[commit: ghc] master: Define chkAppend, and use it (3d81b68)

[Simon Peyton Jones]

Repository : http://darcs.haskell.org/ghc.git/

On branch  : master

https://github.com/ghc/ghc/commit/3d81b68d48e86f9dff032321aff20b76ad3ea081

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

commit 3d81b68d48e86f9dff032321aff20b76ad3ea081
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu May 30 22:02:17 2013 +0100

    Define chkAppend, and use it
    
    Somtimes we need (xs ++ ys) in situations where ys is
    almost always empty.  Utils.chkAppend checks for that
    case first.

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

 compiler/types/FamInstEnv.lhs | 4 +---
 compiler/utils/Util.lhs       | 9 ++++++++-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/compiler/types/FamInstEnv.lhs b/compiler/types/FamInstEnv.lhs
index 34f1701..6ccd44d 100644
--- a/compiler/types/FamInstEnv.lhs
+++ b/compiler/types/FamInstEnv.lhs
@@ -754,13 +754,11 @@ find match_fun match_tys (inst@(FamInst { fi_branches = branches, fi_branched =
               where 
                 match = FamInstMatch { fim_instance = inst
                                      , fim_index    = ind
-                                     , fim_tys      = substTyVars subst tvs `add_on` match_tys2}
+                                     , fim_tys      = substTyVars subst tvs `chkAppend` match_tys2}
       where
         -- Deal with over-saturation
         -- See Note [Over-saturated matches]
         (match_tys1, match_tys2) = splitAtList mb_tcs match_tys
-        add_on tys1 []   = tys1  -- The wildly common case
-        add_on tys1 tys2 = tys1 ++ tys2
 
 lookup_fam_inst_env           -- The worker, local to this module
     :: MatchFun
diff --git a/compiler/utils/Util.lhs b/compiler/utils/Util.lhs
index 90a2077..e2fd0aa 100644
--- a/compiler/utils/Util.lhs
+++ b/compiler/utils/Util.lhs
@@ -18,7 +18,7 @@ module Util (
 
         unzipWith,
 
-        mapFst, mapSnd,
+        mapFst, mapSnd, chkAppend,
         mapAndUnzip, mapAndUnzip3, mapAccumL2,
         nOfThem, filterOut, partitionWith, splitEithers,
 
@@ -259,6 +259,13 @@ splitEithers (e : es) = case e of
                         Left x -> (x:xs, ys)
                         Right y -> (xs, y:ys)
     where (xs,ys) = splitEithers es
+
+chkAppend :: [a] -> [a] -> [a]
+-- Checks for the second arguemnt being empty
+-- Used in situations where that situation is common
+chkAppend xs ys 
+  | null ys   = xs
+  | otherwise = xs ++ ys
 \end{code}
 
 A paranoid @zip@ (and some @zipWith@ friends) that checks the lists