1 unstable release
0.1.0 | Jul 13, 2024 |
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#82 in Parser tooling
21KB
456 lines
Pratt-Gen
A recursive descent parser generator library that use inlined macros.
Json
For example, to parse a json file, you can write the following code, and use parse<Json>
to parse a json formatted string. That's it.
use pratt_gen::*;
#[derive(Clone, Copy, ParserImpl, Space)]
pub enum Json<'a> {
#[parse("{0}")]
Float(f64),
#[parse("{0}")]
Int(i64),
#[parse("{0}")]
Str(&'a str),
#[parse("{{ {0} }}")]
Obj(&'a Obj<'a>),
#[parse("[ {0} ]")]
Arr(&'a Arr<'a>),
}
#[derive(Clone, Copy, ParserImpl, Space)]
pub enum Obj<'a> {
#[parse("{0} : {1} , {2}")]
Next(Ident<'a>, Json<'a>, &'a Obj<'a>),
#[parse("{0} : {1}")]
Just(Ident<'a>, Json<'a>),
#[parse("")]
Null(),
}
#[derive(Clone, Copy, ParserImpl, Space)]
pub enum Arr<'a> {
#[parse("{0} , {1}")]
Next(Json<'a>, &'a Arr<'a>),
#[parse("{0}")]
Just(Json<'a>),
#[parse("")]
Null(),
}
The format string is simple. You can think of it as the reverse of a rust printing format string.
// printing
println!("{0} + {1}", "a", "b");
// when we match {0} to something and {1} to something.
// we fill it as Add({0}, {1})
#[parse("{0} + {1}", precedence=4)]
Add(&'a Expr<'a>, &'a Expr<'a>),
Precedence
To handle binary expression with left recursion and precedence, you can do this:
use pratt_gen::*;
#[derive(Debug, Clone, Copy, ParserImpl, Space)]
pub enum Expr<'a> {
#[parse("{0:2} + {1:1}", precedence=2)]
Add(&'a Expr<'a>, &'a Expr<'a>),
#[parse("{0:4} + {1:3}", precedence=4)]
Mul(&'a Expr<'a>, &'a Expr<'a>),
#[parse("{0}")]
Number(i64),
}
The precedence=...
, gives a precedence to this pattern, and by putting a number {...:n}
after a hole, it means we only allow rules with precedence <n
in that hole.
So Mul
only allow Number
on its left hand side.
Pitfall
A bad thing about recursive descendent parser is that it only supports right association, so a - b - c
is parsed as a - (b - c)
, which is not what we normally think it is. But you can write a transformation to remove that or implement add/sub as iterators.
Dependencies
~4–15MB
~129K SLoC