[plt-scheme] Some fine distinctions

OOPS; I have been skipping this in my readings. Mea culpa


On May 13, 2009, at 9:46 AM, Robby Findler wrote:

> (+ accum (sumlists (first a-list)))
>
> On Wed, May 13, 2009 at 8:44 AM, Matthias Felleisen
> <matthias at ccs.neu.edu> wrote:
>>
>> Which one is not?
>>
>> On May 13, 2009, at 8:40 AM, Robby Findler wrote:
>>
>>> Actually not every call to sumlists is in tail position.
>>>
>>> Robby
>>>
>>> On Wednesday, May 13, 2009, Matthias Felleisen  
>>> <matthias at ccs.neu.edu>
>>> wrote:
>>>>
>>>> On May 13, 2009, at 7:24 AM, wooks wrote:
>>>>
>>>>
>>>> Didn't fully absorb that. So
>>>>
>>>>
>>>> sumlists :: listoflists -> number
>>>> (define sumlists
>>>>   (lambda (a-list)
>>>>      (let sumlists ([a-list a-list] [accum 0])
>>>>        (cond
>>>>         [(empty? a-list) 0]
>>>>         [(list? (first a-list)) (sumlists (rest a-list)
>>>>                                  (+ accum (sumlists (first a- 
>>>> list)))]
>>>>         [else (sumlists (rest a-list) (+ accum (first a- 
>>>> list)))]))))
>>>>
>>>> This is accumulative and I used to think ergo tail recursive.
>>>> If I am right and it is not (because of the evaluation of the list?
>>>> case)...is there any way to write this sort of function tail
>>>> recursively. It looks to me not... at least not in this style of
>>>> programming.
>>>>
>>>>
>>>> Every call to the (inner) sumlist is in tail position, ergo this  
>>>> function
>>>> is tail-recursive.
>>>>
>>>> ;; ---
>>>>
>>>> The accumulator statement is
>>>>
>>>>  ;; accum is the sum of all numbers between a-list0 and a-list
>>>>
>>>> (Of course, you failed to name your parameters properly so go  
>>>> back to the
>>>> original reply and check it out.)
>>>>
>>>> ;; ---
>>>>
>>>> What I am saying is that accumulator-style does NOT imply tail- 
>>>> recursion.
>>>>
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>>
>>