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?
3 Answers
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 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.
Comments
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 a a 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 :)
2 Comments
Daniel Wagner
Indeed, you can even make a Monad instance for such a type.
freestyle
Yes, I known, you are right. But I thing it is not intrestiing for this example :)
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? 🤔
2 Comments
leftaroundabout
This certainly is reasonable, but it's quite different from what the OP was asking about.
Witold Szczerba
I think it solves the main issue hidden behind the question though.
Num sconstraint only at the functions that actually need the it.