From: Nathaniel Wesley Filardo <nwf@cs.jhu.edu>
Date: Thu, 29 Nov 2012 07:10:58 +0000 (-0500)
Subject: Add hrllen to HList utilities module
X-Git-Url: https://hydra-www.ietfng.org/gitweb/?a=commitdiff_plain;h=b15369baae12c38cfb76c75bb74fbbee8b0da735;p=dyna2

Add hrllen to HList utilities module
---

diff --git a/src/Dyna/XXX/HList.hs b/src/Dyna/XXX/HList.hs
index 8eecd93..83bc6e2 100644
--- a/src/Dyna/XXX/HList.hs
+++ b/src/Dyna/XXX/HList.hs
@@ -25,7 +25,7 @@ module Dyna.XXX.HList(
   HList(..), hlmapa,
 
   -- * HLists of a type constructor's images
-  HRList(..), hrlmapa, hrlmap, hrlproj, hrlTravProj,
+  HRList(..), hrlmapa, hrllen, hrlmap, hrlproj, hrlTravProj,
 
   -- * Interoperation between HList and HRList
   HLR(..), hlToHrl,
@@ -87,6 +87,13 @@ instance Show (HRList r '[]) where show _ = "HRN"
 instance (Show (r a), Show (HRList r as)) => Show (HRList r (a ': as)) where
     show (a :++ as) = show a ++ " :+ " ++ show as
 
+hrllen :: HRList r t -> Int
+hrllen = go 0
+ where
+  go :: Int -> HRList r t -> Int
+  go a HRN = a
+  go a (x :++ xs) = a `seq` go (a+1) xs
+
 -- | Shift the type-function of a HRList
 hrlmap :: (forall a . r a -> s a) -> HRList r t -> HRList s t
 hrlmap _  HRN       = HRN