Refactor tuple constraints

Make tuple constraints be handled by a perfectly ordinary
type class, with the component constraints being the
superclasses:
    class (c1, c2) => (c2, c2)

This change was provoked by

  #10359  inability to re-use a given tuple
          constraint as a whole

  #9858   confusion between term tuples
          and constraint tuples

but it's generally a very nice simplification. We get rid of
 -  In Type, the TuplePred constructor of PredTree,
    and all the code that dealt with TuplePreds
 -  In TcEvidence, the constructors EvTupleMk, EvTupleSel

See Note [How tuples work] in TysWiredIn.

Of course, nothing is ever entirely simple. This one
proved quite fiddly.

- I did quite a bit of renaming, which makes this patch
  touch a lot of modules. In partiuclar tupleCon -> tupleDataCon.

- I made constraint tuples known-key rather than wired-in.
  This is different to boxed/unboxed tuples, but it proved
  awkward to have all the superclass selectors wired-in.
  Easier just to use the standard mechanims.

- While I was fiddling with known-key names, I split the TH Name
  definitions out of DsMeta into a new module THNames.  That meant
  that the known-key names can all be gathered in PrelInfo, without
  causing module loops.

- I found that the parser was parsing an import item like
      T( .. )
  as a *data constructor* T, and then using setRdrNameSpace to
  fix it.  Stupid!  So I changed the parser to parse a *type
  constructor* T, which means less use of setRdrNameSpace.

  I also improved setRdrNameSpace to behave better on Exact Names.
  Largely on priciple; I don't think it matters a lot.

- When compiling a data type declaration for a wired-in thing like
  tuples (,), or lists, we don't really need to look at the
  declaration.  We have the wired-in thing!  And not doing so avoids
  having to line up the uniques for data constructor workers etc.
  See Note [Declarations for wired-in things]

- I found that FunDeps.oclose wasn't taking superclasses into
  account; easily fixed.

- Some error message refactoring for invalid constraints in TcValidity

1 files changed, 5 insertions, 5 deletions

diff --git a/compiler/typecheck/TcExpr.hs b/compiler/typecheck/TcExpr.hs
index 155cdb42be..a9622588a0 100644
--- a/compiler/typecheck/TcExpr.hs
+++ b/compiler/typecheck/TcExpr.hs
@@ -16,7 +16,7 @@ module TcExpr ( tcPolyExpr, tcPolyExprNC, tcMonoExpr, tcMonoExprNC,
 #include "HsVersions.h"
 
 import {-# SOURCE #-}   TcSplice( tcSpliceExpr, tcTypedBracket, tcUntypedBracket )
-import DsMeta( liftStringName, liftName )
+import THNames( liftStringName, liftName )
 
 import HsSyn
 import TcHsSyn
@@ -373,7 +373,7 @@ tcExpr (SectionL arg1 op) res_ty
 
 tcExpr (ExplicitTuple tup_args boxity) res_ty
   | all tupArgPresent tup_args
-  = do { let tup_tc = tupleTyCon (boxityNormalTupleSort boxity) (length tup_args)
+  = do { let tup_tc = tupleTyCon boxity (length tup_args)
        ; (coi, arg_tys) <- matchExpectedTyConApp tup_tc res_ty
        ; tup_args1 <- tcTupArgs tup_args arg_tys
        ; return $ mkHsWrapCo coi (ExplicitTuple tup_args1 boxity) }
@@ -383,7 +383,7 @@ tcExpr (ExplicitTuple tup_args boxity) res_ty
     do { let kind = case boxity of { Boxed   -> liftedTypeKind
                                    ; Unboxed -> openTypeKind }
              arity = length tup_args
-             tup_tc = tupleTyCon (boxityNormalTupleSort boxity) arity
+             tup_tc = tupleTyCon boxity arity
 
        ; arg_tys <- newFlexiTyVarTys (tyConArity tup_tc) kind
        ; let actual_res_ty
@@ -1273,14 +1273,14 @@ checkCrossStageLifting id (Brack _ (TcPending ps_var lie_var))
                -- just going to flag an error for now
 
         ; lift <- if isStringTy id_ty then
-                     do { sid <- tcLookupId DsMeta.liftStringName
+                     do { sid <- tcLookupId THNames.liftStringName
                                      -- See Note [Lifting strings]
                         ; return (HsVar sid) }
                   else
                      setConstraintVar lie_var   $
                           -- Put the 'lift' constraint into the right LIE
                      newMethodFromName (OccurrenceOf (idName id))
-                                       DsMeta.liftName id_ty
+                                       THNames.liftName id_ty
 
                    -- Update the pending splices
         ; ps <- readMutVar ps_var