数据声明Haskell中的类型约束 [英] Type Constraints in Data Declaration Haskell

问题描述

我正在使用Haskell并尝试编写以下内容:

I'm using Haskell and trying to write the following:

data Scale s = Scale s s

但是，我要使它成为s必须是Num类型类的东西，例如Int或Double.使用Haskell和GHC可以做到吗?

However, I want to make it so that s must be something that of the Num type class, like Int or Double. Is that possible to do using Haskell and GHC?

推荐答案

是:

{-# LANGUAGE GADTs #-}
data Scale s where
    Scale :: Num s => s -> s -> Scale s

但是，通常认为这样做不是最佳实践.而是将Num约束仅放在使用Scale s和需要 Num约束的函数上.放宽对此类约束的限制，使您可以在适当的时候暂时打破不变式.例如通常希望为这种类型的Functor实例，如果按上述约束构造函数，则是不可能的.

However, it's generally considered best practice not to do this. Instead, put the Num constraint only on the functions that use Scales and need the Num constraint. Being relaxed about such constraints allows you to temporarily break the invariant where appropriate; e.g. it's common to wish for a Functor instance for such a type, which is impossible if you constrain the constructor as above.

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