Fix a long-standing infelicity in the type pretty printer

[ghc-hetmet.git] / compiler / types / FamInstEnv.lhs

diff --git a/compiler/types/FamInstEnv.lhs b/compiler/types/FamInstEnv.lhs
index 983f737..89fd193 100644 (file)
--- a/compiler/types/FamInstEnv.lhs
+++ b/compiler/types/FamInstEnv.lhs
@@ -14,7 +14,7 @@ module FamInstEnv (
        extendFamInstEnv, extendFamInstEnvList, 
        famInstEnvElts, familyInstances,
 
        extendFamInstEnv, extendFamInstEnvList, 
        famInstEnvElts, familyInstances,
 

-       lookupFamInstEnv, lookupFamInstEnvUnify,
+       lookupFamInstEnv, lookupFamInstEnvConflicts,

        
        -- Normalisation
        topNormaliseType
        
        -- Normalisation
        topNormaliseType

@@ -23,7 +23,6 @@ module FamInstEnv (
 #include "HsVersions.h"
 
 import InstEnv
 #include "HsVersions.h"
 
 import InstEnv

-import TcType

 import Unify
 import Type
 import TypeRep
 import Unify
 import Type
 import TypeRep

@@ -37,8 +36,6 @@ import Outputable
 import Maybes
 import Util
 import FastString
 import Maybes
 import Util
 import FastString

-
-import Maybe

 \end{code}
 
 
 \end{code}
 
 

@@ -88,17 +85,18 @@ instance Outputable FamInst where
 pprFamInst :: FamInst -> SDoc
 pprFamInst famInst
   = hang (pprFamInstHdr famInst)
 pprFamInst :: FamInst -> SDoc
 pprFamInst famInst
   = hang (pprFamInstHdr famInst)

-       2 (ptext SLIT("--") <+> pprNameLoc (getName famInst))
+       2 (ptext (sLit "--") <+> pprNameLoc (getName famInst))

 
 pprFamInstHdr :: FamInst -> SDoc
 pprFamInstHdr (FamInst {fi_fam = fam, fi_tys = tys, fi_tycon = tycon})
   = pprTyConSort <+> pprHead
   where
 
 pprFamInstHdr :: FamInst -> SDoc
 pprFamInstHdr (FamInst {fi_fam = fam, fi_tys = tys, fi_tycon = tycon})
   = pprTyConSort <+> pprHead
   where

-    pprHead = pprTypeApp fam (ppr fam) tys
-    pprTyConSort | isDataTyCon tycon = ptext SLIT("data instance")
-                | isNewTyCon  tycon = ptext SLIT("newtype instance")
-                | isSynTyCon  tycon = ptext SLIT("type instance")
-                | otherwise         = panic "FamInstEnv.pprFamInstHdr"
+    pprHead = pprTypeApp fam tys
+    pprTyConSort | isDataTyCon     tycon = ptext (sLit "data instance")
+                | isNewTyCon      tycon = ptext (sLit "newtype instance")
+                | isSynTyCon      tycon = ptext (sLit "type instance")
+                | isAbstractTyCon tycon = ptext (sLit "data instance")
+                | otherwise             = panic "FamInstEnv.pprFamInstHdr"

 
 pprFamInsts :: [FamInst] -> SDoc
 pprFamInsts finsts = vcat (map pprFamInst finsts)
 
 pprFamInsts :: [FamInst] -> SDoc
 pprFamInsts finsts = vcat (map pprFamInst finsts)

@@ -221,51 +219,68 @@ we return the matching instance '(FamInst{.., fi_tycon = :R42T}, Int)'.
 
 \begin{code}
 type FamInstMatch = (FamInst, [Type])           -- Matching type instance
 
 \begin{code}
 type FamInstMatch = (FamInst, [Type])           -- Matching type instance

-
-lookupFamInstEnv :: FamInstEnvs
-                -> TyCon -> [Type]             -- What we are looking for
-                -> [FamInstMatch]              -- Successful matches
-lookupFamInstEnv (pkg_ie, home_ie) fam tys
-  | not (isOpenTyCon fam) 
-  = []
-  | otherwise
-  = home_matches ++ pkg_matches
+  -- See Note [Over-saturated matches]
+
+lookupFamInstEnv
+    :: FamInstEnvs
+    -> TyCon -> [Type]         -- What we are looking for
+    -> [FamInstMatch]          -- Successful matches
+
+lookupFamInstEnv
+   = lookup_fam_inst_env match True
+   where
+     match _ tpl_tvs tpl_tys tys = tcMatchTys tpl_tvs tpl_tys tys
+
+lookupFamInstEnvConflicts
+    :: FamInstEnvs
+    -> FamInst         -- Putative new instance
+    -> [TyVar]         -- Unique tyvars, matching arity of FamInst
+    -> [FamInstMatch]  -- Conflicting matches
+-- E.g. when we are about to add
+--    f : type instance F [a] = a->a
+-- we do (lookupFamInstConflicts f [b])
+-- to find conflicting matches
+-- The skolem tyvars are needed because we don't have a 
+-- unique supply to hand
+
+lookupFamInstEnvConflicts envs fam_inst skol_tvs
+  = lookup_fam_inst_env my_unify False envs fam tys'

   where
   where

-    rough_tcs    = roughMatchTcs tys
-    all_tvs      = all isNothing rough_tcs
-    home_matches = lookup home_ie 
-    pkg_matches  = lookup pkg_ie  
-
-    --------------
-    lookup env = case lookupUFM env fam of
-                  Nothing -> []        -- No instances for this class
-                  Just (FamIE insts has_tv_insts)
-                      -- Short cut for common case:
-                      --   The thing we are looking up is of form (C a
-                      --   b c), and the FamIE has no instances of
-                      --   that form, so don't bother to search 
-                    | all_tvs && not has_tv_insts -> []
-                    | otherwise                   -> find insts
-
-    --------------
-    find [] = []
-    find (item@(FamInst { fi_tcs = mb_tcs, fi_tvs = tpl_tvs, 
-                         fi_tys = tpl_tys, fi_tycon = tycon }) : rest)
-       -- Fast check for no match, uses the "rough match" fields
-      | instanceCantMatch rough_tcs mb_tcs
-      = find rest
-
-        -- Proper check
-      | Just subst <- tcMatchTys tpl_tvs tpl_tys tys
-      = (item, substTyVars subst (tyConTyVars tycon)) : find rest
-
-        -- No match => try next
-      | otherwise
-      = find rest
+    inst_tycon = famInstTyCon fam_inst
+    (fam, tys) = expectJust "FamInstEnv.lookuFamInstEnvConflicts"
+                           (tyConFamInst_maybe inst_tycon)
+    skol_tys = mkTyVarTys skol_tvs
+    tys'     = substTys (zipTopTvSubst (tyConTyVars inst_tycon) skol_tys) tys
+        -- In example above,   fam tys' = F [b]   
+
+    my_unify old_fam_inst tpl_tvs tpl_tys match_tys
+       = ASSERT2( tyVarsOfTypes tys `disjointVarSet` tpl_tvs,
+                 (ppr fam <+> ppr tys) $$
+                 (ppr tpl_tvs <+> ppr tpl_tys) )
+               -- Unification will break badly if the variables overlap
+               -- They shouldn't because we allocate separate uniques for them
+         case tcUnifyTys instanceBindFun tpl_tys match_tys of
+             Just subst | conflicting old_fam_inst subst -> Just subst
+             _other                                      -> Nothing
+
+      -- - In the case of data family instances, any overlap is fundamentally a
+      --   conflict (as these instances imply injective type mappings).
+      -- - In the case of type family instances, overlap is admitted as long as
+      --   the right-hand sides of the overlapping rules coincide under the
+      --   overlap substitution.  We require that they are syntactically equal;
+      --   anything else would be difficult to test for at this stage.
+    conflicting old_fam_inst subst 
+      | isAlgTyCon fam = True
+      | otherwise      = not (old_rhs `tcEqType` new_rhs)
+      where
+        old_tycon = famInstTyCon old_fam_inst
+        old_tvs   = tyConTyVars old_tycon
+        old_rhs   = mkTyConApp old_tycon  (substTyVars subst old_tvs)
+        new_rhs   = mkTyConApp inst_tycon (substTyVars subst skol_tvs)

 \end{code}
 
 While @lookupFamInstEnv@ uses a one-way match, the next function
 \end{code}
 
 While @lookupFamInstEnv@ uses a one-way match, the next function

-@lookupFamInstEnvUnify@ uses two-way matching (ie, unification).  This is
+@lookupFamInstEnvConflicts@ uses two-way matching (ie, unification).  This is

 needed to check for overlapping instances.
 
 For class instances, these two variants of lookup are combined into one
 needed to check for overlapping instances.
 
 For class instances, these two variants of lookup are combined into one

@@ -275,19 +290,45 @@ Moreover, matching is the wildly more frequently used operation in the case of
 indexed synonyms and we don't want to slow that down by needless unification.
 
 \begin{code}
 indexed synonyms and we don't want to slow that down by needless unification.
 
 \begin{code}

-lookupFamInstEnvUnify :: (FamInstEnv, FamInstEnv) -> TyCon -> [Type]
-                     -> [(FamInstMatch, TvSubst)]
-lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
+------------------------------------------------------------
+-- Might be a one-way match or a unifier
+type MatchFun =  FamInst               -- The FamInst template
+             -> TyVarSet -> [Type]     --   fi_tvs, fi_tys of that FamInst
+             -> [Type]                 -- Target to match against
+             -> Maybe TvSubst
+
+type OneSidedMatch = Bool     -- Are optimisations that are only valid for
+                              -- one sided matches allowed?
+
+lookup_fam_inst_env          -- The worker, local to this module
+    :: MatchFun
+    -> OneSidedMatch
+    -> FamInstEnvs
+    -> TyCon -> [Type]         -- What we are looking for
+    -> [FamInstMatch]          -- Successful matches
+lookup_fam_inst_env match_fun one_sided (pkg_ie, home_ie) fam tys

   | not (isOpenTyCon fam) 
   = []
   | otherwise
   | not (isOpenTyCon fam) 
   = []
   | otherwise

-  = home_matches ++ pkg_matches
+  = ASSERT( n_tys >= arity )   -- Family type applications must be saturated
+    home_matches ++ pkg_matches

   where
   where

-    rough_tcs    = roughMatchTcs tys
-    all_tvs      = all isNothing rough_tcs

     home_matches = lookup home_ie 
     pkg_matches  = lookup pkg_ie  
 
     home_matches = lookup home_ie 
     pkg_matches  = lookup pkg_ie  
 

+    -- See Note [Over-saturated matches]
+    arity = tyConArity fam
+    n_tys = length tys
+    extra_tys = drop arity tys
+    (match_tys, add_extra_tys) 
+       | arity > n_tys = (take arity tys, \res_tys -> res_tys ++ extra_tys)
+       | otherwise     = (tys,            \res_tys -> res_tys)
+                -- The second case is the common one, hence functional representation
+
+    --------------
+    rough_tcs = roughMatchTcs match_tys
+    all_tvs   = all isNothing rough_tcs && one_sided
+

     --------------
     lookup env = case lookupUFM env fam of
                   Nothing -> []        -- No instances for this class
     --------------
     lookup env = case lookupUFM env fam of
                   Nothing -> []        -- No instances for this class

@@ -307,26 +348,33 @@ lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
       | instanceCantMatch rough_tcs mb_tcs
       = find rest
 
       | instanceCantMatch rough_tcs mb_tcs
       = find rest
 

-      | otherwise
-      = ASSERT2( tyVarsOfTypes tys `disjointVarSet` tpl_tvs,
-                (ppr fam <+> ppr tys <+> ppr all_tvs) $$
-                (ppr tycon <+> ppr tpl_tvs <+> ppr tpl_tys)
-               )
-               -- Unification will break badly if the variables overlap
-               -- They shouldn't because we allocate separate uniques for them
-        case tcUnifyTys bind_fn tpl_tys tys of
-           Just subst -> let rep_tys = substTyVars subst (tyConTyVars tycon)
-                          in
-                          ((item, rep_tys), subst) : find rest
-           Nothing    -> find rest
+        -- Proper check
+      | Just subst <- match_fun item tpl_tvs tpl_tys match_tys
+      = (item, add_extra_tys $ substTyVars subst (tyConTyVars tycon)) : find rest

 
 

--- See explanation at @InstEnv.bind_fn@.
---
-bind_fn :: TyVar -> BindFlag
-bind_fn tv | isTcTyVar tv && isExistentialTyVar tv = Skolem
-          | otherwise                             = BindMe
+        -- No match => try next
+      | otherwise
+      = find rest

 \end{code}
 
 \end{code}
 

+Note [Over-saturated matches]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's ok to look up an over-saturated type constructor.  E.g.
+     type family F a :: * -> *
+     type instance F (a,b) = Either (a->b)
+
+The type instance gives rise to a newtype TyCon (at a higher kind
+which you can't do in Haskell!):
+     newtype FPair a b = FP (Either (a->b))
+
+Then looking up (F (Int,Bool) Char) will return a FamInstMatch 
+     (FPair, [Int,Bool,Char])
+
+The "extra" type argument [Char] just stays on the end.
+
+
+
+

 %************************************************************************
 %*                                                                     *
                Looking up a family instance
 %************************************************************************
 %*                                                                     *
                Looking up a family instance