我可以有一个名为"/"的值构造函数吗? [英] Can I have a value constructor named "/""?

问题描述

我已经声明了具有以下结构的递归数据类型:

I've declared a recursive data type with the following structure:

data Path = GET | POST | Slash Path String

我真的很想将最后一个值构造函数重命名为/，这样我就可以在诸如GET /"controller"/"action"之类的可爱表达式中插入它.但是，如果我尝试这样做:

I'd really like to rename that last value constructor to / so that I can infix it in cute expressions like GET /"controller"/"action". However, if I try to do so:

import Prelude hiding ((/))
infixr 5 /
data Path = GET | POST | Path / String

...然后我得到了:

...then I get this:

Path.hs:4:30: parse error on input `/'

如果我将/替换为:/或其他任何以:开头的特殊字符序列，那么同样的三行编译也很好.

Those same three lines compile just fine if I replace / with :/ or any other special character sequence beginning with :.

那么，有什么方法可以命名我的值构造函数/?我知道我可以将其命名为Slash，然后声明一个单独的函数:

So, is there any way I can name my value constructor /? I know that I can just name it Slash and then declare a separate function:

(/) :: Path -> String -> Path 
(/) = Slash

...但是那样不会让我进行模式匹配，如下所示:

...but that won't let me pattern match, as in:

request :: Path -> String
request path = case path of GET /"hello" -> "Hello!"
                            GET /"goodbye" -> "Goodbye!"

推荐答案

简短答案:否.

长答案:类型类，类型名称和数据构造函数必须以大写字母或冒号开头(其中某些要求使用语言扩展名).其他所有内容都必须以小写字母或任何其他允许的符号开头.

Long answer: Type classes, type names, and data constructors must begin with either a capital letter or a colon (some of this requires using a language extension). Everything else must begin with a lowercase letter or any other allowed symbol.

请注意，通常是小写字母标识符的变量类型遵循相同的规则，并且不以冒号开头.

Note that type variables, which are normally lowercase identifiers, follow the same rules and do not begin with a colon.

另请参见 GHC用户指南，用于启用类型运算符.我认为，始终允许使用数据构造函数.

See also the GHC user's guide for enabling type operators. Data constructors are always allowed, I think.

就您而言，我个人将使用(:/).看起来还不错，过了一会儿您就习惯了忽略冒号.有些人也喜欢尾随冒号，特别是如果数据在某种意义上是对称的".

Personally, in your case I'd just use (:/). It doesn't look that bad, and after a while you get used to ignoring the colons. Some people like a trailing colon as well, especially if the data is "symmetric" in some sense.

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