X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcDeriv.lhs;h=f86dd64fc269406aa8c8e47a740197b8d1b74d1f;hb=66f73ee4d4130b98d35ec26bba073b2197724011;hp=a273b4ae1795d7593b64c69463aeb5a9f94756f0;hpb=119ede053a062d59dffd63473f126eceff304980;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index a273b4a..f86dd64 100644
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -877,6 +877,26 @@ extendLocalInstEnv dfuns thing_inside
       ; setGblEnv env' thing_inside }
 \end{code}
 
+Note [Deriving context]
+~~~~~~~~~~~~~~~~~~~~~~~
+With -fglasgow-exts, we allow things like (C Int a) in the simplified
+context for a derived instance declaration, because at a use of this
+instance, we might know that a=Bool, and have an instance for (C Int
+Bool)
+
+We nevertheless insist that each predicate meets 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)).
+  
+
 %************************************************************************
 %*									*
 \subsection[TcDeriv-normal-binds]{Bindings for the various classes}