wiki:DefaultSuperclassInstances

Version 6 (modified by simonpj, 3 years ago) (diff)

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A matter of much consternation, here is a proposal to allow type class declarations to include default instance declarations for their superclasses. It's based on Jón Fairbairn's proposal, but it has a more explicit 'off switch' and the policy on corner-cases is rejection. Credit is due also to the class system extension proposal and its ancestors, in particular, John Meacham's class alias proposal.

We may distinguish two uses of superclasses (not necessarily exclusive). A class can widen its superclass, extending its interface with new functionality (e.g., adding an inverse to a monoid to obtain a group -- inversion seldom provides an implementation of composition). A class can deepen its superclass (e.g., an implementation of Traversable f delivers at least enough technology to deliver Foldable f and Functor f). This proposal concerns the latter phenomenon, which is currently such a nuisance that Functor and Applicative are not superclasses of Monad. Nobody wants to be forced to write Functor and Applicative instances, just to access the Monad interface. Moreover, any proposal to refine the library by splitting a type class into depth-layers is (rightly!) greeted with howls of protest as an absence of superclass instances gives rise to breakage of the existing codebase.

Concretely, the proposal is to

  • Allow class declarations to include default superclass instance delcaration for some, none, or all of their given superclass constraints, provided all such instances have distinct classes. We say that superclasses with default implementations are intrinsic superclasses. Yes to
        class Functor f => Applicative f where
          return :: x -> f x
          (<*>) :: f (s -> t) -> f s -> f t
          (>>) :: f s -> f t -> f t
          fs >> ft = return (flip const) <*> fs <*> ft
          instance Functor f where
            fmap = (<*>) . return
    
        class Applicative f => Monad f where
          (>>=) :: f a -> (a -> f b) -> f b
          instance Applicative f where
            ff <*> fs = ff >>= \ f -> fs >>= \ s -> return (f s)
    

but no to

    class (Tweedle dum, Tweedle dee) => Rum dum dee where
      instance Tweedle dum where ...
      instance Tweedle dee where ...
  • let subclass instance declarations spawn intrinsic superclass instances by default -- if we have
        class Bar t[x] => Foo x where
          instance Bar t[x] where ...
    
        instance C => Foo s where ...
    

we automatically acquire a default superclass instance

    instance C => Bar t[s] where ...
  • let subclass instance declarations provide and override the methods of their intrinsic superclasses with no extra delimitation; so we may write
        instance Monad Blah where
          return x = ...
          ba >>= bf = ...
    

and acquire the Monad instance, along with fully formed Applicative and Functor instances. By requiring that intrinsic superclasses be class-distinct, we ensure that the distribution of methods to spawned instances is unambiguous. Moreover, local overrides beat the default. If we write

    instance Monad Blah where
      return x = ...
      ba >>= bf = ...
      bs >> bt = ...

we override the default (>>) but keep the (<*>) in the spawned Applicative instance.

  • to inhibit default-spawning with the syntax
        instance Sub x where
          ...
          hiding instance Super
    

which acts to prevent the generation of instances for Super and all of Super's intrinsic superclasses in turn. We need this, so that we can write

    instance Monad Blah where
      return x = ...
      ba >>= bf = ...
      hiding instance Functor

    instance Traversable Blah where
      traverse f bx = ...  -- inducing a default implementation of Functor

or indeed to turn off all the defaults and provide a standalone Functor instance.

  • while we're about it, to allow multi-headed instance declarations for class-disjoint conjunctions, with the same semantics for constraint duplication and method distribution as for the defaults, so
        instance S => (C x, C' x) where
          methodOfC  = ...
          methodOfC' = ...
    

is short for

    instance S => C x where
      methodOfC  = ...
    instance S => C' x where
      methodOfC' = ...

This proposal fits handily with the kind Fact proposal, which allows multiple constraints to be abbreviated by ordinary type synonyms.

Default superclass instances are implemented in the Strathclyde Haskell Enhancement. They should enable some tidying of the library, with relatively few tears. Moreover, they should allow us to deepen type class hierarchies as we learn. Retaining backward compatibility in relative silence is the motivation for an opt-in default.

Oleg and others note: just because you can make default instances, they are not always the instances you want. A key example is

    instance Monad m => Monad (ReaderT r m) where ...

which would give us by default

    instance Monad m => Applicative (ReaderT r m) where ...

thus preventing us adding the more general

    instance Applicative m => Applicative (ReaderT r m) where ...

The opt-out is crucial, but relatively cheap.

Jón's proposal had a more subtle opt-out policy, namely that an intrinsic superclass can be quietly pre-empted by an instance for the superclass from a prior or the present module. Note that to declare an instance of the subclass, one must produce an instance of the superclass by the same module at the latest. This quiet exclusion policy is not enough to handle the case of multiple candidate instances arising from multiple intrinsic superclasses (e.g., Traversable and Monad giving competing Functor instances), so some explicit form is required. The question for us, then, is what should happen if an intrinsic superclass not explicitly hidden were to clash with an explicit instance from the same or a prior module. We could

  1. reject this as a duplicate instance declaration, which indeed it is, or
  2. allow the explicit to supersede the intrinsic default, but issue a warning suggesting to either remove the explicit instance or add an explicit opt-out, or
  3. allow the explicit to supersede the intrinsic default silently.

As it stands, we propose option 1 as somehow the principled thing to do. We acknowledge that it creates an issue with legacy code, precisely because there are plenty of places where we have written the full stack of instances, often just doing the obvious default thing: these should be cleaned up, sooner or later. Option 3 avoids that problem but risks perplexity: if I make use of some cool package which introduces some Foo :: * -> *, I might notice that Foo is a monad and add a Monad Foo instance in my own code, expecting the Applicative Foo instance to be generated in concert; to my horror, I find my code has subtle bugs because the package introduced a different, non-monadic, Applicative Foo instance which I'm accidentally using instead. Option 2 is certainly worth considering as a pragmatic transitional compromise, although the 'transitional' has a dangerous tendency to be permanent.