make quickcheck tests friendlier to read.

StackSet.hs

@@ -46,7 +46,7 @@ data StackSet i j a =
         , stacks   :: !(M.Map i ([a], [a])) -- ^ screen -> (floating, normal)
         , focus    :: !(M.Map i a)          -- ^ the window focused in each stack
         , cache    :: !(M.Map a i)          -- ^ a cache of windows back to their stacks
-        } deriving (Eq, Show)
+        } deriving (Eq, Show, Read)
 
 -- The cache is used to check on insertion that we don't already have
 -- this window managed on another stack

tests/Properties.hs

@@ -16,6 +16,7 @@ import System.IO
 import System.Random
 import Text.Printf
 import Data.List            (nub,sort,group,sort,intersperse,genericLength)
+import Data.Char            (ord)
 import Data.Map             (keys,elems)
 import qualified Data.Map as M
 
@@ -121,7 +122,7 @@ prop_notpeek_peekStack n x = current x /= n && isJust (peek x) ==> peekStack n x
 
 ------------------------------------------------------------------------
 
-type T = StackSet Int Int Int
+type T = StackSet Int Int Char
 
 prop_delete_uniq i x = not (member i x) ==> delete i x == x
     where _ = x :: T
@@ -249,6 +250,10 @@ noOverlaps xs  = and [ verts a `notOverlap` verts b
 
 ------------------------------------------------------------------------
 
+instance Arbitrary Char where
+    arbitrary = choose ('a','z')
+    coarbitrary n = coarbitrary (ord n)
+
 instance Random Word8 where
   randomR = integralRandomR
   random = randomR (minBound,maxBound)