英文:
Why is my FParsec parser failing to recognize a block comment?
问题
我正在尝试使用FParsec解析C样式的注释。不确定为什么会失败：
我的解析器代码：
let openComment : Parser<_,unit> = pstring "/*"let closeComment : Parser<_,unit> = pstring "*/"let comment = pstring "//" >>. restOfLine true<|> openComment >>. (charsTillString "*/" true System.Int32.MaxValue) >> Comment// <|> openComment >>. manyCharsTill anyChar closeComment >> Commentlet spaceComments = many ((spaces1 >> IgnoreU) <|> comment)let str s = spaceComments >>. pstring s >> spaceComments
测试代码：
let testStr = @"// test comment/* a block comment*/x // another comment"match run (str "x") testStr with| Success(result, _, _) -> printfn "Success: %A" result| Failure(errorMsg, _, _) -> assert false()
错误信息。无论是charsTillString还是manyCharsTill，都是相同的错误信息：
Error in Ln: 6 Col: 4^Note: The error occurred at the end of the input stream.Could not find the string '*/'.
Comment和IgnoreU都是字符串的枚举类型。
英文:
I'm trying to parse C style comments using FParsec. Not sure why this is failing:
My parser code:
let openComment : Parser<_,unit> = pstring "/*"let closeComment : Parser<_,unit> = pstring "*/"let comment = pstring "//" >>. restOfLine true<|> openComment >>. (charsTillString "*/" true System.Int32.MaxValue) |>> Comment//<|> openComment >>. manyCharsTill anyChar closeComment |>> Commentlet spaceComments = many ((spaces1 |>> IgnoreU) <|> comment)let str s = spaceComments >>. pstring s .>> spaceComments
Test Harness:
let testStr = @"// test comment/* a block comment*/x // another comment"match run (str "x") testStr with| Success(result, _, _) -> printfn "Success: %A" result| Failure(errorMsg, _, _) -> assert false()
Error messager. It is the same for both charsTillString and manyCharsTill
Error in Ln: 6 Col: 4^Note: The error occurred at the end of the input stream.Could not find the string '*/'.
Comment and IgnoreU are both a discrimated type of string
答案1
得分: 5
问题在于您的 comment 解析器中的组合子没有您想要的优先级/结合性。您可以通过使用括号进行分组来修复这个问题：
let comment = (pstring "//" >>. restOfLine true)<|> (openComment >>. (charsTillString "*/" true System.Int32.MaxValue)) >>. Comment
我发现对于复杂的解析器，choice 通常比 <|> 更容易阅读：
let comment =choice [pstring "//" >>. restOfLine trueopenComment >>. (charsTillString "*/" true System.Int32.MaxValue)] >>. Comment
英文:
The problem is that the combinators in your comment parser don't have the precedence/associativity that you want. You can fix this by grouping with parens:
let comment = (pstring "//" >>. restOfLine true)<|> (openComment >>. (charsTillString "*/" true System.Int32.MaxValue)) |>> Comment
I find that choice is often easier to read than <|> for complex parsers:
let comment =choice [pstring "//" >>. restOfLine trueopenComment >>. (charsTillString "*/" true System.Int32.MaxValue)] |>> Comment
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