Re: why tail recursion matters and why Java isn't it, was Re: lispperformance was Re: problems with lisp

[Pascal Costanza <address@hidden>]

David Herman wrote:
>> This doesn't prove anything. In Scheme you're relying on a very 
>> specialized feature of tail calls to be sure that your code is efficient.
> 
> 
> This was my first impression of proper tail recursion, too. It even felt 
> like a violation of an abstraction boundary. But in fact, the opposite 
> is true: the expectation that the runtime will blow its stack beyond a 
> fixed depth of recursion is the violation. You should be able to use 
> recursion without having to think about the runtime implementation
[...]

Agreed. Still, proper tail recursion is a very specific feature of 
Scheme, and one of its strong advantages. You don't find it in a lot of 
other languages. That's all I wanted say.

>> In both cases, you have to know the languages well to get your work 
>> done to your satisfaction. In the Common Lisp case, I have at least 
>> three options to express my intent - one of them is straightforward, 
>> the other two are not.
> 
> A particular loop construct may be natural (for some definition thereof) 
> for some problems, but not for others. With recursion and higher-order 
> functions you can build your own loop constructs that fit the problem 
> space better.

If a particular loop construct is natural for a particular problem, why 
shouldn't I use it? If it's not natural I use something else.

I don't care for the general case, I care for the particular case. If a 
particular case is not covered I have to find a better solution and need 
to apply a more general construct.

>> Luke Gorrie mailed me this other solution.
>>
>> (mapcar 'car list)
> 
> Try doing that with iteration, mutation, and integer indexes and still 
> convince yourself that a for-loop is more natural. :)

I didn't say that a for-loop is _generally_ more natural. I also 
wouldn't say that recursion is generally more natural. Again, for a 
particular class of cases, recursion is the best solution.


Pascal

-- 
Pascal Costanza               University of Bonn
mailto:costanza@web.de        Institute of Computer Science III
http://www.pascalcostanza.de  Römerstr. 164, D-53117 Bonn (Germany)