Move Set 11b Ex 5 to Set 13b

exercises/Set11b.hs

@@ -82,34 +82,6 @@ doubleCall op = todo
 compose :: (a -> IO b) -> (c -> IO a) -> c -> IO b
 compose op1 op2 c = todo
 
-------------------------------------------------------------------------------
--- Ex 5: Implement the operation mkCounter that produces the IO operations
--- inc :: IO () and get :: IO Int. These operations should work like this:
---
---   get returns the number of times inc has been called
---
--- In other words, a simple stateful counter. Use an IORef to store the count.
---
--- Note: this is an IO operation that produces two IO operations. Thus
--- the type of mkCounter is IO (IO (), IO Int).
---
--- This exercise is tricky. Feel free to leave it until later.
---
--- An example of how mkCounter works in GHCi:
---
---  *Set11b> (inc,get) <- mkCounter
---  *Set11b> inc
---  *Set11b> inc
---  *Set11b> get
---  2
---  *Set11b> inc
---  *Set11b> inc
---  *Set11b> get
---  4
-
-mkCounter :: IO (IO (), IO Int)
-mkCounter = todo
-
 ------------------------------------------------------------------------------
 -- Ex 6: Reading lines from a file. The module System.IO defines
 -- operations for Handles, which represent open files that can be read

exercises/Set11bTest.hs

@@ -23,7 +23,6 @@ tests = [(1,"appendAll",[ex1])
         ,(2,"swapIORefs",[ex2])
         ,(3,"doubleCall",[ex3_doubleCall, ex3_doubleCall_2])
         ,(4,"composeIO",[ex4_composeIO_1, ex4_composeIO_2])
-        ,(5,"mkCounter",[ex5_mkCounter])
         ,(6,"hFetchLines",[ex6])
         ,(7,"hSelectLines",[ex7_hSelectLines])
         ,(8,"interact'",[ex8_interact_terminates, ex8_interact_example, ex8_interact_loop])]
@@ -80,15 +79,6 @@ ex4_composeIO_2 =
   ($ compose (return . (*2)) (return . (+1)) i) . withNoInput $
     checkResult (?== (i+1)*2)
 
-
-ex5_mkCounter = forAllBlind (choose (0,20)) $ \n ->
-  counterexample "(inc,get) <- mkCounter" $
-  counterexample (" Called inc "++show n++" times, then called get") $
-  monadicIO $ do m <- run $ do (i,g) <- mkCounter
-                               replicateM_ n i
-                               g
-                 stop_ $ m ?== n
-
 ex6 = forAllBlind (listOf1 word) $ \lines ->
   counterexample ("Contents of handle h:\n"++unlines lines) $
   counterexample "hFetchLines h" $

exercises/Set13b.hs

@@ -5,6 +5,7 @@ import Mooc.Todo
 import Control.Monad
 import Control.Monad.Trans.State
 import Data.Char
+import Data.IORef
 import Data.List
 
 
@@ -323,3 +324,31 @@ instance Monad SL where
   -- implement return and >>=
   return = todo
   (>>=) = todo
+
+------------------------------------------------------------------------------
+-- Ex 9: Implement the operation mkCounter that produces the IO operations
+-- inc :: IO () and get :: IO Int. These operations should work like this:
+--
+--   get returns the number of times inc has been called
+--
+-- In other words, a simple stateful counter. Use an IORef to store the count.
+--
+-- Note: this is an IO operation that produces two IO operations. Thus
+-- the type of mkCounter is IO (IO (), IO Int).
+--
+-- This exercise is tricky. Feel free to leave it until later.
+--
+-- An example of how mkCounter works in GHCi:
+--
+--  *Set11b> (inc,get) <- mkCounter
+--  *Set11b> inc
+--  *Set11b> inc
+--  *Set11b> get
+--  2
+--  *Set11b> inc
+--  *Set11b> inc
+--  *Set11b> get
+--  4
+
+mkCounter :: IO (IO (), IO Int)
+mkCounter = todo

exercises/Set13bTest.hs

@@ -11,9 +11,11 @@ import qualified Data.Bits
 import Data.Char
 import Data.Either
 import Data.Function
+import Data.IORef
 import Data.List
 import qualified Data.Map as Map
 import Test.QuickCheck hiding (Result,Failure)
+import Test.QuickCheck.Monadic
 
 import Examples.Bank
 import Set13b hiding (perhapsIncrement,sumBounded,sumNotTwice)
@@ -28,7 +30,8 @@ tests = [(1,"ifM",[ex1_maybe, ex1_state])
         ,(5,"allSums", [ex5_bits])
         ,(6,"foldM", [ex6_sumBounded_ok, ex6_sumBounded_fail, ex6_sumNotTwice])
         ,(7,"Monad Result", [ex7])
-        ,(8,"Monad SL", [ex8_fmap_1, ex8_fmap_2, ex8_1, ex8_2, ex8_stress])]
+        ,(8,"Monad SL", [ex8_fmap_1, ex8_fmap_2, ex8_1, ex8_2, ex8_stress])
+        ,(9,"mkCounter",[ex9_mkCounter])]
 
 -- -- -- -- -- -- --
 
@@ -268,3 +271,11 @@ ex8_stress =
       (incs,msgs) = partitionEithers ops
       state = sum incs
   in counterexample desc $ runSL op 0 ?== ((),state,msgs)
+
+ex9_mkCounter = forAllBlind (choose (0,20)) $ \n ->
+  counterexample "(inc,get) <- mkCounter" $
+  counterexample (" Called inc "++show n++" times, then called get") $
+  monadicIO $ do m <- run $ do (i,g) <- mkCounter
+                               replicateM_ n i
+                               g
+                 stop_ $ m ?== n