From: Nathaniel Wesley Filardo <nwf@cs.jhu.edu>
Date: Fri, 7 Dec 2012 02:18:40 +0000 (-0500)
Subject: Compilation fixes and tweaks to HList utils
X-Git-Url: https://hydra-www.ietfng.org/gitweb/?a=commitdiff_plain;h=652119091df4238f5d1eb150a26a69fb11acdc24;p=dyna2

Compilation fixes and tweaks to HList utils
---

diff --git a/src/Dyna/XXX/HList.hs b/src/Dyna/XXX/HList.hs
index 83bc6e2..ab2983c 100644
--- a/src/Dyna/XXX/HList.hs
+++ b/src/Dyna/XXX/HList.hs
@@ -47,12 +47,25 @@ type family Map (f :: k -> k') (x :: [k]) :: [k']
 type instance Map f '[] = '[]
 type instance Map f (a ': as) = (f a) ': Map f as
 
+-- | Inverse of 'Map'
+type family UnMap (f :: k -> k') (x :: [k']) :: [k]
+type instance UnMap f '[] = '[]
+type instance UnMap f (f a ': fas) = a ': UnMap f fas
+
 -- | Apply a type function yielding a constraint to each element of a
 -- lifted list.
 type family MapConstraint (f :: k -> Constraint) (x :: [k]) :: Constraint
 type instance MapConstraint f '[] = ()
 type instance MapConstraint f (x ': xs) = (f x, MapConstraint f xs)
 
+------------------------------------------------------------------------}}}
+-- Classes                                                              {{{
+
+-- | Assert that two lifted-lists are of the same length
+class EqLen (x :: [k]) (y :: [k']) | x -> y, y -> x
+instance EqLen '[] '[]
+instance (EqLen xs ys) => EqLen (x ': xs) (y ': ys)
+
 ------------------------------------------------------------------------}}}
 -- Data-level HLists                                                    {{{
 
@@ -72,6 +85,15 @@ hlmapa :: (Applicative m, Map c t ~ t')
 hlmapa _ HN = pure HN
 hlmapa f (a :+ as) = liftA2 (:+) (f a) (hlmapa f as)
 
+{-
+ - XXX Why can't I make this work?
+hrlmap' :: (EqLen ras sas, HLR r as ras, HLR s as sas)
+        => (forall a . r a -> s a) -> HList ras -> HList sas
+hrlmap' _  HN       = HN
+hrlmap' f (a :+ as) = f a :+ (hrlmap' f as)
+-}
+
+
 ------------------------------------------------------------------------}}}
 -- HRLists - HLists of a type constructor's images                      {{{
 
@@ -121,8 +143,8 @@ hlToHrl :: (forall b . b -> r b) -> HList a -> HRList r a
 hlToHrl _ HN = HRN
 hlToHrl f (a:+as) = f a :++ (hlToHrl f as)
 
-class (Map r a ~ ra)
-   => HLR (r :: k -> k') (a :: [k]) (ra :: [k']) | ra -> r a
+class (Map r a ~ ra, UnMap r ra ~ a)
+   => HLR (r :: * -> *) (a :: [*]) (ra :: [*])
  where
   -- | Reinterpret an HList as an HRList
   hlAsHrl :: HList ra -> HRList r a