Default superclass instances

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  superclass defaults proposal,  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). The subclass methods provide extra functionality, but do not induce a standard implementation of the superclass. Num provides arithmetic operations, widening Eq, but does not induce an implementation of (==).
• A class can deepen its superclass, providing methods at least as expressive, admitting a default implementation of superclass functionality (e.g., the compare function from an Ord instance can be used to give an Eq instance; traverse from Traversable f can be instantiated to foldMapDefault for Foldable f and fmapDefault for Functor f).

(SLPJ I don't understand this distinction clearly.)

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.

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.

Design goals

The major design goal is this:

• Design goal 1: a class C can be re-factored into a class C with a superclass S, without disturbing any clients.

The difficulty is that if you start with

class C a where
   f :: ...
   g :: ...

and lots of clients write instances of C and functions that use it:

instance C ClientType
  f = ...blah...
  g = ...blah...

foo :: C a => ...
foo = ...

Now you want to refactor C thus:

class S a where
  f :: ...
class S a => C a where
  g :: ...

Design goal 1 is that this change should not force clients to change their code. Haskell 98 does not satisfy this goal. In Haskell 98 the client function foo is fine unmodified, but the instance declaration would have to be split into two.

SLPJ: are there any other design goals?

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The proposal

Concretely, the proposal is as follows.

Default superclass instances

First, we allow a class declaration to include a default superclass instance declaration for some, none, or all of its superclass constraints. We say that superclasses with default implementations are intrinsic superclasses. Example:

    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

Note the instance declaration nested inside the class declaration. This is the default superclass instance declaration, and Functor thereby becomes an intrinsic superclass of Applicative. Moreover, note that the definition of fmap uses the <*> operation of Applicative; that is the whole point!

Here is another example:

    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)

Here, Applicative is an intrinsic superclass of Monad.

Instance declarations

A default superclass instance in a class declaration for class C has an effect on the instance declarations for C.

Specifically:

• An instance declaration

  instance Q => C ty where ...defs...
  

  for class C generates an extra instance declaration

  instance Q => Si ty where ....
  

  for each intrinsic superclass Si of C

• The method definitions in ...defs... are distributed to the appropriate instance declaration, according to which class the method belongs to.

• Any methods that are not specified explicitly are "filled in" from the default definition given in the default superclass instance. (If there is no default definition, then a warning is produced, and a definition that calls error is used instead.)

For example, assume the class declaration for Monad above. Then this instance declaration:

  instance Monad m where
    (>>=) = ...blah...
    (<*)  = ...bleh...

would generate an extra instance declaration for the instrinsic superclass Applicative, with the methods distributed appropriately:

  instance Monad m where
    (>>=) = ...blah...

  instance Applicative m where
    (<*) = ...bleh...  -- Programmer specified

    ff <*> fs = ff >>= \ f -> fs >>= \ s -> return (f s)
                       -- From the default superclass instance

We call these extra instance declarations an intrinsic instance declaration. (The term "derived instance" is already taken!)

This process is recursive. Since Functor is an intrinsic superclass of Applicative, the intrinsic instance for Applicative recursively generates an intrinsic instance for Functor:

  instance Functor m where
    fmap = (<*>) . return	-- From default superclass instance

The opt-out mechanism

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 the intrinsic instance

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

thus preventing us adding the more general

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

To inhibit the generation of an intrinsic instance declaration, one can use a hiding clause in the instance declaration:

    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. For example: write

    instance Monad m => Monad (ReaderT r m) where ...
      return x = ...
      ba >>= bf = ...
      hiding instance Applicative

The hiding clause applies to all the intrinsic instances generated from an instance declaration. For example, we might write

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

Note that Functor is only an indirect intrinsic superclass of Monad, via Applicative. So the above instance would generate an intrinsic instance for Applicative but not for Functor.

See below for more about the opt-out mechanism.

Details

Each default superclass instance declaration in a class declaration must be for a distinct class. So one of these is OK and the other is not:

    -- This is ILLEGAL
    class (Tweedle dum, Tweedle dee) => Rum dum dee where
      instance Tweedle dum where ...
      instance Tweedle dee where ...

    -- But this is OK 
    class (Tweedle dum, Tweedle dee) => Rum dum dee where
      instance Tweedle dee where ...

By requiring that intrinsic superclasses be class-distinct, we ensure that the distribution of methods to spawned instances is unambiguous.

Flags

The declaration of a class with a default superclass instance would require a language extension flag; but the instances of such a class would not. Again Design Goal 1 means that we want to impact client code as little as possible.

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Possible variations

The design of the opt-out mechanism

Jón's proposal had a more subtle opt-out policy, namely that an intrinsic superclass can be silently pre-empted by an instance for the superclass from a prior or the present module. For example, instead of

  instance Functor T where ...

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

you would simply say

  instance Functor T where ...

  instance Monad T where
    return x = ...
    ba >>= bf = ...

Of course, the instance of Functor T might be in a different module entirely. 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 intrinsic instances arising from multiple intrinsic superclasses (e.g., Traversable and Monad giving competing Functor instances), so some explicit hiding form is required even under the "silent pre-emption" plan.

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. We acknowledge that it creates an issue with legacy code --- that is, it contradicts Design Goal 1 --- precisely because there are plenty of places where we have written the full stack of instances, often just doing the obvious default thing.

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.

3. Allow the explicit to supersede the intrinsic default silently. This fits with Design Goal 1, 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.

There is considerable support in the  email discussion thread for Option 2 or 3, on the grounds that Option 1 contradicts Design Goal 1.

Perhaps Option 2 is the pragmatic choice.

Opting in rather than out

The [  http://www.haskell.org/haskellwiki/Superclass_defaults superclass default proposal] deals with the question of opt-outs by intead requiring you to opt in. A Monad instance would look like

  instance (Functor T, Applicative T, Monad T) where
    (>>=) = ...blah...
    return = ...bleh...

where we explicitly ask the compiler to generate an instance of Applicative T and Functor T. The disadvantage is that you have to know to do so, which contradicts Design Goal 1.

Multi-headed instance declarations

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. So we might write

  type Stringy x = (Read x, Show s)
  instance Stringy Int where
    read = ...
    show = ...

The common factor is that one instance declaration is expanded into several with the method definitions distributed appropriately among them.

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Applications

If you want superclass instances, add yourself below with a quick sketch of what your application is

For years, I have been pretending that Monads are not Functors. When applicative functors showed up, I could accept them as functors, but my internal contradictions prevented me to accept them as monads. This is a cry for help!

-- Haskell 98