Make tuple constraints into a class

At the moment GHC treats tuple constraints specially. If you grep for TuplePred you'll see this. But the special treatment is strange; see the confusion between constraint tuples and ordinary tuples in #9858 (closed).

But I now realise that we can sweep away all this nonsense. Suppose we declare

module GHC.Classes where
  class    (c1, c2) => (,) c1 c2
  instance (c1, c2) => (,) c1 c2

  class    (c1, c2, c3) => (,) c1 c2 c3
  instance (c1, c2, c3) => (,) c1 c2 c3

and so on. (This is analogous the tuple data type declarations in GHC.Tuple.) Notice that:

GHC.Classes,(,) is a class, of kind Constraint -> Constraint -> Constraint.

quite distinct from GHC.Tuple.(,), whose kind is * -> * -> *.

GHC.Classes.(,) is a perfectly ordinary class, with no methods and two superclasses.
So all the usual superclass stuff applies.

* If you have a given Ord a then you have a given Eq a (its superclass). Similarly if you have a given (,) c1 c2 then you also have given c1 and given c2`.
- If you want to construct a dictionary of type Ord a (a "wanted"), you must supply a dictionary of type Eq a. Similarly, if you want to construct a dictionary of type (,) c1 c2 then you must supply c1 and c2.
- I have written (,) c1 c2 to stress that there is a class GHC.Classes.(,), but we'll also allow the concrete syntax (c1,c2) instead.
- Nevertheless the syntactic form (c1, c2) =\> blah is just sugar for c1 =\> c2 =\> blah (a type with two, curried constraints); it does not stand a type with a single constraint. Otherwise the instance {{{ instance (c1,c2) => (c1,c2) }}} would be the identity function!
The key thing is that, aside from special syntax, (,) is a perfectly ordinary class, so we can simply delete all the special treatment of TuplePred. (Implicit parameters are also treated as a special class, incidentally.)

There should be no user-visible effects. But I think it would cure the worst aspects of #10359 (closed), as well as cleaning up TypeRep\ confusion.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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Make tuple constraints into a class

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