斯卡拉。困難的表達式解析器。 OutOfMemoryError

Question

我想創建一個解析器，用於操作順序的困難表達式。我有一些例子，但它工作非常緩慢，並拋出異常OutOfMemoryError。我該如何改進它？

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) } 

Answer 1

基本上，它速度慢，消耗的內存太多，因爲你的語法是非常低效的。

讓我們考慮的第二行：B = A:(1+2)。它會嘗試解析此行是這樣的：

1. term4 OR term4然後term4。
2. term3 AND term3然後term3。
3. term2 <> term2，然後=，然後NE然後EQ然後term2。
4. term1 8個不同的運營商term1，then term1。
5. 長期+來看，長期-來看，長期:項，然後項。
6. 因子*因子，因子/因子，因子MOD因子，然後因子。
7. 括號表達式，一元因子，函數，數字文字，字符串文字，ident。

的第一次嘗試是這樣的：

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

我跳過括號，一元，功能，數字和字符串，因爲它們不匹配A - 雖然function可能做，但它的定義不可用。現在，由於:不匹配*，它會在下一個嘗試：

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 

現在轉到下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 

而接下來：

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！我們終於在term1上獲得了一場比賽！但(不匹配<，所以它必須嘗試下一個詞條2：

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

等等

所有，因爲每行的第一項每學期會始終保持一致！要匹配一個簡單的數字，它必須解析factor 2 * 2 * 5 * 9 * 4 * 4 = 2880次！

但這不是故事的一半！你看，因爲termX重複了兩次，它會在這兩個兩側重複所有這些東西。例如，對於A:(1+2)的第一場比賽是這樣的：

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

這是不正確，所以它會嘗試>代替<，然後<=，等等，等等

我把一個記錄版本下面的解析器。試着運行它，看看它試圖分析的所有東西。

同時，有很多關於如何編寫這些可用解析器的例子。使用sbaz，嘗試：

sbaz install scala-devel-docs 

然後看看doc/scala-devel-docs/examples/parsing目錄Scala發行的內部，你會發現幾個例子。

這是你的解析器的版本（無function），它記錄它會嘗試一切：

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)) 
} 

Answer 2

我的第一個改進是這樣的：

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

它的工作原理沒有OutOfMemoryError異常，但放緩。查看DOC /斯卡拉-devel的-文檔/例子/分析/λ/ TestParser.scala後，我得到這個來源：

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") 

它的工作原理快。我很抱歉，但我不明白chainl1的功能如何提高我的來源。我不明白它是如何工作的。