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message-driven function: more explanation

• To: mathgroup at smc.vnet.net
• Subject: [mg95603] message-driven function: more explanation
• From: "Bert Aerts (rm X)" <bert.aertsX at advalvas.beX>
• Date: Thu, 22 Jan 2009 07:03:16 -0500 (EST)

In the book Programming Paradigms with Mathematica 
http://library.wolfram.com/infocenter/MathSource/1847/
in the notebook Hw21.nb there is following function:

Clear[makeCounter];
makeCounter::usage="makeCounter[] returns a message-driven function
which manipulates a \"hidden\" counter variable.  The initial value
of the counter variable may be set via an optional argument to
makeCounter (default is 0).  Valid messages for the function are
\"++\" (increment), \"--\" (decrement), \"value\", and \"reset\".
For example, ctr = makeCounter[100]; ctr[\"value\"] returns 100.";

makeCounter[
   defaultValue_Integer:0
] :=
   Module[
     { counter = defaultValue,
       changeCounter
     },
     changeCounter[msg_String:"++"] :=
       Switch[msg,
         "++",
           counter = counter + 1; counter,
         "--",
           counter = counter - 1; counter,
         "value",
           counter,
         "reset",
           counter = defaultValue; counter,
         _,
           Print["Unknown message to counter:", msg];
           counter
       ];
     changeCounter
   ];

ctr = makeCounter[100]
ctr["value"]
ctr["++"]

gives as output

changeCounter$723
100
101

Where can I find more explanation of this type of "message-driven function"?