[英]Backtracking in scala parser combinators?
It seems like scala's parser combinators don't backtrack. 似乎scala的解析器组合器不会回溯。 I have a grammar (see bottom) which can't parse the following "stmt" correctly:
我有一个语法(见底部),无法正确解析以下“stmt”:
copy in to out .
That should be easy to parse with backtracking: 这应该很容易用回溯解析:
stmt: (to out(copy in))
or am I missing something? 还是我错过了什么?
Parser: 分析器:
type ExprP = Parser[Expr]
type ValueP = Parser[ValExpr]
type CallP = Parser[Call]
type ArgsP = Parser[Seq[Expr]]
val ident = "[a-zA-Z\\+\\-\\*/%><\\\\\\=]+".r
val sqstart = "\\[" .r
val sqend = "\\]" .r
val del = "," .r
val end = "\\." .r
def stmt: ExprP = expr <~ end
def expr: ExprP = ucall | call | value
def value: ValueP = ident ^^ {str => IdentExpr(str)}
def call: CallP = (args ~ ident ~ expr) ^^ {case args ~ method ~ upon => Call(args, method, upon)}
def ucall: CallP = (ident ~ expr) ^^ {case method ~ upon => Call(Seq(), method, upon)}
def args: ArgsP = advargs | smplargs
def smplargs: ArgsP = expr ^^ {e => Seq(e)}
def advargs: ArgsP = (sqstart ~> repsep(expr, del) <~ sqend) ^^ {seq => seq}
You want to use PackratParsers
in 2.8. 你想在2.8中使用
PackratParsers
。 I think the packrat parser is the only backtracking parser. 我认为packrat解析器是唯一的回溯解析器。
Edit: as of mid-year 2015, you should use fastparse instead. 编辑:截至2015年中期,您应该使用fastparse 。 It's not only much faster, but also easier to use (especially when building data structures from parsing).
它不仅更快,而且更易于使用(特别是在从解析构建数据结构时)。
Your problem is not backtracking. 你的问题不是回溯。 The standard
|
标准
|
operator in scala.util.parsing.combinator
will do backtracking. scala.util.parsing.combinator
运算符将执行回溯。 Your problem is left-recursion ( expr
→ call
→ args
→ smplargs
→ expr
). 你的问题是左递归(
expr
→ call
→ args
→ smplargs
→ expr
)。 Packrat parsing may indeed help with that. Packrat解析可能确实有助于此。
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