# [plt-scheme] Some fine distinctions

 From: Robby Findler (robby at eecs.northwestern.edu) Date: Wed May 13 09:46:40 EDT 2009 Previous message: [plt-scheme] Some fine distinctions Next message: [plt-scheme] Some fine distinctions Messages sorted by: [date] [thread] [subject] [author]

```(+ 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.
>>>
>>> _________________________________________________