Question

所以我试图用F＃编写一个编译器，并且一直在查看F＃powerpack附带的Fslex和Fsyacc工具。有一个示例项目负责我一直试图理解的外部构建工具。它可以下载here。该示例为我编译并运行，但我认为语法中存在一个微妙的错误。我说微妙，因为语法看起来类似于我在龙书中看到的解析表达式，我没有经验可以发现它。

输入“4 * 5 + 3”正确评估为23。

然而，输入4 * 5-3会生成解析错误。这是Fsyacc生成的代码中的错误。

我将非常感谢您的帮助，以便更好地了解问题所以我可以更好地了解并对Fsyacc更有信心。我在下面发布了* .fsy文件。

// This is the type of the data produced by a successful reduction of the 'start'
// symbol:
%type < Ast.Equation > start

%%

// These are the rules of the grammar along with the F# code of the 
// actions executed as rules are reduced.  In this case the actions 
// produce data using F# data construction terms.
start: Prog { Equation($1) }

Prog:
    | Expr EOF                  { $1 }

Expr: 
    | Expr PLUS  Term           { Plus($1, $3)  }
    | Expr MINUS Term           { Minus($1, $3) }
    | Term                      { Term($1)      }

Term:
    | Term ASTER Factor         { Times($1, $3)  }
    | Term SLASH Factor         { Divide($1, $3) }
    | Factor                    { Factor($1)     }

Factor:
    | FLOAT                     { Float($1)  }
    | INT32                     { Integer($1) }
    | LPAREN Expr RPAREN        { ParenEx($2) }

这是AST数据类型的定义

namespace Ast
open System

type Factor =
    | Float   of Double
    | Integer of Int32
    | ParenEx of Expr

and Term =
    | Times  of Term * Factor
    | Divide of Term * Factor
    | Factor of Factor

and Expr =
    | Plus  of Expr * Term
    | Minus of Expr * Term
    | Term  of Term

and Equation =
    | Equation of Expr

修改

我发布了词法分析器定义和驱动解析器的代码，以帮助理解错误。

{
module Lexer
open System
open Parser
open Microsoft.FSharp.Text.Lexing

let lexeme lexbuf =
    LexBuffer<char>.LexemeString lexbuf
}

// These are some regular expression definitions
let digit = ['0'-'9']
let whitespace = [' ' '\t' ]
let 

newline = ('\n' | '\r' '\n')

rule tokenize = parse
| whitespace    { tokenize lexbuf }
| newline       { tokenize lexbuf }
// Operators
| "+"           { PLUS }
| "-"           { MINUS }
| "*"           { ASTER }
| "/"           { SLASH }
// Misc
| "("           { LPAREN }
| ")"           { RPAREN }
// Numberic constants
| ['-']?digit+                                  { INT32 (Int32.Parse(lexeme lexbuf)) }
| ['-']?digit+('.'digit+)?(['e''E']digit+)?     { FLOAT (Double.Parse(lexeme lexbuf)) }
// EOF
| eof   { EOF }

最后，驱动解析器的代码。

    // This project type requires the F# PowerPack at http://fsharppowerpack.codeplex.com/releases
    // Learn more about F# at http://fsharp.net
    // Original project template by Jomo Fisher based on work of Brian McNamara, Don Syme and Matt Valerio
    // This posting is provided "AS IS" with no warranties, and confers no rights.

    open System
    open Microsoft.FSharp.Text.Lexing

    open Ast
    open Lexer
    open Parser

    /// Evaluate a factor
    let rec evalFactor factor =
        match factor with
        | Float x   -> x
        | Integer x -> float x
        | ParenEx x -> evalExpr x

    /// Evaluate a term
    and evalTerm term =
        match term with
        | Times (term1, term2)  -> (evalTerm term1) * (evalTerm term2)
        | Divide (term1, term2)  -> (evalTerm term1) / (evalTerm term2)
        | Factor fact         -> evalFactor fact

    /// Evaluate an expression
    and evalExpr expr =
        match expr with
        | Plus (expr1, expr2)  -> (evalExpr expr1) + (evalExpr expr2)
        | Minus (expr1, expr2)  -> (evalExpr expr1) - (evalExpr expr2)
        | Term term          -> evalTerm term

    /// Evaluate an equation
    and evalEquation eq =
        match eq with
        | Equation expr -> evalExpr expr

    printfn "Calculator"

    let rec readAndProcess() =
        printf ":"
        match Console.ReadLine() with
        | "quit" -> ()
        | expr ->
            try
                printfn "Lexing [%s]" expr
                let lexbuff = LexBuffer<char>.FromString(expr)

                printfn "Parsing..."
                let equation = Parser.start Lexer.tokenize lexbuff

                printfn "Evaluating Equation..."
                let result = evalEquation equation

                printfn "

Result: %s" (result.ToString())

        with ex ->
            printfn "Unhandled Exception: %s" ex.Message

        readAndProcess()

readAndProcess()

编辑：词法分析器中的可选减号是问题所在。移除后，样本按预期工作。

Answer 1

我只是瞥了一眼，看起来lexer可能正在治疗

// Numberic constants 
| ['-']?digit+                                  { INT32 (Int32.Parse(lexeme lexbuf)) } 
etc

这里的减号

4*5-3

作为一元，常数“-3”的一部分而不是二进制减号。所以我同意这是样本中的错误。我将摆脱词法分析器中的可选减号，并在因子的行中在解析器中添加一个规则。 “减去INT32”。

只是一个如何解决它的草图，希望这将引导你，或者你将获得另一个更全面的代码更深入的答案。

Fsyacc的示例语法错误？

1 个答案: