proper tail calls

Neil Mix nmix at pandora.com
Tue Jan 22 07:45:37 PST 2008


>> Thanks. That would work. But I can still see the "average" user being
>> confused when debugging, and not knowing what is going on.
>
> Would you think an explicit keyword syntax for mandatory tail call
> would help such a user?

I do.

> To the claim that debugging in the face of PTCs will become madness-
> inducing, Schemers and others retort "do you want to see every state
> of an iteration?" (A loop is a tail call in Scheme.) The right answer
> is "yes". Yes, I want a debugger that remembers all program states
> (http://code.google.com/p/chronomancer/) and runs in near real-time
> (not chronomancer, alas -- not yet). I want the moon, as a debugger
> user (and yet I still suffer in this day and age with gdb!).
>
> My point is that debugging is a specialized task with immature
> (frozen in the last days of disco!) tools; the debugger tail should
> not wag the dog.

I can't argue the sentiment.  But exactly how soon will such advanced  
debugging tools be generally available and in the hands of the  
programmers we're discussing?  Before ES4 has given way to ES5/6/7?

> Separately, poring over crashdumps (which is not the same as
> debugging, and not a task for "average" users), many C++ hackers have
> had to deal with good old "TCO". It's a pain, but we keep the
> optimization levels high for production builds and suffer the entrail-
> reading horror when investigating crashes.

Without forcing you to declare your age ;) I must point out that it's  
common these days for programmers to have over a decade's experience  
without any coredump debugging experience.  (I'm *almost* an example  
of this.)  I'm having a hard time swallowing the argument that it's  
OK for a modern language like ES4 to require the skillsets used for  
assembly++.  (I'm not arguing that those aren't good skillsets to  
have, just pointing out the reality of next-generation programmers.)

> I've heard Schemers testify that tail calls seldom impair debugging,
> but I'll invite those Schemer among the many on this list who are so
> inclined to re-testify.

I think that's skewing the sample.  We're not talking about schemers  
here, we're talking about scripters.  They don't read language specs,  
they use tutorials and references, and they just so happen to vastly  
outnumber the people on this list.  Advanced PLT concepts are "over  
engineered" from their perspective.  They have no frame of reference  
for incomplete stack traces.  Implicit PTC will confuse the heck out  
of them, they'll go straight to bugzilla and file a bug on the  
interpreter.  ;)  At least Explicit gives them a fighting chance.   
They might just ask, "what the heck does that keyword mean, anyway?"  
and go look it up in their favorite reference.

I think the implicit-hurts-debugability argument has a lot more  
weight than you're giving it, especially in the near-term.




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