# ParseIt
### Description
This library provides a very simple and lightweight parser (recursive descendant ll(1)) to combine and express a grammar.
The library uses [Logos](https://github.com/maciejhirsz/logos) as a lexical analyzer and tokenizer.
### The premise
This library major incentives were:
- lightweight : very small and does not require a deep dive
- transparency : literally 3 structs with a handful of methods
- speed : good speed (with a gratitude to [Logos](https://github.com/maciejhirsz/logos))
### The steps to implement
#### Create a set of tokens using Logos
#### Add logos to dependency
```cargo
logos = "*"
```
```rust
use logos::Logos;
#[derive(Logos, Debug, PartialEq)]
enum Token {
// Tokens can be literal strings, of any length.
#[token("fast")]
Fast,
#[token(".")]
Period,
// Or regular expressions.
#[regex("[a-zA-Z]+")]
Text,
// Logos requires one token variant to handle errors,
// it can be named anything you wish.
#[error]
// We can also use this variant to define whitespace,
// or any other matches we wish to skip.
#[regex(r"[ \t\n\f]+", logos::skip)]
Error,
}
```
#### Create a parser that will be able to parse the given set of tokens
The library provides `ParseIt<'a,T>` instance that encompasses a set of tokens and auxiliary methods
```rust
struct Parser<'a> {
inner: ParseIt<'a, Token<'a>>,
}
```
#### Implement a parsing functions using `ParseIt` instance and auxiliary methods from the `Step`
The helpers:
- the macros token! that alleviates comparing and matching single tokens
- methods `then`, `then_zip` and others from `Step`
- methods `one_or_more`, `zero_or_more` from `ParseIt`
#### Transform the result into `Result<Structure, ParserError<'a>>`
```rust
fn text(&self, pos: usize) -> Result<Vec<Sentence<'a>>, ParseError<'a>> {
self.inner.zero_or_more(pos, |p| self.sentence(p)).into()
}
```
### Complete example
```rust
use crate::parser::ParseIt;
use crate::token;
use crate::step::Step;
use crate::parser::EmptyToken;
use crate::error::ParseError;
use logos::Logos;
#[derive(Logos, Debug, Copy, Clone, PartialEq)]
pub enum Token<'a> {
#[regex(r"[a-zA-Z-]+")]
Word(&'a str),
#[token(",")]
Comma,
#[token(".")]
Dot,
#[token("!")]
Bang,
#[token("?")]
Question,
#[regex(r"[ \t\r\n\u000C\f]+", logos::skip)]
Whitespace,
#[error]
Error,
}
#[derive(Debug, Copy, Clone, PartialEq)]
enum Item<'a> {
Word(&'a str),
Comma,
}
#[derive(Debug, Clone, PartialEq)]
enum Sentence<'a> {
Sentence(Vec<Item<'a>>),
Question(Vec<Item<'a>>),
Exclamation(Vec<Item<'a>>),
}
struct Parser<'a> {
inner: ParseIt<'a, Token<'a>>,
}
impl<'a> Parser<'a> {
fn new(text: &'a str) -> Parser<'a> {
let delegate: ParseIt<Token> = ParseIt::new(text).unwrap();
Parser { inner: delegate }
}
fn sentence(&self, pos: usize) -> Step<'a, Sentence<'a>> {
let items = |p| self.inner.one_or_more(p, |p| self.word(p));
let sentence = |p| items(p)
.then_zip(|p| token!(self.inner.token(p) => Token::Dot))
.take_left()
.map(Sentence::Sentence);
let exclamation = |p| items(p)
.then_zip(|p| token!(self.inner.token(p) => Token::Bang))
.take_left()
.map(Sentence::Exclamation);
let question = |p| items(p)
.then_zip(|p| token!(self.inner.token(p) => Token::Question))
.take_left()
.map(Sentence::Question);
sentence(pos)
.or_from(pos)
.or(exclamation)
.or(question)
.into()
}
fn word(&self, pos: usize) -> Step<'a, Item<'a>> {
token!(self.inner.token(pos) =>
Token::Word(v) => Item::Word(v),
Token::Comma => Item::Comma
)
}
fn text(&self, pos: usize) -> Result<Vec<Sentence<'a>>, ParseError<'a>> {
self.inner.zero_or_more(pos, |p| self.sentence(p)).into()
}
}
#[test]
fn test() {
let parser = Parser::new(r#"
I have a strange addiction,
It often sets off sparks!
I really cannot seem to stop,
Using exclamation marks!
Anyone heard of the interrobang?
The poem is for kids.
"#);
let result = parser.text(0).unwrap();
println!("{:?}",result);
}
```
### The base auxiliary methods
#### On parser
- `token` - gives a possibility to pull out a curren token
- `one_or_more` - gives a one or more semantic
- `zero_or_more` - gives a zero or more semantic
- `validate_eof` - ensure the parser reaches end of the input
#### Macros
- `token!` - parses the current token. In general, it is used the following `token!(p.token(pos) => T::Bang => "!")`
- `wrap!` - implements a simple pattern in grammar like `left value right`, for instance `[1,2,3]` or `(a,b)`
- can handle the default value like `wrap!(0 => left; value or default; right)`
- can handle the option value like `wrap!(0 => left; value ?; right)`
- `seq!` - implements a simple pattern of sequence like `el sep el ... `, for instance `1,2,3`
- can have a `,` at the end signaling the separator can be at the ned of the seq like `1,2,3 (,)?`
#### On step
##### To alternate
- `or` - gives an alternative in a horizon of one token
- `or_from` - gives a backtracking option
##### To combine
- `then` - gives a basic combination with a next rule omitting the current one
- `then_zip` - combines a current result and a next one into a pair
- `then_skip` - parses the next one but drops the result keeping only current one
- `then_or_none` -combines a next one in an option with a current one or return a none otherwise
##### To collect
- `take_left` - drops a right value from a pair
- `take_right` - drops a left value from a pair
- `merge` - merge a value into a list
- `to_map` - transforms a list of pairs into a map
##### To transform
- `or_val` - replaces a value with a default value if it is not presented
- `or_none` - replaces a value with a none if it is not presented
##### To work with value
- `ok` - transforms a value into an option
- `error` - transforms an error into an option
- `map` - transforms a value
- `combine` - combines a value with another value from a given step
- `validate` - validates a given value and transforms into an error if a validation failed
##### To print
- `print` - print a step
- `print_with` - print a step with a given prefix
- `print_as` - print a step with a transformation of value
- `print_with_as` - print a step with a transformation of value with a given prefix
### Testing
#### Lexer
To test a lexer there are methods from `crate::parsit::test::lexer_test::*` for service
```rust
use logos::Logos;
use crate::parsit::test::lexer_test::*;
#[derive(Logos, Debug, PartialEq)]
pub enum T<'a> {
#[regex(r"[a-zA-Z-]+")]
Word(&'a str),
#[token(",")]
Comma,
#[token(".")]
Dot,
#[token("!")]
Bang,
#[token("?")]
Question,
#[regex(r"[ \t\r\n]+", logos::skip)]
Whitespace,
#[error]
Error,
}
#[test]
fn test() {
expect::<T>("abc, bcs!", vec![T::Word("abc"), T::Comma, T::Word("bcs"), T::Bang]);
expect_succeed::<T>("abc, bcs!");
expect_failed::<T>("abc, bcs >> !");
expect_failed_with::<T,_>("abc, bcs > !", |e| e.is_bad_token_on(">") );
}
```
#### Parser
To test a parser there are methods from `crate::parsit::test::parser_test::*` for service
```rust
use logos::Logos;
use crate::parsit::test::parser_test::fail;
use crate::parsit::test::parser_test::parsit;
use crate::parsit::token;
use crate::parsit::parser::ParseIt;
use crate::parsit::step::Step;
#[derive(Logos, Debug, PartialEq)]
pub enum T<'a> {
#[regex(r"[a-zA-Z-]+")]
Word(&'a str),
#[token(",")]
Comma,
#[token(".")]
Dot,
#[token("!")]
Bang,
#[token("?")]
Question,
#[regex(r"[ \t\r\n]+", logos::skip)]
Whitespace,
#[error]
Error,
}
#[test]
fn test_expect() {
let p = parsit("abc!");
let bang = |pos:usize| token!(p.token(pos) => T::Bang => "!");
let word = |pos:usize| token!(p.token(pos) => T::Word(v) => *v);
let step =
word(0)
.then_or_val_zip(bang, "")
.map(|(a,b)| format!("{}{}",a,b));
expect(step,"abc!".to_string());
}
#[test]
fn test_pos() {
let p = parsit("abc!");
let bang = |pos:usize| token!(p.token(pos) => T::Bang => "!");
let word = |pos:usize| token!(p.token(pos) => T::Word(v) => v);
let step =word(0).then_or_val_zip(bang, "");
expect_pos(step,2); // the next position to parse
}
#[test]
fn test_fail() {
let p = parsit("abc?!");
let bang = |pos:usize| token!(p.token(pos) => T::Bang => "!");
let word = |pos:usize| token!(p.token(pos) => T::Word(v) => v);
let step =word(0).then_zip(bang);
fail(step);
}
#[test]
fn test_fail_on() {
let p = parsit("abc?!");
let bang = |pos:usize| token!(p.token(pos) => T::Bang => "!");
let word = |pos:usize| token!(p.token(pos) => T::Word(v) => v);
let step =word(0).then_zip(bang);
fail_on(step,1);
}
```