Fix a deriving bug, arising from recent refactoring

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

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index a273b4a..f86dd64 100644 (file)
--- 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}