scala - 斯卡拉。困难的表达式解析器。内存不足错误

Question

我想为具有操作顺序的困难表达式创建一个解析器。我有一些例子，但它工作得非常缓慢并抛出异常 OutOfMemoryError。我该如何改进它？

def expr: Parser[Expression] = term5
def term5: Parser[Expression] =
    (term4 ~ "OR" ~ term4) ^^ { case lhs~o~rhs => BinaryOp("OR", lhs, rhs) } |
    term4
def term4: Parser[Expression] =
    (term3 ~ "AND" ~ term3) ^^ { case lhs~a~rhs => BinaryOp("AND", lhs, rhs) } |
    term3
def term3: Parser[Expression] =
    (term2 ~ "<>" ~ term2) ^^ { case lhs~ne~rhs => BinaryOp("NE", lhs, rhs) } |
    (term2 ~ "=" ~ term2) ^^ { case lhs~eq~rhs => BinaryOp("EQ", lhs, rhs) } |
    (term2 ~ "NE" ~ term2) ^^ { case lhs~ne~rhs => BinaryOp("NE", lhs, rhs) } |
    (term2 ~ "EQ" ~ term2) ^^ { case lhs~eq~rhs => BinaryOp("EQ", lhs, rhs) } |
    term2
def term2: Parser[Expression] =
    (term1 ~ "<" ~ term1) ^^ { case lhs~lt~rhs => BinaryOp("LT", lhs, rhs) } |
    (term1 ~ ">" ~ term1) ^^ { case lhs~gt~rhs => BinaryOp("GT", lhs, rhs) } |
    (term1 ~ "<=" ~ term1) ^^ { case lhs~le~rhs => BinaryOp("LE", lhs, rhs) } |
    (term1 ~ ">=" ~ term1) ^^ { case lhs~ge~rhs => BinaryOp("GE", lhs, rhs) } |
    (term1 ~ "LT" ~ term1) ^^ { case lhs~lt~rhs => BinaryOp("LT", lhs, rhs) } |
    (term1 ~ "GT" ~ term1) ^^ { case lhs~gt~rhs => BinaryOp("GT", lhs, rhs) } |
    (term1 ~ "LE" ~ term1) ^^ { case lhs~le~rhs => BinaryOp("LE", lhs, rhs) } |
    (term1 ~ "GE" ~ term1) ^^ { case lhs~ge~rhs => BinaryOp("GE", lhs, rhs) } |
    term1
def term1: Parser[Expression] =
    (term ~ "+" ~ term) ^^ { case lhs~plus~rhs => BinaryOp("+", lhs, rhs) } |
    (term ~ "-" ~ term) ^^ { case lhs~minus~rhs => BinaryOp("-", lhs, rhs) } |
    (term ~ ":" ~ term) ^^ { case lhs~concat~rhs => BinaryOp(":", lhs, rhs) } |
    term 
def term: Parser[Expression] =
    (factor ~ "*" ~ factor) ^^ { case lhs~times~rhs => BinaryOp("*", lhs, rhs) } |
    (factor ~ "/" ~ factor) ^^ { case lhs~div~rhs => BinaryOp("/", lhs, rhs) } |
    (factor ~ "MOD" ~ factor) ^^ { case lhs~div~rhs => BinaryOp("MOD", lhs, rhs) } |
    factor
def factor: Parser[Expression] =
    "(" ~> expr <~ ")" |
    ("+" | "-") ~ factor ^^ { case op~rhs => UnaryOp(op, rhs) } |
    function |
    numericLit ^^ { case x => Number(x/*.toFloat*/) } |
    stringLit ^^ { case s => Literal(s) } |
    ident ^^ { case id => Variable(id) }

score 4 · Accepted Answer

基本上，它很慢并且消耗太多内存，因为您的语法效率非常低。

让我们考虑第二行：B = A:(1+2). 它将尝试像这样解析这一行：

term4 ORterm4 然后是 term4。
term3 ANDterm3 然后是 term3。
术语 2<>术语 2，然后=，然后NE，EQ然后是术语 2。
term1 8 个不同的运算符 term1，然后是 term1。
术语+术语，术语-术语，术语:术语，然后术语。
因子*因子，因子/因子，因子MOD因子，再因子。
括号表达式、一元因子、函数、数字文字、字符串文字、标识。

第一次尝试是这样的：

ident * factor + term < term1 <> term2 AND term3 OR term4

我跳过括号、一元、函数、数字和字符串文字，因为它们不匹配A——尽管function可能匹配，但它的定义不可用。现在，由于:不匹配*，它将尝试下一个：

ident / factor + term < term1 <> term2 AND term3 OR term4
ident MOD factor + term < term1 <> term2 AND term3 OR term4
ident + term < term1 <> term2 AND term3 OR term4

现在它进入下一个term1：

ident * factor - term < term1 <> term2 AND term3 OR term4
ident / factor - term < term1 <> term2 AND term3 OR term4
ident MOD factor - term < term1 <> term2 AND term3 OR term4
ident - term < term1 <> term2 AND term3 OR term4

接下来：

ident * factor : term < term1 <> term2 AND term3 OR term4
ident / factor : term < term1 <> term2 AND term3 OR term4
ident MOD factor : term < term1 <> term2 AND term3 OR term4
ident : term < term1 <> term2 AND term3 OR term4

啊哈！我们终于在第 1 学期得到了一场比赛！但是(不匹配<，所以它必须尝试下一个term2：

ident * factor + term > term1 <> term2 AND term3 OR term4

ETC...

这一切都是因为每个术语的每行中的第一个术语将始终匹配！要匹配一个简单的数字，它必须解析factor2 * 2 * 5 * 9 * 4 * 4 = 2880 次！

但这还不是故事的一半！你看，因为 termX 重复了两次，它会在两边重复所有这些东西。例如，第一个匹配A:(1+2)是这样的：

ident : term < term1 <> term2 AND term3 OR term4
where ident = A
and   term  = (1+2)

这是不正确的，所以它会尝试>代替<, 然后<=, 等等等等。

我在下面放了这个解析器的日志版本。尝试运行它并查看它试图解析的所有内容。

同时，有很好的例子说明如何编写这些可用的解析器。使用sbaz，尝试：

sbaz install scala-devel-docs

然后查看doc/scala-devel-docs/examples/parsingScala 发行版的目录，您会发现几个示例。

这是您的解析器的一个版本（没有function），它记录了它尝试的所有内容：

sealed trait Expression
case class Variable(id: String) extends Expression
case class Literal(s: String) extends Expression
case class Number(x: String) extends Expression
case class UnaryOp(op: String, rhs: Expression) extends Expression
case class BinaryOp(op: String, lhs: Expression, rhs: Expression) extends Expression

object TestParser extends scala.util.parsing.combinator.syntactical.StdTokenParsers {
    import scala.util.parsing.combinator.lexical.StdLexical
    type Tokens = StdLexical
    val lexical = new StdLexical
    lexical.delimiters ++= List("(", ")", "+", "-", "*", "/", "=", "OR", "AND", "NE", "EQ", "LT", "GT", "LE", "GE", ":", "MOD")
    def stmts: Parser[Any] = log(expr.*)("stmts")
    def stmt: Parser[Expression] = log(expr <~ "\n")("stmt")
    def expr: Parser[Expression] = log(term5)("expr")
    def term5: Parser[Expression] = (
        log((term4 ~ "OR" ~ term4) ^^ { case lhs~o~rhs => BinaryOp("OR", lhs, rhs) })("term5 OR")
      | log(term4)("term5 term4")
    )
    def term4: Parser[Expression] = (
        log((term3 ~ "AND" ~ term3) ^^ { case lhs~a~rhs => BinaryOp("AND", lhs, rhs) })("term4 AND")
      | log(term3)("term4 term3")
    )
    def term3: Parser[Expression] = (
        log((term2 ~ "<>" ~ term2) ^^ { case lhs~ne~rhs => BinaryOp("NE", lhs, rhs) })("term3 <>")
      | log((term2 ~ "=" ~ term2) ^^ { case lhs~eq~rhs => BinaryOp("EQ", lhs, rhs) })("term3 =")
      | log((term2 ~ "NE" ~ term2) ^^ { case lhs~ne~rhs => BinaryOp("NE", lhs, rhs) })("term3 NE")
      | log((term2 ~ "EQ" ~ term2) ^^ { case lhs~eq~rhs => BinaryOp("EQ", lhs, rhs) })("term3 EQ")
      | log(term2)("term3 term2")
    )
    def term2: Parser[Expression] = (
        log((term1 ~ "<" ~ term1) ^^ { case lhs~lt~rhs => BinaryOp("LT", lhs, rhs) })("term2 <")
      | log((term1 ~ ">" ~ term1) ^^ { case lhs~gt~rhs => BinaryOp("GT", lhs, rhs) })("term2 >")
      | log((term1 ~ "<=" ~ term1) ^^ { case lhs~le~rhs => BinaryOp("LE", lhs, rhs) })("term2 <=")
      | log((term1 ~ ">=" ~ term1) ^^ { case lhs~ge~rhs => BinaryOp("GE", lhs, rhs) })("term2 >=")
      | log((term1 ~ "LT" ~ term1) ^^ { case lhs~lt~rhs => BinaryOp("LT", lhs, rhs) })("term2 LT")
      | log((term1 ~ "GT" ~ term1) ^^ { case lhs~gt~rhs => BinaryOp("GT", lhs, rhs) })("term2 GT")
      | log((term1 ~ "LE" ~ term1) ^^ { case lhs~le~rhs => BinaryOp("LE", lhs, rhs) })("term2 LE")
      | log((term1 ~ "GE" ~ term1) ^^ { case lhs~ge~rhs => BinaryOp("GE", lhs, rhs) })("term2 GE")
      | log(term1)("term2 term1")
    )
    def term1: Parser[Expression] = (
        log((term ~ "+" ~ term) ^^ { case lhs~plus~rhs => BinaryOp("+", lhs, rhs) })("term1 +")
      | log((term ~ "-" ~ term) ^^ { case lhs~minus~rhs => BinaryOp("-", lhs, rhs) })("term1 -")
      | log((term ~ ":" ~ term) ^^ { case lhs~concat~rhs => BinaryOp(":", lhs, rhs) })("term1 :")
      | log(term)("term1 term")
    )
    def term: Parser[Expression] = (
        log((factor ~ "*" ~ factor) ^^ { case lhs~times~rhs => BinaryOp("*", lhs, rhs) })("term *")
      | log((factor ~ "/" ~ factor) ^^ { case lhs~div~rhs => BinaryOp("/", lhs, rhs) })("term /")
      | log((factor ~ "MOD" ~ factor) ^^ { case lhs~div~rhs => BinaryOp("MOD", lhs, rhs) })("term MOD")
      | log(factor)("term factor")
    )
    def factor: Parser[Expression] = (
        log("(" ~> expr <~ ")")("factor (expr)")
      | log(("+" | "-") ~ factor ^^ { case op~rhs => UnaryOp(op, rhs) })("factor +-")
      //| function |
      | log(numericLit ^^ { case x => Number(x/*.toFloat*/) })("factor numericLit")
      | log(stringLit ^^ { case s => Literal(s) })("factor stringLit")
      | log(ident ^^ { case id => Variable(id) })("factor ident")
    )
    def parse(s: String) = stmts(new lexical.Scanner(s))
}

score 0 · Accepted Answer

我的第一个改进是这样的：

def term3: Parser[Expression] =
log((term2 ~ ("<>" | "=" | "NE" | "EQ") ~ term2) ^^ { case lhs~op~rhs => BinaryOp(op, lhs, rhs) })("term3 <>,=,NE,EQ") |
log(term2)("term3 term2")

它在没有 OutOfMemoryError 的情况下工作，但速度很慢。查看 doc/scala-devel-docs/examples/parsing/lambda/TestParser.scala 后，我得到了这个来源：

def expr: Parser[Expression] = term5
def term5: Parser[Expression] =
    log(chainl1(term4, term5, "OR" ^^ {o => (a: Expression, b: Expression) => BinaryOp(o, a, b)}))("term5 OR")
def term4: Parser[Expression] =
    log(chainl1(term3, term4, "AND" ^^ {o => (a: Expression, b: Expression) => BinaryOp(o, a, b)}))("term4 AND")
def term3: Parser[Expression] =
    log(chainl1(term2, term3, ("<>" | "=" | "NE" | "EQ") ^^ {o => (a: Expression, b: Expression) => BinaryOp(o, a, b)}))("term3 <>,=,NE,EQ")
def term2: Parser[Expression] =
    log(chainl1(term1, term2, ("<" | ">" | "<=" | ">=" | "LT" | "GT" | "LE" | "GE") ^^ {o => (a: Expression, b: Expression) => BinaryOp(o, a, b)}))("term2 <,>,...")
def term1: Parser[Expression] =
    log(chainl1(term, term1, ("+" | "-" | ":") ^^ {o => (a: Expression, b: Expression) => BinaryOp(o, a, b)}))("term1 +,-,:")
def term: Parser[Expression] =
    log(chainl1(factor, term, ("*" | "/" | "MOD") ^^ {o => (a: Expression, b: Expression) => BinaryOp(o, a, b)}))("term *,/,MOD")
def factor: Parser[Expression] =
    log("(" ~> expr <~ ")")("factor ()") |
    log(("+" | "-") ~ factor ^^ { case op~rhs => UnaryOp(op, rhs) })("factor unary") |
    log(function)("factor function") |
    log(numericLit ^^ { case x => Number(x/*.toFloat*/) })("factor numLit") |
    log(stringLit ^^ { case s => Literal(s) })("factor strLit") |
    log(ident ^^ { case id => Variable(id) })("factor ident")

它工作得很快。很抱歉，但我无法理解 chainl1 函数如何改进我的源代码。我不明白它是如何工作的。

scala - 斯卡拉。困难的表达式解析器。内存不足错误

2 回答 2

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