数据记录的类限制 [英] Class constraints for data records

问题描述

我有一个 data 类型：

I have a data type :

data BuildException a = KillBuild JobID a Stage
                      | FailBuild JobID a Stage
                      | CancelBuild JobID a Stage
                      | StopBuild JobID a Stage
                         deriving Typeable

其中a必须具有Foo类的实例。我记得阅读（也许在RWH中）
，而在 data 定义中可能有类限制，这是不可取的。
然后执行此操作的正确方法是什么？

where a has to have an instance for class Foo. I remember reading (in RWH, maybe) that while it was possible to have class constraints in a data definition, it was not desirable. What's the proper way to do this then?

推荐答案

简单地将类型约束应用于需要它的函数。例如：

Simply apply the type constraints on functions that need it. For example:

makeException :: Foo a => String -> BuildException a

甚至可能有一些函数不需要 Foo 约束，这可能使API的客户端可以使用它的一个子集而不必定义 Foo 约束！这是不可能的，如果它是数据记录定义的一部分。

There might even be some functions that don't need the Foo constraint, which might make it possible for the client of your API to use a subset of it without having to define the Foo constraint! That wouldn't be possible if it was part of the data record definition.

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