从Haskell中的字符串中推断出类型

发布于 2025-02-13 10:22:26 字数 942 浏览 1 评论 0原文

我正在研究Haskell中的JSON数据编码器！（更具体地说，我正在尝试移植 jo 到haskell！）。我已经有很多工作，但是我遇到了一些皱纹。我会在这里努力简化我的问题，我试图剥夺尽可能多的不需要的上下文。

目标：构建value给定的字符串。

这些字符串来自命令行：用户以＆lt; key＆gt; =＆lt; value＆gt;的形式输入键/值对。将它们分开后，我被留在一个String中，即未知类型的值数据。

示例案例：

let s = "someString" -- use the `String` constructor
let s = "1234"       -- use the `Number` constructor
let s = "True"       -- use the `Bool` constructor

问题：如何推断s的内容是String vs a numbern，bool /code>等？

这是Aeson value type的相关类型 +构造函数（编辑为简短）。

data Value = Object Object
           | Array Array
           | String Text
           | Number Scientific
           | Bool Bool
           | Null

原文

I'm working on a JSON data encoder in Haskell! (to be more specific, I am trying to port Jo into Haskell!). I've gotten a lot of it working, but I'm running into a little wrinkle. I'll try to be concise with my question here, I've tried to strip away as much unneeded context as possible.

Goal: Construct a Value given a String.

These strings come from the command line: Users enter in key/value pairs in the form <key>=<value>. After splitting them apart I am left in a String that is the value data of unknown type.

Example cases:

let s = "someString" -- use the `String` constructor
let s = "1234"       -- use the `Number` constructor
let s = "True"       -- use the `Bool` constructor

Question: How might I infer that the contents of s is a String vs a Number, Bool, etc?

This is the relevant type + constructors for the Aeson Value type (edited for brevity).

data Value = Object Object
           | Array Array
           | String Text
           | Number Scientific
           | Bool Bool
           | Null

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南冥有猫 2025-02-20 10:22:26

由于您已经在使用 aeson 软件包，因此可以使用 decode 。之所以起作用，是因为value也是bytestring实例：

Prelude Data.Aeson> decode "\"someString\"" :: Maybe Value
Just (String "someString")
Prelude Data.Aeson> decode "1234" :: Maybe Value
Just (Number 1234.0)
Prelude Data.Aeson> decode "true" :: Maybe Value
Just (Bool True)

注意（如n。1.8e9-mys-my-share m。在评论）必须引用字符串。

那么，您可以做的是将您的未知价值拿出来，并首先用引号包围，并试图解析它。然后尝试在没有周围的引号的情况下再次解析它。

现在，您有两个也许值值。选择第一个仅值（并准备处理nothing案例）。

Since you're already using the aeson package, you could use decode. This works because Value is also a ByteString instance:

Prelude Data.Aeson> decode "\"someString\"" :: Maybe Value
Just (String "someString")
Prelude Data.Aeson> decode "1234" :: Maybe Value
Just (Number 1234.0)
Prelude Data.Aeson> decode "true" :: Maybe Value
Just (Bool True)

Notice (as n. 1.8e9-where's-my-share m. points out in the comments) that strings must be quoted.

What you could do, then, is to take your unknown value and first surround it with quotes and attempt to parse it. Then try to parse it again without surrounding quotes.

You now have two Maybe Value values. Pick the first Just value (and be prepared to deal with the Nothing case).

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