From 02ec37666737ddca8d59ad4ba89ca8b0f12244e2 Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Fri, 13 Aug 2010 16:10:19 +0000 Subject: [PATCH] Comments only --- compiler/typecheck/TcInstDcls.lhs | 26 +++++++++++++++++--------- 1 file changed, 17 insertions(+), 9 deletions(-) diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs index c04f4a2..d7aafc1 100644 --- a/compiler/typecheck/TcInstDcls.lhs +++ b/compiler/typecheck/TcInstDcls.lhs @@ -197,14 +197,15 @@ a top-level axiom: df :: forall a. C a => C [a] {-# NOINLINE df DFun[ $cop_list ] #-} - df = /\a. \d. MkD ($cop_list a d) + df = /\a. \d. MkC ($cop_list a d) - $cop_list :: forall a. C a => a -> a + $cop_list :: forall a. C a => [a] -> [a] $cop_list = -The "constructor" MkD expands to a cast, as does the class-op selector. +The "constructor" MkC expands to a cast, as does the class-op selector. The RULE works just like for multi-field dictionaries: - * (df a d) returns (Just (MkD,..,[$cop_list a d])) + + * (df a d) returns (Just (MkC,..,[$cop_list a d])) to exprIsConApp_Maybe * The RULE for op picks the right result @@ -214,18 +215,25 @@ application. But it works just fine in this case, exprIsConApp_maybe is otherwise used only when we hit a case expression which will have a real data constructor in it. -The biggest reason for doing it this way, apart form uniformity, is +The biggest reason for doing it this way, apart from uniformity, is that we want to be very careful when we have instance C a => C [a] where {-# INLINE op #-} op = ... -then we'll get an INLINE pragma on $cop_list. The danger is that -we'll get something like - foo = /\a.\d. $cop_list a d +then we'll get an INLINE pragma on $cop_list but it's important that +$cop_list only inlines when it's applied to *two* arguments (the +dictionary and the list argument + +The danger is that we'll get something like + op_list :: C a => [a] -> [a] + op_list = /\a.\d. $cop_list a d and then we'll eta expand, and then we'll inline TOO EARLY. This happened in -Trac #3772 and I spent far too long fiddling arond trying to fix it. +Trac #3772 and I spent far too long fiddling around trying to fix it. Look at the test for Trac #3772. + (Note: re-reading the above, I can't see how using the + uniform story solves the problem.) + Note [Subtle interaction of recursion and overlap] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this -- 1.7.10.4