Implement overlapping type family instances.

An ordered, overlapping type family instance is introduced by 'type
instance
where', followed by equations. See the new section in the user manual
(7.7.2.2) for details. The canonical example is Boolean equality at the
type
level:

type family Equals (a :: k) (b :: k) :: Bool
type instance where
  Equals a a = True
  Equals a b = False

A branched family instance, such as this one, checks its equations in
order
and applies only the first the matches. As explained in the note
[Instance
checking within groups] in FamInstEnv.lhs, we must be careful not to
simplify,
say, (Equals Int b) to False, because b might later unify with Int.

This commit includes all of the commits on the overlapping-tyfams
branch. SPJ
requested that I combine all my commits over the past several months
into one
monolithic commit. The following GHC repos are affected: ghc, testsuite,
utils/haddock, libraries/template-haskell, and libraries/dph.

Here are some details for the interested:

- The definition of CoAxiom has been moved from TyCon.lhs to a
  new file CoAxiom.lhs. I made this decision because of the
  number of definitions necessary to support BranchList.

- BranchList is a GADT whose type tracks whether it is a
  singleton list or not-necessarily-a-singleton-list. The reason
  I introduced this type is to increase static checking of places
  where GHC code assumes that a FamInst or CoAxiom is indeed a
  singleton. This assumption takes place roughly 10 times
  throughout the code. I was worried that a future change to GHC
  would invalidate the assumption, and GHC might subtly fail to
  do the right thing. By explicitly labeling CoAxioms and
  FamInsts as being Unbranched (singleton) or
  Branched (not-necessarily-singleton), we make this assumption
  explicit and checkable. Furthermore, to enforce the accuracy of
  this label, the list of branches of a CoAxiom or FamInst is
  stored using a BranchList, whose constructors constrain its
  type index appropriately.

I think that the decision to use BranchList is probably the most
controversial decision I made from a code design point of view.
Although I provide conversions to/from ordinary lists, it is more
efficient to use the brList... functions provided in CoAxiom than
always to convert. The use of these functions does not wander far
from the core CoAxiom/FamInst logic.

BranchLists are motivated and explained in the note [Branched axioms] in
CoAxiom.lhs.

- The CoAxiom type has changed significantly. You can see the new
  type in CoAxiom.lhs. It uses a CoAxBranch type to track
  branches of the CoAxiom. Correspondingly various functions
  producing and consuming CoAxioms had to change, including the
  binary layout of interface files.

- To get branched axioms to work correctly, it is important to have a
  notion
  of type "apartness": two types are apart if they cannot unify, and no
  substitution of variables can ever get them to unify, even after type
family
  simplification. (This is different than the normal failure to unify
because
  of the type family bit.) This notion in encoded in tcApartTys, in
Unify.lhs.
  Because apartness is finer-grained than unification, the tcUnifyTys
now
  calls tcApartTys.

- CoreLinting axioms has been updated, both to reflect the new
  form of CoAxiom and to enforce the apartness rules of branch
  application. The formalization of the new rules is in
  docs/core-spec/core-spec.pdf.

- The FamInst type (in types/FamInstEnv.lhs) has changed
  significantly, paralleling the changes to CoAxiom. Of course,
  this forced minor changes in many files.

- There are several new Notes in FamInstEnv.lhs, including one
  discussing confluent overlap and why we're not doing it.

- lookupFamInstEnv, lookupFamInstEnvConflicts, and
  lookup_fam_inst_env' (the function that actually does the work)
  have all been more-or-less completely rewritten. There is a
  Note [lookup_fam_inst_env' implementation] describing the
  implementation. One of the changes that affects other files is
  to change the type of matches from a pair of (FamInst, [Type])
  to a new datatype (which now includes the index of the matching
  branch). This seemed a better design.

- The TySynInstD constructor in Template Haskell was updated to
  use the new datatype TySynEqn. I also bumped the TH version
  number, requiring changes to DPH cabal files. (That's why the
  DPH repo has an overlapping-tyfams branch.)

- As SPJ requested, I refactored some of the code in HsDecls:

 * splitting up TyDecl into SynDecl and DataDecl, correspondingly
   changing HsTyDefn to HsDataDefn (with only one constructor)

 * splitting FamInstD into TyFamInstD and DataFamInstD and
   splitting FamInstDecl into DataFamInstDecl and TyFamInstDecl

 * making the ClsInstD take a ClsInstDecl, for parallelism with
   InstDecl's other constructors

 * changing constructor TyFamily into FamDecl

 * creating a FamilyDecl type that stores the details for a family
   declaration; this is useful because FamilyDecls can appear in classes
but
   other decls cannot

 * restricting the associated types and associated type defaults for a
 * class
   to be the new, more restrictive types

 * splitting cid_fam_insts into cid_tyfam_insts and cid_datafam_insts,
   according to the new types

 * perhaps one or two more that I'm overlooking

None of these changes has far-reaching implications.

- The user manual, section 7.7.2.2, is updated to describe the new type
  family
  instances.

1 files changed, 65 insertions, 47 deletions

diff --git a/src/Haddock/Convert.hs b/src/Haddock/Convert.hs
index aca12188..8894793d 100644
--- a/src/Haddock/Convert.hs
+++ b/src/Haddock/Convert.hs
@@ -18,7 +18,7 @@ module Haddock.Convert where
 
 
 import HsSyn
-import TcType ( tcSplitTyConApp_maybe, tcSplitSigmaTy )
+import TcType ( tcSplitSigmaTy )
 import TypeRep
 import Type(isStrLitTy)
 import Kind ( splitKindFunTys, synTyConResKind )
@@ -26,6 +26,7 @@ import Name
 import Var
 import Class
 import TyCon
+import CoAxiom
 import DataCon
 import BasicTypes ( TupleSort(..) )
 import TysPrim ( alphaTyVars )
@@ -53,7 +54,14 @@ tyThingToLHsDecl t = noLoc $ case t of
   -- later in the file (also it's used for class associated-types too.)
   ATyCon tc
     | Just cl <- tyConClass_maybe tc -- classes are just a little tedious
-    -> TyClD $ ClassDecl
+    -> let extractFamilyDecl :: TyClDecl a -> LFamilyDecl a
+           extractFamilyDecl (FamDecl d) = noLoc d
+           extractFamilyDecl _           =
+             error "tyThingToLHsDecl: impossible associated tycon"
+
+           atTyClDecls = [synifyTyCon at_tc | (at_tc, _) <- classATItems cl]
+           atFamDecls  = map extractFamilyDecl atTyClDecls in
+       TyClD $ ClassDecl
          { tcdCtxt = synifyCtx (classSCTheta cl)
          , tcdLName = synifyName cl
          , tcdTyVars = synifyTyVars (classTyVars cl)
@@ -64,7 +72,7 @@ tyThingToLHsDecl t = noLoc $ case t of
                          (classMethods cl)
          , tcdMeths = emptyBag --ignore default method definitions, they don't affect signature
          -- class associated-types are a subset of TyCon:
-         , tcdATs = [noLoc (synifyTyCon at_tc) | (at_tc, _) <- classATItems cl]
+         , tcdATs = atFamDecls
          , tcdATDefs = [] --ignore associated type defaults
          , tcdDocs = [] --we don't have any docs at this point
          , tcdFVs = placeHolderNames }
@@ -73,36 +81,40 @@ tyThingToLHsDecl t = noLoc $ case t of
 
   -- type-constructors (e.g. Maybe) are complicated, put the definition
   -- later in the file (also it's used for class associated-types too.)
-  ACoAxiom ax -> InstD (FamInstD { lid_inst = synifyAxiom ax })
+  ACoAxiom ax -> InstD (TyFamInstD { tfid_inst = synifyAxiom ax })
 
   -- a data-constructor alone just gets rendered as a function:
   ADataCon dc -> SigD (TypeSig [synifyName dc]
     (synifyType ImplicitizeForAll (dataConUserType dc)))
 
-synifyATDefault :: TyCon -> LFamInstDecl Name
+synifyATDefault :: TyCon -> LTyFamInstDecl Name
 synifyATDefault tc = noLoc (synifyAxiom ax)
   where Just ax = tyConFamilyCoercion_maybe tc
 
-synifyAxiom :: CoAxiom -> FamInstDecl Name
-synifyAxiom (CoAxiom { co_ax_tvs = tkvs, co_ax_lhs = lhs, co_ax_rhs = rhs })
-  | Just (tc, args) <- tcSplitTyConApp_maybe lhs
-  = let name      = synifyName tc
-        typats    = map (synifyType WithinType) args
-        hs_rhs_ty = synifyType WithinType rhs
+synifyAxBranch :: TyCon -> CoAxBranch -> TyFamInstEqn Name
+synifyAxBranch tc (CoAxBranch { cab_tvs = tkvs, cab_lhs = args, cab_rhs = rhs })
+  = let name       = synifyName tc
+        typats     = map (synifyType WithinType) args
+        hs_rhs     = synifyType WithinType rhs
         (kvs, tvs) = partition isKindVar tkvs
-    in FamInstDecl { fid_tycon = name 
-                   , fid_pats = HsWB { hswb_cts = typats
-                                     , hswb_kvs = map tyVarName kvs
-                                     , hswb_tvs = map tyVarName tvs }
-                   , fid_defn = TySynonym hs_rhs_ty, fid_fvs = placeHolderNames }
-  | otherwise
-  = error "synifyAxiom" 
+    in TyFamInstEqn { tfie_tycon = name
+                    , tfie_pats  = HsWB { hswb_cts = typats
+                                        , hswb_kvs = map tyVarName kvs
+                                        , hswb_tvs = map tyVarName tvs }
+                    , tfie_rhs   = hs_rhs }
+
+synifyAxiom :: CoAxiom br -> TyFamInstDecl Name
+synifyAxiom (CoAxiom { co_ax_tc = tc, co_ax_branches = branches })
+  = let eqns = brListMap (noLoc . synifyAxBranch tc) branches
+    in TyFamInstDecl { tfid_eqns  = eqns
+                     , tfid_group = (brListLength branches /= 1)
+                     , tfid_fvs   = placeHolderNames }
 
 synifyTyCon :: TyCon -> TyClDecl Name
 synifyTyCon tc
   | isFunTyCon tc || isPrimTyCon tc 
-  = TyDecl { tcdLName = synifyName tc
-           , tcdTyVars =       -- tyConTyVars doesn't work on fun/prim, but we can make them up:
+  = DataDecl { tcdLName = synifyName tc
+             , tcdTyVars =       -- tyConTyVars doesn't work on fun/prim, but we can make them up:
                          let mk_hs_tv realKind fakeTyVar 
                                 = noLoc $ KindedTyVar (getName fakeTyVar) 
                                                       (synifyKindSig realKind)
@@ -111,37 +123,44 @@ synifyTyCon tc
                                                                 alphaTyVars --a, b, c... which are unfortunately all kind *
                                    }
                             
-           , tcdTyDefn = TyData { td_ND = DataType  -- arbitrary lie, they are neither 
+           , tcdDataDefn = HsDataDefn { dd_ND = DataType  -- arbitrary lie, they are neither 
                                                     -- algebraic data nor newtype:
-                                , td_ctxt = noLoc []
-                                , td_cType = Nothing
-                                , td_kindSig = Just (synifyKindSig (tyConKind tc))
+                                      , dd_ctxt = noLoc []
+                                      , dd_cType = Nothing
+                                      , dd_kindSig = Just (synifyKindSig (tyConKind tc))
                                                -- we have their kind accurately:
-                                , td_cons = []  -- No constructors
-                                , td_derivs = Nothing }
+                                      , dd_cons = []  -- No constructors
+                                      , dd_derivs = Nothing }
            , tcdFVs = placeHolderNames }
   | isSynFamilyTyCon tc 
-  = TyFamily TypeFamily (synifyName tc) (synifyTyVars (tyConTyVars tc))
-               (Just (synifyKindSig (synTyConResKind tc)))
+  = case synTyConRhs_maybe tc of
+        Just (SynFamilyTyCon {}) ->
+          FamDecl (FamilyDecl TypeFamily (synifyName tc) (synifyTyVars (tyConTyVars tc))
+                              (Just (synifyKindSig (synTyConResKind tc))))
+        _ -> error "synifyTyCon: impossible open type synonym?"
   | isDataFamilyTyCon tc 
   = --(why no "isOpenAlgTyCon"?)
     case algTyConRhs tc of
         DataFamilyTyCon ->
-          TyFamily DataFamily (synifyName tc) (synifyTyVars (tyConTyVars tc))
-               Nothing --always kind '*'
-               -- placeHolderKind
+          FamDecl (FamilyDecl DataFamily (synifyName tc) (synifyTyVars (tyConTyVars tc))
+                              Nothing) --always kind '*'
         _ -> error "synifyTyCon: impossible open data type?"
+  | isSynTyCon tc
+  = case synTyConRhs_maybe tc of
+        Just (SynonymTyCon ty) ->
+          SynDecl { tcdLName = synifyName tc
+                  , tcdTyVars = synifyTyVars (tyConTyVars tc)
+                  , tcdRhs = synifyType WithinType ty
+                  , tcdFVs = placeHolderNames }
+        _ -> error "synifyTyCon: impossible synTyCon"
   | otherwise =
-  -- (closed) type, newtype, and data
+  -- (closed) newtype and data
   let
-  -- alg_ only applies to newtype/data
-  -- syn_ only applies to type
-  -- others apply to both
   alg_nd = if isNewTyCon tc then NewType else DataType
   alg_ctx = synifyCtx (tyConStupidTheta tc)
   name = synifyName tc
   tyvars = synifyTyVars (tyConTyVars tc)
-  alg_kindSig = Just (tyConKind tc)
+  kindSig = Just (tyConKind tc)
   -- The data constructors.
   --
   -- Any data-constructors not exported from the module that *defines* the
@@ -158,19 +177,18 @@ synifyTyCon tc
   -- That seems like an acceptable compromise (they'll just be documented
   -- in prefix position), since, otherwise, the logic (at best) gets much more
   -- complicated. (would use dataConIsInfix.)
-  alg_use_gadt_syntax = any (not . isVanillaDataCon) (tyConDataCons tc)
-  alg_cons = map (synifyDataCon alg_use_gadt_syntax) (tyConDataCons tc)
+  use_gadt_syntax = any (not . isVanillaDataCon) (tyConDataCons tc)
+  cons = map (synifyDataCon use_gadt_syntax) (tyConDataCons tc)
   -- "deriving" doesn't affect the signature, no need to specify any.
   alg_deriv = Nothing
-  defn | Just (_, syn_rhs) <- synTyConDefn_maybe tc 
-       = TySynonym (synifyType WithinType syn_rhs)
-       | otherwise = TyData { td_ND = alg_nd, td_ctxt = alg_ctx
-                            , td_cType = Nothing
-                            , td_kindSig = fmap synifyKindSig alg_kindSig
-                            , td_cons    = alg_cons 
-                            , td_derivs  = alg_deriv }
- in TyDecl { tcdLName = name, tcdTyVars = tyvars, tcdTyDefn = defn
-           , tcdFVs = placeHolderNames }
+  defn = HsDataDefn { dd_ND      = alg_nd
+                    , dd_ctxt    = alg_ctx
+                    , dd_cType   = Nothing
+                    , dd_kindSig = fmap synifyKindSig kindSig
+                    , dd_cons    = cons 
+                    , dd_derivs  = alg_deriv }
+ in DataDecl { tcdLName = name, tcdTyVars = tyvars, tcdDataDefn = defn
+             , tcdFVs = placeHolderNames }
 
 -- User beware: it is your responsibility to pass True (use_gadt_syntax)
 -- for any constructor that would be misrepresented by omitting its