提高FParsec解析器的可读性

Question

我在C#中完成了一个手写的CSS解析器,这个解析器变得难以管理,并且正在尝试使用FParsec来使其更具可持续性.这是一个解析用regexes制作的css选择器元素的片段:

var tagRegex = @"(?<Tag>(?:[a-zA-Z][_\-0-9a-zA-Z]*|\*))";
var idRegex = @"(?:#(?<Id>[a-zA-Z][_\-0-9a-zA-Z]*))";
var classesRegex = @"(?<Classes>(?:\.[a-zA-Z][_\-0-9a-zA-Z]*)+)";
var pseudoClassRegex = @"(?::(?<PseudoClass>link|visited|hover|active|before|after|first-line|first-letter))";
var selectorRegex = new Regex("(?:(?:" + tagRegex + "?" + idRegex + ")|" +
                                 "(?:" + tagRegex + "?" + classesRegex + ")|" +
                                  tagRegex + ")" +
                               pseudoClassRegex + "?");

var m = selectorRegex.Match(str);

if (m.Length != str.Length) {
    cssParserTraceSwitch.WriteLine("Unrecognized selector: " + str);
    return null;
}

string tagName = m.Groups["Tag"].Value;

string pseudoClassString = m.Groups["PseudoClass"].Value;
CssPseudoClass pseudoClass;
if (pseudoClassString.IsEmpty()) {
    pseudoClass = CssPseudoClass.None;
} else {
    switch (pseudoClassString.ToLower()) {
        case "link":
            pseudoClass = CssPseudoClass.Link;
            break;
        case "visited":
            pseudoClass = CssPseudoClass.Visited;
            break;
        case "hover":
            pseudoClass = CssPseudoClass.Hover;
            break;
        case "active":
            pseudoClass = CssPseudoClass.Active;
            break;
        case "before":
            pseudoClass = CssPseudoClass.Before;
            break;
        case "after":
            pseudoClass = CssPseudoClass.After;
            break;
        case "first-line":
            pseudoClass = CssPseudoClass.FirstLine;
            break;
        case "first-letter":
            pseudoClass = CssPseudoClass.FirstLetter;
            break;
        default:
            cssParserTraceSwitch.WriteLine("Unrecognized selector: " + str);
            return null;
    }
}

string cssClassesString = m.Groups["Classes"].Value;
string[] cssClasses = cssClassesString.IsEmpty() ? EmptyArray<string>.Instance : cssClassesString.Substring(1).Split('.');
allCssClasses.AddRange(cssClasses);

return new CssSelectorElement(
    tagName.ToLower(),
    cssClasses,
    m.Groups["Id"].Value,
    pseudoClass);

我的第一次尝试产生了这个:

type CssPseudoClass =
    | None = 0
    | Link = 1
    | Visited = 2
    | Hover = 3
    | Active = 4
    | Before = 5
    | After = 6
    | FirstLine = 7
    | FirstLetter = 8

type CssSelectorElement = 
    { Tag : string
      Id : string
      Classes : string list    
      PseudoClass : CssPseudoClass } 
with
    static member Default = 
        { Tag = "";
          Id = "";
          Classes = [];
          PseudoClass = CssPseudoClass.None; }

open FParsec

let ws = spaces
let str = skipString
let strWithResult str result = skipString str >>. preturn result

let identifier =
    let isIdentifierFirstChar c = isLetter c || c = '-'
    let isIdentifierChar c = isLetter c || isDigit c || c = '_' || c = '-'    
    optional (str "-") >>. many1Satisfy2L isIdentifierFirstChar isIdentifierChar "identifier"

let stringFromOptional strOption =
    match strOption with
    | Some(str) -> str
    | None -> ""

let pseudoClassFromOptional pseudoClassOption =
    match pseudoClassOption with
    | Some(pseudoClassOption) -> pseudoClassOption
    | None -> CssPseudoClass.None

let parseCssSelectorElement =
    let tag = identifier <?> "tagName"
    let id = str "#" >>. identifier <?> "#id"
    let classes = many1 (str "." >>. identifier) <?> ".className"
    let parseCssPseudoClass =
        choiceL [ strWithResult "link" CssPseudoClass.Link;
                  strWithResult "visited" CssPseudoClass.Visited;
                  strWithResult "hover" CssPseudoClass.Hover;
                  strWithResult "active" CssPseudoClass.Active;
                  strWithResult "before" CssPseudoClass.Before;
                  strWithResult "after" CssPseudoClass.After;
                  strWithResult "first-line" CssPseudoClass.FirstLine;
                  strWithResult "first-letter" CssPseudoClass.FirstLetter]
                 "pseudo-class"    
    // (tag?id|tag?classes|tag)pseudoClass?
    pipe2 ((pipe2 (opt tag) 
                  id
                  (fun tag id -> 
                      { CssSelectorElement.Default with 
                          Tag = stringFromOptional tag;
                          Id = id })) |> attempt
           <|>
           (pipe2 (opt tag) 
                  classes
                  (fun tag classes -> 
                      { CssSelectorElement.Default with 
                          Tag = stringFromOptional tag;
                          Classes = classes })) |> attempt
           <|>
           (tag |>> (fun tag -> { CssSelectorElement.Default with Tag = tag })))
           (opt (str ":" >>. parseCssPseudoClass) |> attempt)
           (fun selectorElem pseudoClass -> { selectorElem with PseudoClass = pseudoClassFromOptional pseudoClass })

但我并不是很喜欢它的形成方式.我期待得到一些更容易理解的东西,但是部分解析(tag？id | tag？classes | tag)pseudoClass？有几个pipe2和尝试是非常糟糕的.

来自有更多FParsec经验的人教会我更好的方法来实现这一目标吗？我正在考虑尝试FSLex/Yacc或Boost.Spirit而不是FParsec,看看我是否可以用它们提出更好的代码

Answer 1

您可以将该复杂解析器的某些部分提取到变量中,例如:

let tagid = 
    pipe2 (opt tag) 
      id
      (fun tag id -> 
          { CssSelectorElement.Default with 
              Tag = stringFromOptional tag
              Id = id })

您也可以尝试使用应用程序界面,我个人觉得它比pipe2更容易使用和思考:

let tagid =
    (fun tag id -> 
          { CssSelectorElement.Default with 
              Tag = stringFromOptional tag
              Id = id })
    <!> opt tag
    <*> id