lrparlrpar provides a Yacc-compatible parser (where grammars can be generated at
compile-time or run-time). It can take in traditional .y files and convert
them into an idiomatic Rust parser. More details can be found in the grmtools
book which includes a
quickstart guide.
Let's assume we want to statically generate a parser for a simple calculator
language (and let's also assume we are able to use
lrlex for the
lexer). We need to add a build.rs file to our project which tells lrpar to
statically compile the lexer and parser files:
```rust extern crate lrlex; extern crate lrpar;
use lrlex::LexerBuilder; use lrpar::{CTParserBuilder, ActionKind};
fn main() -> Result<(), Box
where src/calc.l is as follows:
%%
[0-9]+ "INT"
\+ "PLUS"
\* "MUL"
\( "LBRACK"
\) "RBRACK"
[\t ]+ ;
and src/calc.y is as follows:
``` %start Expr // Define the Rust type that is to be returned by the actions. %type u64 %% Expr: Term 'PLUS' Expr { $1 + $3 } | Term { $1 } ;
Term: Factor 'MUL' Term { $1 * $3 } | Factor { $1 } ;
Factor: 'LBRACK' Expr 'RBRACK' { $2 } | 'INT' { parseint($lexer.lexemestr(&$1.unwrap())) } ; %% // Any functions here are in scope for all the grammar actions above.
fn parseint(s: &str) -> u64 {
match s.parse::
A simple src/main.rs is as follows:
```rust // The cfgrammar import will not be needed once the 2018 edition is stable. extern crate cfgrammar; // We import lrpar and lrlex with macros so that lrlexmod! and lrparmod! are in scope.
use std::io::{self, BufRead, Write};
// Using lrlex_mod! brings the lexer for calc.l into scope.
lrlexmod!(calcl);
// Using lrpar_mod! brings the lexer for calc.l into scope.
lrparmod!(calcy);
fn main() {
// We need to get a LexerDef for the calc language in order that we can lex input.
let lexerdef = calcl::lexerdef();
let stdin = io::stdin();
loop {
print!(">>> ");
io::stdout().flush().ok();
match stdin.lock().lines().next() {
Some(Ok(ref l)) => {
if l.trim().isempty() {
continue;
}
// Now we create a lexer with the lexer method with which we can lex an input.
let mut lexer = lexerdef.lexer(l);
// Pass the lexer to the parser and lex and parse the input.
let (res, errs) = calcy::parse(&mut lexer);
for e in errs {
println!("{}", e.pp(&lexer, &calcy::token_epp));
}
match res {
Some(r) => println!("Result: {}", r),
_ => eprintln!("Unable to evaluate expression.")
}
}
_ => break
}
}
}
```
We can now cargo run our project and evaluate simple expressions:
```
2 + 3 Result: 5 2 + 3 * 4 Result: 14 (2 + 3) * 4 Result: 20 ```
lrpar also comes with advanced error
recovery built-in:
```
2 + + 3 Parsing error at line 1 column 5. Repair sequences found: 1: Delete + 2: Insert INT Result: 5 2 + 3 3 Parsing error at line 1 column 7. Repair sequences found: 1: Delete 3 2: Insert PLUS 3: Insert MUL Result: 5 2 + 3 4 5 Parsing error at line 1 column 7. Repair sequences found: 1: Insert MUL, Delete 4 2: Insert PLUS, Delete 4 3: Delete 4, Delete 5 4: Insert MUL, Shift 4, Delete 5 5: Insert MUL, Shift 4, Insert PLUS 6: Insert MUL, Shift 4, Insert MUL 7: Insert PLUS, Shift 4, Delete 5 8: Insert PLUS, Shift 4, Insert PLUS 9: Insert PLUS, Shift 4, Insert MUL Result: 17 ```