[commit: ghc] master: Add a comment to the mapFB rules (665cefe)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/665cefe80d112ed2e4fb9617d277a1466e83f9bd/ghc

>---------------------------------------------------------------

commit 665cefe80d112ed2e4fb9617d277a1466e83f9bd
Author: Joachim Breitner <mail at joachim-breitner.de>
Date:   Thu Mar 9 16:13:08 2017 +0100

    Add a comment to the mapFB rules
    
    to amend 2fa44217c1d9722227297eefb0d6c6aed7e128ca.


>---------------------------------------------------------------

665cefe80d112ed2e4fb9617d277a1466e83f9bd
 libraries/base/GHC/Base.hs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libraries/base/GHC/Base.hs b/libraries/base/GHC/Base.hs
index 6f9d454..a678c22 100644
--- a/libraries/base/GHC/Base.hs
+++ b/libraries/base/GHC/Base.hs
@@ -964,10 +964,12 @@ mapFB c f = \x ys -> c (f x) ys
 -- (along with build's unfolding) else we'd get an infinite loop
 -- in the rules.  Hence the activation control below.
 --
--- The "mapFB" rule optimises compositions of map.
---
 -- This same pattern is followed by many other functions:
 -- e.g. append, filter, iterate, repeat, etc.
+--
+-- The "mapFB" rule optimises compositions of map and
+-- the "mapFB/id" rule get rids of 'map id' calls.
+-- (Any similarity to the Functor laws for [] is expected.)
 
 {-# RULES
 "map"       [~1] forall f xs.   map f xs                = build (\c n -> foldr (mapFB c f) n xs)