Version 2 (modified by pigworker, 5 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.

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 embed instance declarations 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 toclass 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.