parsing - 如何用 Rebol PARSE 方言表达分支？

Question

我有一个像下面这样的 mysql 架构:

data: {
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `name` varchar(10) DEFAULT '' COMMENT 'the name',
    `content` text COMMENT 'something',
}

现在我想从中提取一些信息:文件名，类型和注释(如果有的话)。见下文:

["id" "int" "" "name" "varchar" "the name" "content" "text" "something" ]

我的代码是:

parse data [
    any [ 
        thru {`} copy field to {`} {`}
        thru some space copy field-type to [ {(} | space]
        (comm: "")
        opt [ thru {COMMENT} thru some space thru {'} copy comm to {'}]
        (repend temp field repend temp field-type either comm [ repend temp comm ][ repend temp ""])
    ]
]

但我得到这样的东西:

["id" "int" "the name" "content" "text" "something"]

我知道线路 opt ..是不正确的。

我要 express 如果找到 COMMENT先关键字，再提取评论信息；如果首先找到lf，则继续下一个循环。 但是我不知道怎么表达。任何人都可以帮忙吗？

Answer 1

我非常喜欢(在可能的情况下)用肯定的术语来构建一组语法规则来匹配目标输入——我发现它更有文字、更精确、更灵活并且更容易调试。在上面的代码段中，我们可以确定五个核心组件:

space: use [space][
    space: charset "^-^/ "
    [some space]
]

word: use [letter][
    letter: charset [#"a" - #"z" #"A" - #"Z" "_"]
    [some letter]
]

id: use [letter][
    letter: complement charset "`"
    [some letter]
]

number: use [digit][
    digit: charset "0123456789"
    [some digit]
]

string: use [char][
    char: complement charset "'"
    [any [some char | "''"]]
]

定义了术语后，编写一个描述输入语法的规则就相对简单了:

result: collect [
    parsed?: parse/all data [ ; parse/all for Rebol 2 compatibility
        opt space
        some [
            (field: type: none comment: copy "")
            "`" copy field id "`"
            space 
            copy type word opt ["(" number ")"]
            any [
                space [
                    "COMMENT" space "'" copy comment string "'"
                    | word | "'" string "'" | number
                ]
            ]
            opt space "," (keep reduce [field type comment])
            opt space
        ]
    ]
]

作为额外的奖励，我们可以验证输入。

if parsed? [new-line/all/skip result true 3]

new-line 的一个小应用来让事情变得更聪明一点应该会产生:

== [
    "id" "int" "" 
    "name" "varchar" "the name" 
    "content" "text" "something"
]