SwiftUI View protocol in Haskell

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#1

I am trying to implement a Haskell counterpart of the View protocol of SwiftUI using type class and type family.

The View protocol in SwiftUI is something like:

protocol View {
  associatedtype Body: View
  var body: Body { get }
}

Through type class and the associated type family, I can have:

{-# LANGUAGE TypeFamilies, AllowAmbiguousTypes, FlexibleContexts, U
ndecidableSuperClasses #-}

class (View (Body x)) => View x where
  type Body x
  body :: Body x

which fails to compile: solveWanteds: too many iterations (limit = 4).

My question is if there is a way to achieve such ‘recursive’ type class definition in Haskell?

#2

The problem doesn’t seem to be recursive definition, although the errors make it out to be like that.
Rather, it is this part:

body :: Body x

The function is considered as body :: View x => Body x. Compiler cannot infer x even with desired result type Body x, since you cannot know x from Body x. For instance, you might have Body Foo = Int, Body Bar = Int and you have no way of knowing if x should be Foo or Bar.

instead, I think what you should have is this:

class (View (Body x)) => View x where
  type Body x
  body :: x -> Body x

It seems like Swift protocol assumes this instance. In haskell, typeclasses does not have this or anything referring to the self instance. So, you need to provide it yourself.

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#3

Thank you for your reply! This makes a lot of sense to me.