id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5590	"""guarded instances"": instance selection can add extra parameters to the class"	nfrisby	simonpj	"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 `Pred`icate `p` and use the `Sat` class to find a path leading through a tree of `Either`s to a type that satisfies that `Pred`icate.

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: [http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap 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
…
}}}"	feature request	new	normal	7.6.2	Compiler (Type checker)	7.2.1				Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown