Tut 10.4.3 Reading the counter

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+module tut10_4_3
+
+// ********************************************************************************
+// Clean tutorial example program.
+//
+// This program defines a Controls component that implements a manually settable
+// counter.
+// A bi-directional receiver is added to give external access to the counter value.
+// ********************************************************************************
+
+import StdEnv, StdIO
+
+Start :: *World -> *World
+Start world = startIO NDI Void initialise [] world
+where
+    initialise pst
+    # (displayId, pst)  = accPIO openId pst
+    # (resetId, pst)    = accPIO openRId pst
+    # (readId, pst)     = accPIO openR2Id pst
+    # (error, pst)      = openDialog Void (dialog displayId resetId readId) pst
+    | error <> NoError  = abort "Counter could not open counter dialog."
+    # (displayId, pst)  = accPIO openId pst
+    # (error, pst)      = openDialog Void (display displayId readId) pst
+    | error <> NoError  = abort "Counter could not open display dialog."
+    | otherwise         = pst
+
+dialog displayId resetId readId = Dialog "Counter" (counter :+: resetButton) [WindowClose (noLS closeProcess)]
+where
+    counter = { newLS = initcount,
+                newDef = LayoutControl (EditControl (toString initcount) (PixelWidth (hmm 50.0)) 1 [ControlId displayId, ControlSelectState Unable] :+:
+                                        ButtonControl "-" [ControlFunction (count(-1)), ControlWidth (PixelWidth (hmm 25.0)), ControlPos (BelowPrev, zero)] :+:
+                                        ButtonControl "+" [ControlFunction (count  1 ), ControlWidth (PixelWidth (hmm 25.0))] :+:
+                                        Receiver resetId reset [] :+:
+                                        Receiver2 readId read [])
+                                       [ControlPos (Center, zero), ControlHMargin 0 0, ControlVMargin 0 0, ControlItemSpace 0 0] }
+    where
+        initcount = 0
+
+        count :: Int (Int, PSt .l) -> (Int, PSt .l)
+        count dx (count, pst) = (count + dx, appPIO (setControlText displayId (toString (count+dx))) pst)
+
+        reset :: m (Int, PSt .l) -> (Int, PSt .l)
+        reset _ (_, pst) = (initcount, appPIO (setControlText displayId (toString initcount)) pst)
+
+        read :: m (Int, PSt .l) -> (Int, (Int, PSt .l))
+        read _ (count, pst) = (count, (count, pst))
+
+    resetButton = ButtonControl "Reset" [ControlFunction (noLS (snd o syncSend resetId undef)), ControlPos (Center, zero)]
+
+display displayId readId = Dialog "Read" (EditControl "" (PixelWidth (hmm 50.0)) 1 [ControlId displayId, ControlSelectState Unable] :+:
+                                          ButtonControl "Read" [ControlFunction (noLS read), ControlPos (Center, zero)])
+                                         [WindowClose (noLS closeProcess)]
+where
+    read pst
+    # ((error, value), pst) = syncSend2 readId undef pst
+    | error <> SendOk   = abort "Could not read counter value"
+    | otherwise = appPIO (setControlText displayId (toString (fromJust value))) pst
+