/// Next up is number parsing. We're keeping it simple here by accepting any number (> 1)
/// of digits but ending the program if it doesn't fit into an i32.
fn parse_num<'a>(i: &'a str) -> IResult<&'a str, Atom, VerboseError<&'a str>> {
alt((
map_res(digit1, |digit_str: &str| digit_str.parse::<i32>().map(Atom::Num)),
map(preceded(tag("-"), digit1), |digit_str: &str| {
Atom::Num(-1 * digit_str.parse::<i32>().unwrap())
}),
))(i)
}
/// Now we take all these simple parsers and connect them.
/// We can now parse half of our language!
fn parse_atom<'a>(i: &'a str) -> IResult<&'a str, Atom, VerboseError<&'a str>> {
alt((parse_num, parse_bool, map(parse_builtin, Atom::BuiltIn), parse_keyword))(i)
}
/// We then add the Expr layer on top
fn parse_constant<'a>(i: &'a str) -> IResult<&'a str, Expr, VerboseError<&'a str>> {
map(parse_atom, |atom| Expr::Constant(atom))(i)
}
/// Before continuing, we need a helper function to parse lists.
/// A list starts with `(` and ends with a matching `)`.
/// By putting whitespace and newline parsing here, we can avoid having to worry about it
/// in much of the rest of the parser.
///
/// Unlike the previous functions, this function doesn't take or consume input, instead it
/// takes a parsing function and returns a new parsing function.
fn s_exp<'a, O1, F>(inner: F) -> impl Fn(&'a str) -> IResult<&'a str, O1, VerboseError<&'a str>>
where
F: Fn(&'a str) -> IResult<&'a str, O1, VerboseError<&'a str>>,
{
delimited(
char('('),
preceded(multispace0, inner),
context("closing paren", cut(preceded(multispace0, char(')')))),
)
}
/// We can now use our new combinator to define the rest of the `Expr`s.
///
/// Starting with function application, we can see how the parser mirrors our data
/// definitions: our definition is `Application(Box<Expr>, Vec<Expr>)`, so we know
/// that we need to parse an expression and then parse 0 or more expressions, all
/// wrapped in an S-expression.
///
/// `tuple` is used to sequence parsers together, so we can translate this directly
/// and then map over it to transform the output into an `Expr::Application`
fn parse_application<'a>(i: &'a str) -> IResult<&'a str, Expr, VerboseError<&'a str>> {
let application_inner = map(tuple((parse_expr, many0(parse_expr))), |(head, tail)| {
Expr::Application(Box::new(head), tail)
});
// finally, we wrap it in an s-expression
s_exp(application_inner)(i)
}
/// Because `Expr::If` and `Expr::IfElse` are so similar (we easily could have
/// defined `Expr::If` to have an `Option` for the else block), we parse both
/// in a single function.
///
/// In fact, we define our parser as if `Expr::If` was defined with an Option in it,
/// we have the `opt` combinator which fits very nicely here.
fn parse_if<'a>(i: &'a str) -> IResult<&'a str, Expr, VerboseError<&'a str>> {
let if_inner = context(
"if expression",
map(
preceded(
// here to avoid ambiguity with other names starting with `if`, if we added
// variables to our language, we say that if must be terminated by at least
// one whitespace character
terminated(tag("if"), multispace1),
cut(tuple((parse_expr, parse_expr, opt(parse_expr)))),
),
|(pred, true_branch, maybe_false_branch)| {
if let Some(false_branch) = maybe_false_branch {
Expr::IfElse(Box::new(pred), Box::new(true_branch), Box::new(false_branch))
} else {
Expr::If(Box::new(pred), Box::new(true_branch))
}
},
),
);
s_exp(if_inner)(i)
}
/// A quoted S-expression is list data structure.
///
/// This example doesn't have the symbol atom, but by adding variables and changing
/// the definition of quote to not always be around an S-expression, we'd get them
/// naturally.
fn parse_quote<'a>(i: &'a str) -> IResult<&'a str, Expr, VerboseError<&'a str>> {
// this should look very straight-forward after all we've done:
// we find the `'` (quote) character, use cut to say that we're unambiguously
// looking for an s-expression of 0 or more expressions, and then parse them
map(context("quote", preceded(tag("'"), cut(s_exp(many0(parse_expr))))), |exprs| {
Expr::Quote(exprs)
})(i)
}
/// We tie them all together again, making a top-level expression parser!
fn parse_expr<'a>(i: &'a str) -> IResult<&'a str, Expr, VerboseError<&'a str>> {
preceded(multispace0, alt((parse_constant, parse_application, parse_if, parse_quote)))(i)
}
/// And that's it!
/// We can now parse our entire lisp language.
///
/// But in order to make it a little more interesting, we can hack together
/// a little interpreter to take an Expr, which is really an
/// [Abstract Syntax Tree](https://en.wikipedia.org/wiki/Abstract_syntax_tree) (AST),
/// and give us something back
/// To start we define a couple of helper functions
fn get_num_from_expr(e: Expr) -> Option<i32> {
if let Expr::Constant(Atom::Num(n)) = e {
Some(n)
} else {
None
}
}
fn get_bool_from_expr(e: Expr) -> Option<bool> {
if let Expr::Constant(Atom::Boolean(b)) = e {
Some(b)
} else {
None
}
}
/// This function tries to reduce the AST.
/// This has to return an Expression rather than an Atom because quoted s_expressions
/// can't be reduced
fn eval_expression(e: Expr) -> Option<Expr> {
match e {
// Constants and quoted s-expressions are our base-case
Expr::Constant(_) | Expr::Quote(_) => Some(e),
// we then recursively `eval_expression` in the context of our special forms
// and built-in operators
Expr::If(pred, true_branch) => {
let reduce_pred = eval_expression(*pred)?;
if get_bool_from_expr(reduce_pred)? {
eval_expression(*true_branch)
} else {
None
}
}
Expr::IfElse(pred, true_branch, false_branch) => {
let reduce_pred = eval_expression(*pred)?;
if get_bool_from_expr(reduce_pred)? {
eval_expression(*true_branch)
} else {
eval_expression(*false_branch)
}
}
Expr::Application(head, tail) => {
let reduced_head = eval_expression(*head)?;
let reduced_tail = tail.into_iter().map(|expr| eval_expression(expr)).collect::<Option<Vec<Expr>>>()?;
if let Expr::Constant(Atom::BuiltIn(bi)) = reduced_head {
Some(Expr::Constant(match bi {
BuiltIn::Plus => Atom::Num(
reduced_tail
.into_iter()
.map(get_num_from_expr)
.collect::<Option<Vec<i32>>>()?
.into_iter()
.sum(),
),
BuiltIn::Times => Atom::Num(
reduced_tail
.into_iter()
.map(get_num_from_expr)
.collect::<Option<Vec<i32>>>()?
.into_iter()
.product(),
),
BuiltIn::Equal => Atom::Boolean(reduced_tail.iter().zip(reduced_tail.iter().skip(1)).all(|(a, b)| a == b)),
BuiltIn::Not => {
if reduced_tail.len() != 1 {
return None;
} else {
Atom::Boolean(!get_bool_from_expr(reduced_tail.first().cloned().unwrap())?)
}
}
BuiltIn::Minus => Atom::Num(if let Some(first_elem) = reduced_tail.first().cloned() {
let fe = get_num_from_expr(first_elem)?;
reduced_tail
.into_iter()
.map(get_num_from_expr)
.collect::<Option<Vec<i32>>>()?
.into_iter()
.skip(1)
.fold(fe, |a, b| a - b)
} else {
Default::default()
}),
BuiltIn::Divide => Atom::Num(if let Some(first_elem) = reduced_tail.first().cloned() {
let fe = get_num_from_expr(first_elem)?;
reduced_tail
.into_iter()
.map(get_num_from_expr)
.collect::<Option<Vec<i32>>>()?
.into_iter()
.skip(1)
.fold(fe, |a, b| a / b)
} else {
Default::default()
}),
}))
} else {
None
}
}
}
}
/// And we add one more top-level function to tie everything together, letting
/// us call eval on a string directly
fn eval_from_str(src: &str) -> Result<Expr, String> {
parse_expr(src)
.map_err(|e: nom::Err<VerboseError<&str>>| format!("{:#?}", e))
.and_then(|(_, exp)| eval_expression(exp).ok_or("Eval failed".to_string()))
}
fn main() {
let expression_1 = "((if (= (+ 3 (/ 9 3))
(* 2 3))
*
/)
456 123)";
println!("\"{}\"\nevaled gives us: {:?}", expression_1, eval_from_str(expression_1));
}
You can’t perform that action at this time.