From a57b6d8f9eab3c50d8691d2a55c54ccccac2018c Mon Sep 17 00:00:00 2001
From: simonpj <unknown>
Date: Tue, 3 Sep 2002 11:27:33 +0000
Subject: [PATCH] [project @ 2002-09-03 11:27:33 by simonpj] More comments

---
 ghc/compiler/typecheck/TcMType.lhs    |    4 ++++
 ghc/compiler/typecheck/TcMonoType.lhs |    3 +++
 2 files changed, 7 insertions(+)

diff --git a/ghc/compiler/typecheck/TcMType.lhs b/ghc/compiler/typecheck/TcMType.lhs
index c13993a..d4d506a 100644
--- a/ghc/compiler/typecheck/TcMType.lhs
+++ b/ghc/compiler/typecheck/TcMType.lhs
@@ -936,6 +936,10 @@ check_valid_theta ctxt []
 check_valid_theta ctxt theta
   = getDOptsTc					`thenNF_Tc` \ dflags ->
     warnTc (notNull dups) (dupPredWarn dups)	`thenNF_Tc_`
+	-- Actually, in instance decls and type signatures, 
+	-- duplicate constraints are eliminated by TcMonoType.hoistForAllTys,
+	-- so this error can only fire for the context of a class or
+	-- data type decl.
     mapTc_ (check_source_ty dflags ctxt) theta
   where
     (_,dups) = removeDups tcCmpPred theta
diff --git a/ghc/compiler/typecheck/TcMonoType.lhs b/ghc/compiler/typecheck/TcMonoType.lhs
index eedad78..ac86e09 100644
--- a/ghc/compiler/typecheck/TcMonoType.lhs
+++ b/ghc/compiler/typecheck/TcMonoType.lhs
@@ -644,6 +644,9 @@ hoistForAllTys :: Type -> Type
 -- e.g.  T -> forall a. a        ==>   forall a. T -> a
 --	 T -> (?x::Int) -> Int   ==>   (?x::Int) -> T -> Int
 --
+-- Also: eliminate duplicate constraints.  These can show up
+-- when hoisting constraints, notably implicit parameters.
+--
 -- We want to 'look through' type synonyms when doing this
 -- so it's better done on the Type than the HsType
 
-- 
1.7.10.4