view n . shift n . view i . shift i) x == x --> shift + view is invertible

darcs-hash:20070430062901-9c5c1-dd591516cb05072be929aa68d42be3b27476bd5a

1 files changed, 8 insertions, 4 deletions

diff --git a/tests/Properties.hs b/tests/Properties.hs
index b49cbd3..ba82fdc 100644
--- a/tests/Properties.hs
+++ b/tests/Properties.hs
@@ -142,10 +142,12 @@ prop_view_idem (x :: T) r =
         sz = size x
     in view i (view i x) == (view i x)
 
-prop_shiftshift r x =
-    let n  = current x
-    in shift n (shift r x) == x
-    where _ = x :: T
+prop_shift_reversible r (x :: T) =
+    let i  = fromIntegral $ r `mod` sz
+        sz = size x
+        n  = current x
+    in height n x > 0 ==> (view n . shift n . view i . shift i) x == x
+
 
 prop_fullcache x = cached == allvals where
     cached  = sort . keys $ cache x
@@ -323,6 +325,8 @@ main = do
         ,("view/view        ", mytest prop_viewview)
         ,("view idem        ", mytest prop_view_idem)
 
+        ,("shift reversible ", mytest prop_shift_reversible)
+
         ,("fullcache        ", mytest prop_fullcache)
         ,("currentwsvisible ", mytest prop_currentwsvisible)
         ,("ws screen mapping", mytest prop_ws2screen_screen2ws)