Fix type signature of 'append'

1 files changed, 7 insertions, 1 deletions

diff --git a/src/Data/Array/Mixed.hs b/src/Data/Array/Mixed.hs
index d9eb5f0..9a8ccfd 100644
--- a/src/Data/Array/Mixed.hs
+++ b/src/Data/Array/Mixed.hs
@@ -252,7 +252,13 @@ index xarr i
   = let XArray arr' = indexPartial xarr i :: XArray '[] a
     in S.unScalar arr'
 
-append :: forall sh a. (KnownShapeX sh, Storable a) => XArray sh a -> XArray sh a -> XArray sh a
+type family AddMaybe n m where
+  AddMaybe Nothing _ = Nothing
+  AddMaybe (Just _) Nothing = Nothing
+  AddMaybe (Just n) (Just m) = Just (n + m)
+
+append :: forall n m sh a. (KnownShapeX sh, Storable a)
+       => XArray (n : sh) a -> XArray (m : sh) a -> XArray (AddMaybe n m : sh) a
 append (XArray a) (XArray b)
   | Dict <- lemKnownINatRankSSX (knownShapeX @sh)
   , Dict <- knownNatFromINat (Proxy @(Rank sh))