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YACC for V

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# vyacc

[yacc](https://en.wikipedia.org/wiki/Yacc) for V. It was adapted from
https://pkg.go.dev/golang.org/x/tools/cmd/goyacc. See
[vyacc.v](https://github.com/elliotchance/vyacc/blob/main/vyacc.v) for full
license and credits.

# Example

```
v run vyacc.v examples/calc.y
```

Will generate a `y.v` from the grammar. Then compiler/run as usual:

```
v run y.v
```

# Documentation

Any existing docs for yacc should be relevant. Read below for some specifics of
this implementation.

## Tokens

All tokens in the grammar are converted to lower-case with a `token_` prefix
to work with V language rules. So `KEYWORD_ABS` is referenced in code as
`token_keyword_abs`.

You will need to provide an implementation for `YYSymType` (this is what is used
for all `$` tokens):

```
struct YYSymType {
mut:
  // Your custom fields here.
  yys int
}
```

## Lexer

You will need to provide your own lexer that implements:

```v
interface YYLexer {
mut:
	lex(mut lval YYSymType) int
	error(s string)
}
```

## Parser

The original (and Go version) of yacc depend on global/module variables for
parser output. Since that is not allowed in V, you have to assign `yyrcvr.lval`
in the grammar:

```
root_rule: subrule { yyrcvr.lval = $1 }
```

Then cast the parser to get access to it:

```v
mut parser := yy_new_parser()
parser.parse(mut lex)
result := (parser as YYParserImpl).lval
```

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