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, 32 insertions, 30 deletions

diff --git a/src/Haddock/Backends/Xhtml/Decl.hs b/src/Haddock/Backends/Xhtml/Decl.hs
index 59be34f7..db39ccca 100644
--- a/src/Haddock/Backends/Xhtml/Decl.hs
+++ b/src/Haddock/Backends/Xhtml/Decl.hs
@@ -39,10 +39,9 @@ ppDecl :: Bool -> LinksInfo -> LHsDecl DocName ->
           DocForDecl DocName -> [DocInstance DocName] -> [(DocName, DocForDecl DocName)] ->
           Bool -> Qualification -> Html
 ppDecl summ links (L loc decl) (mbDoc, fnArgsDoc) instances subdocs unicode qual = case decl of
-  TyClD d@(TyFamily {})          -> ppTyFam summ False links loc mbDoc d unicode qual
-  TyClD d@(TyDecl{ tcdTyDefn = defn })   
-      | isHsDataDefn defn        -> ppDataDecl summ links instances subdocs loc mbDoc d unicode qual
-      | otherwise                -> ppTySyn summ links loc (mbDoc, fnArgsDoc) d unicode qual
+  TyClD (FamDecl d)              -> ppTyFam summ False links loc mbDoc d unicode qual
+  TyClD d@(DataDecl {})          -> ppDataDecl summ links instances subdocs loc mbDoc d unicode qual
+  TyClD d@(SynDecl {})           -> ppTySyn summ links loc (mbDoc, fnArgsDoc) d unicode qual
   TyClD d@(ClassDecl {})         -> ppClassDecl summ links instances loc mbDoc subdocs d unicode qual
   SigD (TypeSig lnames (L _ t))  -> ppFunSig summ links loc (mbDoc, fnArgsDoc) (map unLoc lnames) t unicode qual
   ForD d                         -> ppFor summ links loc (mbDoc, fnArgsDoc) d unicode qual
@@ -115,8 +114,8 @@ ppFor _ _ _ _ _ _ _ = error "ppFor"
 -- we skip type patterns for now
 ppTySyn :: Bool -> LinksInfo -> SrcSpan -> DocForDecl DocName -> TyClDecl DocName -> Bool
         -> Qualification -> Html
-ppTySyn summary links loc doc (TyDecl { tcdLName = L _ name, tcdTyVars = ltyvars
-                                      , tcdTyDefn = TySynonym { td_synRhs = ltype } }) 
+ppTySyn summary links loc doc (SynDecl { tcdLName = L _ name, tcdTyVars = ltyvars
+                                       , tcdRhs = ltype }) 
         unicode qual
   = ppTypeOrFunSig summary links loc [name] (unLoc ltype) doc
                    (full, hdr, spaceHtml +++ equals) unicode qual
@@ -145,10 +144,10 @@ ppTyName name
 --------------------------------------------------------------------------------
 
 
-ppTyFamHeader :: Bool -> Bool -> TyClDecl DocName -> Bool -> Qualification -> Html
-ppTyFamHeader summary associated decl unicode qual =
-
-  (case tcdFlavour decl of
+ppTyFamHeader :: Bool -> Bool -> FamilyDecl DocName -> Bool -> Qualification -> Html
+ppTyFamHeader summary associated d@(FamilyDecl { fdFlavour = flav
+                                               , fdKindSig = mkind }) unicode qual =
+  (case flav of
      TypeFamily
        | associated -> keyword "type"
        | otherwise  -> keyword "type family"
@@ -157,22 +156,22 @@ ppTyFamHeader summary associated decl unicode qual =
        | otherwise  -> keyword "data family"
   ) <+>
 
-  ppTyClBinderWithVars summary decl <+>
+  ppFamDeclBinderWithVars summary d <+>
 
-  case tcdKindSig decl of
+  case mkind of
     Just kind -> dcolon unicode  <+> ppLKind unicode qual kind
     Nothing   -> noHtml
 
 
 ppTyFam :: Bool -> Bool -> LinksInfo -> SrcSpan -> Documentation DocName ->
-              TyClDecl DocName -> Bool -> Qualification -> Html
+              FamilyDecl DocName -> Bool -> Qualification -> Html
 ppTyFam summary associated links loc doc decl unicode qual
 
   | summary   = ppTyFamHeader True associated decl unicode qual
   | otherwise = header_ +++ docSection qual doc +++ instancesBit
 
   where
-    docname = tcdName decl
+    docname = unLoc $ fdLName decl
 
     header_ = topDeclElem links loc [docname] (ppTyFamHeader summary associated decl unicode qual)
 
@@ -187,23 +186,25 @@ ppTyFam summary associated links loc doc decl unicode qual
 --------------------------------------------------------------------------------
 
 
-ppAssocType :: Bool -> LinksInfo -> DocForDecl DocName -> LTyClDecl DocName -> Bool
+ppAssocType :: Bool -> LinksInfo -> DocForDecl DocName -> LFamilyDecl DocName -> Bool
             -> Qualification -> Html
 ppAssocType summ links doc (L loc decl) unicode qual =
-  case decl of
-    TyFamily  {} -> ppTyFam summ True links loc (fst doc) decl unicode qual
-    _            -> error "declaration type not supported by ppAssocType"
+   ppTyFam summ True links loc (fst doc) decl unicode qual
 
 
 --------------------------------------------------------------------------------
 -- * TyClDecl helpers
 --------------------------------------------------------------------------------
 
+-- | Print a type family and its variables 
+ppFamDeclBinderWithVars :: Bool -> FamilyDecl DocName -> Html
+ppFamDeclBinderWithVars summ (FamilyDecl { fdLName = lname, fdTyVars = tvs }) =
+  ppAppDocNameNames summ (unLoc lname) (tyvarNames tvs)
 
--- | Print a type family / newtype / data / class binder and its variables 
-ppTyClBinderWithVars :: Bool -> TyClDecl DocName -> Html
-ppTyClBinderWithVars summ decl =
-  ppAppDocNameNames summ (unLoc $ tcdLName decl) (tyvarNames $ tcdTyVars decl)
+-- | Print a newtype / data binder and its variables 
+ppDataBinderWithVars :: Bool -> TyClDecl DocName -> Html
+ppDataBinderWithVars summ decl =
+  ppAppDocNameNames summ (tcdName decl) (tyvarNames $ tcdTyVars decl)
 
 
 --------------------------------------------------------------------------------
@@ -303,7 +304,7 @@ ppShortClassDecl summary links (ClassDecl { tcdCtxt = lctxt, tcdLName = lname, t
       +++ shortSubDecls
           (
             [ ppAssocType summary links doc at unicode qual | at <- ats
-              , let doc = lookupAnySubdoc (tcdName $ unL at) subdocs ]  ++
+              , let doc = lookupAnySubdoc (unL $ fdLName $ unL at) subdocs ]  ++
 
                 -- ToDo: add associated type defaults
 
@@ -336,14 +337,14 @@ ppClassDecl summary links instances loc d subdocs
       | null lsigs = topDeclElem links loc [nm] (hdr unicode qual)
       | otherwise  = topDeclElem links loc [nm] (hdr unicode qual <+> keyword "where")
 
-    nm   = unLoc $ tcdLName decl
+    nm   = tcdName decl
 
     hdr = ppClassHdr summary lctxt (unLoc lname) ltyvars lfds
 
     -- ToDo: add assocatied typ defaults
     atBit = subAssociatedTypes [ ppAssocType summary links doc at unicode qual
                       | at <- ats
-                      , let doc = lookupAnySubdoc (tcdName $ unL at) subdocs ]
+                      , let doc = lookupAnySubdoc (unL $ fdLName $ unL at) subdocs ]
 
     methodBit = subMethods [ ppFunSig summary links loc doc names typ unicode qual
                            | L _ (TypeSig lnames (L _ typ)) <- lsigs
@@ -401,7 +402,7 @@ ppShortDataDecl summary _links _loc dataDecl unicode qual
     doConstr c con = toHtml [c] <+> ppShortConstr summary (unLoc con) unicode qual
     doGADTConstr con = ppShortConstr summary (unLoc con) unicode qual
 
-    cons      = td_cons (tcdTyDefn dataDecl)
+    cons      = dd_cons (tcdDataDefn dataDecl)
     resTy     = (con_res . unLoc . head) cons
 
 
@@ -415,8 +416,8 @@ ppDataDecl summary links instances subdocs loc doc dataDecl unicode qual
   | otherwise = header_ +++ docSection qual doc +++ constrBit +++ instancesBit
 
   where
-    docname   = unLoc . tcdLName $ dataDecl
-    cons      = td_cons (tcdTyDefn dataDecl)
+    docname   = tcdName dataDecl
+    cons      = dd_cons (tcdDataDefn dataDecl)
     resTy     = (con_res . unLoc . head) cons
 
     header_ = topDeclElem links loc [docname] (ppDataHeader summary dataDecl unicode qual
@@ -570,14 +571,15 @@ ppShortField summary unicode qual (ConDeclField (L _ name) ltype _)
 -- | Print the LHS of a data\/newtype declaration.
 -- Currently doesn't handle 'data instance' decls or kind signatures
 ppDataHeader :: Bool -> TyClDecl DocName -> Bool -> Qualification -> Html
-ppDataHeader summary decl@(TyDecl { tcdTyDefn = TyData { td_ND = nd, td_ctxt = ctxt } }) 
+ppDataHeader summary decl@(DataDecl { tcdDataDefn = HsDataDefn { dd_ND = nd
+                                                               , dd_ctxt = ctxt } }) 
              unicode qual
   = -- newtype or data
     (case nd of { NewType -> keyword "newtype"; DataType -> keyword "data" }) <+>
     -- context
     ppLContext ctxt unicode qual <+>
     -- T a b c ..., or a :+: b
-    ppTyClBinderWithVars summary decl
+    ppDataBinderWithVars summary decl
 ppDataHeader _ _ _ _ = error "ppDataHeader: illegal argument"