Set 9a Ex 9: clarify text, add test

for #31

exercises/Set9a.hs

@@ -163,6 +163,10 @@ data Text = Text String
 --
 -- Hint: remember the function `lookup` from Prelude?
 --
+-- Note! The order of arguments to `compose` is the other way around
+-- compared to e.g. (.): `compose f g` should apply `f` first, then
+-- `g`, but `f.g` applies `g` first, then `f`.
+--
 -- Examples:
 --   composing two mappings of size 1:
 --     compose [("a",1)] [(1,True)]

exercises/Set9aTest.hs

@@ -21,7 +21,7 @@ tests = [(1,"workload",[ex1_small, ex1_medium, ex1_big])
         ,(6,"Lock",[ex6_examples, ex6_gen])
         ,(7,"Eq Text",[ex7])
         ,(8,"compose",[ex8_one_one, ex8_many_one, ex8_bijection, ex8_surjective])
-        ,(9,"permute",[ex9_id, ex9_compose])
+        ,(9,"permute",[ex9_id, ex9_simple, ex9_compose])
         ]
 
 -- -- -- -- --
@@ -191,6 +191,11 @@ ex9_id = property $ do
   xs <- shuffle (take n "abcdefghij")
   return $ $(testing [|permute id xs|]) (?== xs)
 
+ex9_simple = forAllShrink_ (sublistOf "pqrstuv") $ \sorted ->
+  forAllBlind (shuffle sorted) $ \cs ->
+  let res = [(i,j) | (i,c) <- zip [0..] cs, let Just j = elemIndex c sorted]
+  in $(testing [|permute res cs|]) (?==sorted)
+
 ex9_compose = property $ do
   n  <- choose (2,5)
   p  <- return . zip [0..n-1] =<< shuffle [0..n-1]