我可以在 GHCi 中添加实例声明吗

发布于 2024-12-21 12:27:29 字数 458 浏览 3 评论 0原文

我正在摆弄 HashMap 并尝试使用 Data.Bson.ObjectId 作为键。当然，我发现该结构没有 Hashable 实例。没关系，因为编写一个代码很简单。¹

instance Hashable ObjectId where hash (Oid x y) = hash (x,y)

我在 GHCi 中输入了这一行，并被告知“在输入‘实例’上解析错误”。这实际上是有意义的，因为 GHCi 提示符的运行方式就好像这些行被键入到 IO monad 中的 do 块中，并且无法在此上下文中定义实例。

那么我的问题是，有没有办法在 GHCi 中定义新实例？

¹ 为什么库不提供这个实例是另一回事。我相信答案是限制依赖关系，除了 bson 包已经依赖于阳光下的一切。

原文

I was messing around with HashMap and tried to use a Data.Bson.ObjectId as a key. I, of course, discovered that there is not a Hashable instance for that structure. That's ok, because writing one is trivial.¹

instance Hashable ObjectId where hash (Oid x y) = hash (x,y)

I typed that line into GHCi and was told "parse error on input `instance'". This actually makes sense as the GHCi prompt operates as if the lines were being typed into a do block in the IO monad and an instance can not be defined in this context.

My question then, is there a way to define a new instance within GHCi?

¹ Why this instance is not provided by the library is another matter. I would believe the answer was to limit dependencies except that the bson package already depends on everything under the sun.

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一身仙ぐ女味 2024-12-28 12:27:29

好消息：是的，有一种方法可以在 GHCi 中定义新实例。

坏消息：目前，这样做的第一步是“安装 GHC 的开发快照”。

很长一段时间以来，这一直是 GHCi 中明显缺失的功能。它的缺席并没有什么内在原因，但我认为实施起来有些困难，所以它被搁置了。

然而，似乎从版本 7.4.1 开始，现在可用：

在 GHCi 提示符下，您还可以输入任何顶级 Haskell 声明，包括数据、类型、newtype、类、实例、派生和外部声明。例如：
前奏>数据 T = A |乙| C 导出（Eq、Ord、Show、Enum）
前奏> [一..]
[A、B、C]
前奏> ：它
数据 T = A |乙| C -- 定义于 :2:6
实例枚举 T -- 定义于 :2:45
实例 Eq T -- 定义于 :2:30
实例 Ord T -- 定义于 :2:34
实例 Show T -- 定义于 :2:39

您是否认为现在值得安装 GHC 的非发行版本，这取决于您。

The good news: Yes, there is a way to define a new instance within GHCi.

The bad news: At the moment, the first step in doing so is "install a development snapshot of GHC".

This has been an obvious bit of missing functionality in GHCi for quite a while. There was no inherent reason for it to be absent, but I assume it was somewhat difficult to implement and so it got set aside.

However, it seems that as of version 7.4.1, it's now available:

At the GHCi prompt you can also enter any top-level Haskell declaration, including data, type, newtype, class, instance, deriving, and foreign declarations. For example:
Prelude> data T = A | B | C deriving (Eq, Ord, Show, Enum)
Prelude> [A ..]
[A,B,C]
Prelude> :i T
data T = A | B | C      -- Defined at <interactive>:2:6
instance Enum T -- Defined at <interactive>:2:45
instance Eq T -- Defined at <interactive>:2:30
instance Ord T -- Defined at <interactive>:2:34
instance Show T -- Defined at <interactive>:2:39

Whether you think having that right now worth the hassle of installing a non-release version of GHC is up to you.

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