LALRPOP parser for procedural macro input
=========================================
[](https://travis-ci.org/dtolnay/lalrproc)
This crate demostrates a proof of concept of using the [LALRPOP] parser
generator framework to parse input tokens in a Rust procedural macro.
[LALRPOP]: https://github.com/nikomatsakis/lalrpop
## Macros
The procedural macros in this example are `s_expr!` and `s_type!` which expand
to a string literal S-expression representation of the Rust expression or type
given in the input.
```rust
extern crate lalrproc;
use lalrproc::{s_expr, s_type};
fn main() {
// Expands to "(+ (* 1 2) (* 3 4))"
let e = s_expr!(1 * 2 + 3 * 4);
// Expands to "(Reference 'a mut (Path module T))"
let t = s_type!(&'a mut module::T);
}
```
## Parser
The input parsing is handled entirely by LALRPOP. In particular, [Syn] and
[proc-macro2] are not involved.
[Syn]: https://github.com/dtolnay/syn
[proc-macro2]: https://github.com/alexcrichton/proc-macro2
All of the features of LALRPOP are available to the LALRPOP grammar. For example
here is a subset of the expression grammar illustrating the parsing of something
like `1 * 2 + 3 * 4` into the syntax tree `(+ (* 1 2) (* 3 4))`.
```rust
pub Expr: Box<Expr> = {
Expr ExprOp Factor => Box::new(Expr::Binary(<>)),
Factor,
};
ExprOp: BinOp = {
"+" => BinOp::Add,
"-" => BinOp::Sub,
};
Factor: Box<Expr> = {
Factor FactorOp Component => Box::new(Expr::Binary(<>)),
Component,
};
FactorOp: BinOp = {
"*" => BinOp::Mul,
"/" => BinOp::Div,
};
Component: Box<Expr> = {
Literal => Box::new(Expr::Lit(<>)),
Path => Box::new(Expr::Path(<>)),
"(" <Expr> ")",
};
```
## Error reporting
If the input does not conform to the grammar expected by the procedural macro,
the error from LALRPOP is rendered in a way that indicates the problematic token
and gives the tokens that would have been accepted by the grammar in that
position.
```
error: failed to parse macro input
--> tests/test.rs:17:25
|
|
= note: unrecognized token `:`
= note: expected one of `(`, identifier, or literal
```
<br>
#### License
<sup>
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
</sup>
<br>
<sub>
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.
</sub>