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From: jans@tekgvs.GVS.TEK.COM (Jan Steinman)
Newsgroups: comp.lang.forth,comp.lang.smalltalk
Subject: Re: Dynamic Bindings
Message-ID: <4033@tekgvs.GVS.TEK.COM>
Date: 4 Oct 88 19:53:51 GMT
Organization: Tektronix Inc., Beaverton, Or.
Lines: 43

<<Why is dynamic binding bad?>>

Because of the run-time overhead only, NOT because:

<...general dynamic binding is... one of the worst types of hidden dependency I 
know of.  Let me give an example. (example deleted)... Now with this fairly 
straightforward typing error, we have produced a virtually undetectable bug.  
If 'a' is supposed to be bound to a procedure that returns an integer... (but 
is actually) a string, since the bindings all change dynamically, this will set 
off a chain of errors too large to comprehend!>

(It's so easy to detect the Smalltalk detractors who are not actually Smalltalk 
users!)  I'm not sure what you're argument is here.  Any reasonable combination 
of dynamic binding, message passing, and incremental compilation should have no 
trouble detecting the compile-time inconsistency.  Although your example is 
rather difficult to envision as object-oriented code, I think Smalltalk would 
catch your "typing error" at compile time.

If you are referring to run-time behavior (that is, if you told the compiler to 
accept unbound selectors), the combination of message sending and dynamic 
binding (often referred to as "polymorphism") causes a rather simple-to-debug 
run-time error.  In your case, the String object would report that it does not 
understand how to perform the Integer message it just received.  Of course, if 
you want to guarantee that both Strings and Integers respond to a given set of 
messages, it is quite simple to override the default error handling to enable 
coercion.  (This "lazy mutation" is how Tektronix Smalltalk allows you to 
change a class definition, and have that change propagate to existing 
instances.)  Hybrid languages, like Ada, C++, and Modula-2 require you to 
overload the operator for each class or type.  Can you spell "maintenance 
headache"?

<...I think that if you required explicit marking by the programmer that this 
identifier would have a variable binding (perhaps even stating the set of 
possible values that it could take) then you'd be onto a winner...  Static 
binding is best in all cases except for its usefulness in inheritance...>

It's not really useful with inheritance if the language employs "copy down" as 
opposed to "look up" inheritance.  Where it is useful is in combination with 
true message passing, when it can implement polymorphism.

:::::: Software Productivity Technologies -- Experiment Manager Project ::::::
:::::: Jan Steinman N7JDB	Box 500, MS 50-383	(w)503/627-5881 ::::::
:::::: jans@tekcrl.TEK.COM	Beaverton, OR 97077	(h)503/657-7703 ::::::