[commit: dph] master: Comments for packByTag (ea09388)

[Ben Lippmeier]

Repository : ssh://darcs.haskell.org//srv/darcs/packages/dph

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/ea09388bd1761fc2c0eebdc21a76e46500938dad

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

commit ea09388bd1761fc2c0eebdc21a76e46500938dad
Author: Ben Lippmeier <benl at ouroborus.net>
Date:   Tue May 3 16:52:08 2011 +1000

    Comments for packByTag

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

 dph-prim-interface/interface/DPH_Interface.h |   20 ++++++++++++++++++--
 1 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/dph-prim-interface/interface/DPH_Interface.h b/dph-prim-interface/interface/DPH_Interface.h
index 6d55f75..3b5c54b 100644
--- a/dph-prim-interface/interface/DPH_Interface.h
+++ b/dph-prim-interface/interface/DPH_Interface.h
@@ -586,9 +586,25 @@ tagsToSel2 tags = let rep = mkSelRep2 tags
 
 
 -- Packing and Picking --------------------------------------------------------
-packByTag :: Elt a => Array a -> Array Tag -> Tag -> Array a
+
+-- | Select the elements of an array that have a corresponding tag.
+--   
+-- @
+-- packByTag [12, 24, 42, 93] [1, 0, 0, 1] 0
+--  = [24, 42]
+-- @
+--
+packByTag 
+        :: Elt a
+        => Array a      -- ^ data values
+        -> Array Tag    -- ^ tag values
+        -> Tag          -- ^ the tag of values to select
+        -> Array a      -- ^ data values that had that tag
+
 {-# INLINE_BACKEND packByTag #-}
-packByTag xs tags !tag = fsts (filter (\p -> Prelude.snd p == tag) (zip xs tags))
+packByTag xs tags !tag
+        = fsts (filter (\p -> Prelude.snd p == tag) (zip xs tags))
+
 
 pick :: (Elt a, Eq a) => Array a -> a -> Array Bool
 {-# INLINE pick #-}