Haskell 数据声明中的类型约束

Question

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

data Scale s = Scale s s

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?

Answer 1

Yes:

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

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

Answer 2

I had a similar situation with Point type. But I thought not about constraints, I thought about how to do generalizing element type of my point. Then I understood if I'll have point type like this data Point a = Point aa then I can do it instance of Functor, Applicative, Foldable and Traversable. And I'll can design function by standart general way. For example:

dist :: Floating a => Point a -> Point a -> a
dist a b = sqrt $ sum $ (^2) <$> ((-) <$> a <*> b)

And I had question. What is going on? :) If I add constraint (as you asked) I would not can design by this way and I would need to implement a lot of functions like pointSub .

So, there is something to think about :)

Answer 3

What about this:

data Scale' s = Scale s s
type Scale = Scale' Int Int

I did not try this, I do not know Haskell that well, just (mostly) reading about it, but it seems reasonable, isn't it? 🤔