解析 if / else / if 语句

发布于 2024-11-15 02:35:27 字数 687 浏览 2 评论 0原文

我试图复制一个简单的 if 语句的结构：

if (paren) { block } [else ({ block } | rec if (paren)) ]

对于 if (paren) 块，我创建一个 IfBlock AST 节点。否则，它递归地填充 IfElseBlock 节点。

我已经尝试了很多替代结构

let parse_if = 
    suffixparen .>>. suffixblock |>> IfBlock 
    //>>? attempt (str "else" >>. ifParser) |>> IfElseBlock 
    //<|> preturn IfBlock
    // .>>? attempt (str "else" >>. ifParser) |>> IfElseBlock
    // suffixparen .>>. suffixblock |>> IfBlock 
    // <|> ifParser |>> IfElseBlock

let inlineIf = str_ws "if" >>. parse_if
do ifParserR := inlineIf

建议？

原文

I'm trying to replicate the structure of a simple if statement:

if (paren) { block } [else ({ block } | rec if (paren)) ]

for if (paren) block, I create a IfBlock AST node. Otherwise, it recursively fills a IfElseBlock node.

I've tried quite a few alternate constructions

let parse_if = 
    suffixparen .>>. suffixblock |>> IfBlock 
    //>>? attempt (str "else" >>. ifParser) |>> IfElseBlock 
    //<|> preturn IfBlock
    // .>>? attempt (str "else" >>. ifParser) |>> IfElseBlock
    // suffixparen .>>. suffixblock |>> IfBlock 
    // <|> ifParser |>> IfElseBlock

let inlineIf = str_ws "if" >>. parse_if
do ifParserR := inlineIf

Suggestions?

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沧笙踏歌 2024-11-22 02:35:27

你看过我的 GLSL 解析器（它是一种类似 C 的语言）吗？
http://laurent.le-brun.eu/fsharp/glsl_parse.fs

从代码示例中，这里是 if 语句的相关部分：

let statement, stmtRef = createParserForwardedToRef()

let ifStatement =
    pipe3 (keyword "if" >>. parenExp) statement (opt (keyword "else" >>. statement))
      (fun cond stmt1 stmt2 -> Ast.If(cond, stmt1, stmt2))

stmtRef := choice [
  simpleStatement
  block
  ifStatement
  forLoop
  //...
  ]

我认为您的问题是您使用的是 attempt 而不是 opt 。 opt 表示 else 部分是可选的（如果不存在，则为 None）。 attempt 是完全不同的：

解析器尝试p应用
解析器p。如果 p 更改后失败
解析器状态或带有致命错误
错误，attempt p 将回溯到
原始解析器状态并报告
非致命错误。

当 attempt 中的解析器失败时，仍然会出现错误，但输入不会被消耗（与 <|> 或 choice 运算符）。

Have you had a look at my GLSL parser (it's a C-like language)?
http://laurent.le-brun.eu/fsharp/glsl_parse.fs

From the code example, here is the relevant part for the if statement:

let statement, stmtRef = createParserForwardedToRef()

let ifStatement =
    pipe3 (keyword "if" >>. parenExp) statement (opt (keyword "else" >>. statement))
      (fun cond stmt1 stmt2 -> Ast.If(cond, stmt1, stmt2))

stmtRef := choice [
  simpleStatement
  block
  ifStatement
  forLoop
  //...
  ]

I think your problem is that you're using attempt instead of opt. opt means the else part is optional (if it's not there, you get None). attempt is quite different:

The parser attempt p applies the
parser p. If p fails after changing
the parser state or with a fatal
error, attempt p will backtrack to the
original parser state and report a
non‐fatal error.

When the parser in attempt fails, there's still an error but the input is not consumed (it's useful when combined with the <|> or choice operators).

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