Revised signature of tcLookupFamInst and lookupFamInstEnv

[ghc-hetmet.git] / compiler / typecheck / TcEnv.lhs

diff --git a/compiler/typecheck/TcEnv.lhs b/compiler/typecheck/TcEnv.lhs
index 787616a..0f9bf23 100644 (file)
--- a/compiler/typecheck/TcEnv.lhs
+++ b/compiler/typecheck/TcEnv.lhs
@@ -42,7 +42,10 @@ module TcEnv(
        topIdLvl, 
 
        -- New Ids
-       newLocalName, newDFunName, newFamInstTyConName
+       newLocalName, newDFunName, newFamInstTyConName,
+
+        -- Errors
+        famInstNotFound
   ) where
 
 #include "HsVersions.h"
@@ -159,7 +162,21 @@ tcLookupLocatedTyCon :: Located Name -> TcM TyCon
 tcLookupLocatedTyCon = addLocM tcLookupTyCon
 
 -- Look up the representation tycon of a family instance.
--- Return the rep tycon and the corresponding rep args
+--
+-- The match must be unique - ie, match exactly one instance - but the 
+-- type arguments used for matching may be more specific than those of 
+-- the family instance declaration.
+--
+-- Return the instance tycon and its type instance.  For example, if we have
+--
+--  tcLookupFamInst 'T' '[Int]' yields (':R42T', 'Int')
+--
+-- then we have a coercion (ie, type instance of family instance coercion)
+--
+--  :Co:R42T Int :: T [Int] ~ :R42T Int
+--
+-- which implies that :R42T was declared as 'data instance T [a]'.
+--
 tcLookupFamInst :: TyCon -> [Type] -> TcM (TyCon, [Type])
 tcLookupFamInst tycon tys
   | not (isOpenTyCon tycon)
@@ -169,20 +186,8 @@ tcLookupFamInst tycon tys
        ; eps <- getEps
        ; let instEnv = (eps_fam_inst_env eps, tcg_fam_inst_env env)
        ; case lookupFamInstEnv instEnv tycon tys of
-
-          [(subst, fam_inst)] | variable_only_subst -> 
-            return (rep_tc, substTyVars subst (tyConTyVars rep_tc))
-               where   -- NB: assumption is that (tyConTyVars rep_tc) is in 
-                       --     the domain of the substitution
-                 rep_tc              = famInstTyCon fam_inst
-                 subst_domain        = varEnvElts . getTvSubstEnv $ subst
-                 tvs                 = map (Type.getTyVar "tcLookupFamInst") 
-                                           subst_domain
-                 variable_only_subst = all Type.isTyVarTy subst_domain &&
-                                       sizeVarSet (mkVarSet tvs) == length tvs
-                                       -- renaming may have no repetitions
-
-          other -> famInstNotFound tycon tys other
+          [(fam_inst, rep_tys)] -> return (famInstTyCon fam_inst, rep_tys)
+          other                 -> famInstNotFound tycon tys other
        }
 \end{code}