"guarded instances": instance selection can add extra parameters to the class

Disclaimer: the same semantics can currently be achieved without this syntax. So this is mostly a Parser request, though I've made it a Type Checker request because some type errors would probably need to be aware of the language extension. More on this at the bottom.

We'll start with a demonstration. Just some ancillary declarations for now.

class Sat t where dict :: t

data True; data False
type family Pred (p :: * -> *) a

type family Left  a; type instance (Either l r) = l
type family Right a; type instance (Either l r) = r

data Path p a where
  Here :: p a -> Path p a
  L :: Path p l -> Path p (Either l r)
  R :: Path p r -> Path p (Either l r)

The objective of these declarations is to allow us to define some Predicate p and use the Sat class to find a path leading through a tree of Eithers to a type that satisfies that Predicate.

These next three declarations use the new syntax, as I'm imagining it.

-- NB new syntax: `guard' keyword, the pipe after the instance head,
-- and a comma-separated list of types after that
instance guard Sat (Path a)
  | Pred p a, Pred (Path p) (Left a), Pred (Path p) (Right a)

-- now we match on the instance guards, using the same pipe syntax
instance Sat (p a)      => Sat (Path p a)            | True , satl , satr where
  dict = Here dict
instance Sat (Path p l) => Sat (Path p (Either l r)) | False, True , satr where
  dict = SL dict
instance Sat (Path p r) => Sat (Path p (Either l r)) | False, False, True where
  dict = SR dict

The guard declaration asserts that any instance of Sat with a head that would overlap a la OverlappingInstances with Path a shall be disambiguated via the comma-separated list of types following the pipe. In this example, the subsequent three instances, which would traditionally overlap, are indeed disambiguated by their additional "instance head guards" (cf. HList's type-level programming style: AdvancedOverlap).

We can currently simulate this syntax by declaring a variant class of `Sat' which takes an extra parameter and thread the instance guards through that. Unfortunately, this workaround is repetitive, misses out on the better type error messages possible with specific Type Checker support, and it's just a bother.

{{{ class Sat_ a anno where dict_ :: anno -> a

instance (anno ~ (Pred p a, Pred (Path p) (Left a), Pred (Path p) (Right a)), Sat_ (Found a) anno) => Sat (Path p a) where dict = dict_ (undefined :: anno)

instance Sat (p a) => Sat_ (Path p a) (True, satl, satr) where dict_ _ = Here dict …

}}}

In the spirit of #4259, [TypeFunctions/TotalFamilies total type families], and [http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap AdvancedOverlap], this syntax could be enriched and thereby promoted to an actual Type Checker extension. Replacing the comma-separated list of types in the guard declaration with a sequence of contexts would be appropriate syntax for explicitly making instance selection sensitive to those contexts. The instance head guards could then just be a type boolean (wired-in to the compiler, now) indicating whether the context was satisfied. A True would bring that context's consequences to bear within both the instance's own context and its declarations. For example, we could do without the Left and Right\ type families.

instance guard Sat (Path a)
  | (Pred p a) (a ~ Either l r, Pred (Path p) l) (a ~ Either l r, Pred (Path p) r)

instance Sat (p a) => Sat (Path p a) | True satl satr where
  dict = Here dict
…

Trac metadata

Trac field	Value
Version	7.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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