Goffrie Plex Save

a parser and lexer generator as a Rust procedural macro

Project README

plex, a parser and lexer generator

This crate provides a couple syntax extensions:

  • lexer!, which creates a DFA-based lexer that uses maximal munch. It works a bit like the lex tool. You write regular expressions defining your tokens, together with Rust expressions that create your tokens from slices of input.
  • parser!, which creates an LALR(1) parser. It works a bit like yacc. You write a context-free grammar, together with expressions for each rule. You give each nonterminal a Rust type, allowing you to build an AST recursively. It also supports spans, giving you convenient source location reporting.

You can find a demo in examples/demo.rs.


First, include the plex macros.

use plex::{lexer, parser};

Creating a lexer

To define a lexer, use the lexer! macro.

lexer! {
    fn take_token(tok: 'a) -> Token<'a>;

First declare the name of the function, the name of the token you will be able to access within the lexer, and the return type of your lexer. You can also optionally declare a lifetime for the strings you accept (here, 'a).

Note that this will declare a function with the actual signature fn take_token<'a>(text: &mut &'a str) -> Option<Token<'a>>. The lexer will modify the text slice to remove the consumed text. This is designed to make it easier to create an iterator of Tokens out of a string slice.

    r"[ \t\r\n]" => Token::Whitespace,
    "[0-9]+" => Token::IntegerLiteral(tok.parse().unwrap()),
    r#""[^"]*""# => Token::StringLiteral(&tok[1..tok.len()-1]),

The rest of your lexer should consist of rules. The left hand side should be a literal string (raw string literals are OK) corresponding to a regular expression. You can use the typical regular expression syntax, including parentheses for grouping, square brackets for character classes, and the usual ., |, *, and +. (? is currently not supported.) You can also use some extra operators, like ~ for negation and & for conjunction:

    r"/\*~(.*\*/.*)\*/" => Token::Comment(tok),

The above regular expression will match a C-style comment with /* */ delimiters, but won't allow */ to appear inside the comment. (.*\*/.* matches any string containing */, ~(.*\*/.*) matches any string that does not.) This is important because the lexer uses maximal munch. If you had written simply r"/\*.*\*/", then the lexer would consume the longest matching substring. That would interpret /* comment */ not comment? /* comment */ as one large comment.

    "let" => Token::Let,
    "[a-zA-Z]+" => Token::Ident(tok),
    "." => panic!("unexpected character"),

Note that if multiple rules could apply, the one declared first wins. This lets you declare keywords (which have precedence over identifiers) by putting them first.

Creating a parser

plex uses the LALR(1) construction for parsers. This section, and plex in general, will assume you understand LR parsing, along with its associated vocabulary.

To define a parser, use the parser! macro.

parser! {
    fn parse(Token, Span);

This declares the name of the parser (in this case, parse) and the input types that it takes. In this case, parse will take any iterator of pairs (Token, Span). The token type must be an enum whose variants are in scope. (This is a current limitation of plex that might be fixed later.). Those variants are the terminals of your grammar. plex-generated parsers also keep track of source locations ("spans") that are fed into it, so you'll need to mention your span type here. If you don't want to keep track of source locations, you can use the unit type ().

Next, tell plex how to combine two spans:

    (a, b) {
        Span {
            lo: a.lo,
            hi: b.hi,

Here, a and b are Spans. In this case we've defined Span as a structure with two fields, lo and hi, representing the byte offsets of the beginning and end of the span. Note that the extra braces are necessary here: the body of the function has to be a block.

Now you write your grammar. For each nonterminal, write its name, together with its type. This indicates the kind of data that the nonterminal parses into.

    statements: Vec<Expr> {

Note that the first nonterminal is special: it's the start symbol of your grammar, and its type is the return type (more or less) of the parser.

Then write the rules for this nonterminal. (The left-hand side of each rule is implied to be statements.)

        statements[mut st] expr[e] Semi => {

Write the rule's right-hand side, an arrow =>, and the code to handle this rule. The right-hand side is a sequence of nonterminals or terminals to match. Here, statements and expr are nonterminals. Square brackets assign a pattern to the result of a nonterminal, allowing us to use the data returned by that nonterminal. Terminals must be enum variants brought in scope. The expression must evaluate to the type of the left-hand side: in this case, Vec<Expr>.

        => vec![],

Empty rules are allowed: just don't write anything before the arrow.

If a terminal (i.e. a token) is a tuple-like enum variant, and so holds data, you should destructure it using round brackets:

    expr: Expr {
        Ident(s) => Expr::Var(span!(), s)

Inside a rule, the span!() macro evaluates to the span of the current right-hand-side. However, this only works if at least one token was matched. If the rule matched an empty sequence, span!() will panic, so avoid using it in nullable rules.

The return type of this parser is Result<Vec<Expr>, (Option<(Token, Span)>, &'static str)>. The error type is a pair consisting of the unexpected token, or None for EOF, and a message describing the tokens that were expected.

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