Improve the handling of deriving, in error cases

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

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 32fe6cf..3f25a4c 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -2522,19 +2522,60 @@ tcSimplifyDeriv orig tyvars theta
        ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
        ; (irreds, _) <- tryHardCheckLoop doc wanteds
 
+       ; let (tv_dicts, others) = partition isTyVarDict irreds
+       ; addNoInstanceErrs others
+
        ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
-             simpl_theta = substTheta rev_env (map dictPred irreds)
+             simpl_theta = substTheta rev_env (map dictPred tv_dicts)
                -- This reverse-mapping is a pain, but the result
                -- should mention the original TyVars not TcTyVars
 
-       -- NB: the caller will further check the tv_dicts for
-       --     legal instance-declaration form
-
        ; return simpl_theta }
   where
     doc = ptext SLIT("deriving classes for a data type")
 \end{code}
 
+Note [Exotic derived instance contexts]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+       data T a b c = MkT (Foo a b c) deriving( Eq )
+       instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
+
+Notice that this instance (just) satisfies the Paterson termination 
+conditions.  Then we *could* derive an instance decl like this:
+
+       instance (C Int a, Eq b, Eq c) => Eq (T a b c) 
+
+even though there is no instance for (C Int a), because there just
+*might* be an instance for, say, (C Int Bool) at a site where we
+need the equality instance for T's.  
+
+However, this seems pretty exotic, and it's quite tricky to allow
+this, and yet give sensible error messages in the (much more common)
+case where we really want that instance decl for C.
+
+So for now we simply require that the derived instance context
+should have only type-variable constraints.
+
+Here is another example:
+       data Fix f = In (f (Fix f)) deriving( Eq )
+Here, if we are prepared to allow -fallow-undecidable-instances we
+could derive the instance
+       instance Eq (f (Fix f)) => Eq (Fix f)
+but this is so delicate that I don't think it should happen inside
+'deriving'. If you want this, write it yourself!
+
+NB: if you want to lift this condition, make sure you still meet the
+termination conditions!  If not, the deriving mechanism generates
+larger and larger constraints.  Example:
+  data Succ a = S a
+  data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
+
+Note the lack of a Show instance for Succ.  First we'll generate
+  instance (Show (Succ a), Show a) => Show (Seq a)
+and then
+  instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
+and so on.  Instead we want to complain of no instance for (Show (Succ a)).
 
 
 @tcSimplifyDefault@ just checks class-type constraints, essentially;