[plt-scheme] guido on tail recursion

From: Nadeem Abdul Hamid (nadeem at acm.org)
Date: Fri Apr 24 16:55:55 EDT 2009

Hmm... I see what you mean about the TCO. In Python, one could write:

def readline(func):
     '''readline : (string -> a) -> a'''

def loop():
     def callback(line):
         print "you said %s" % line
     print "Say something"

And it definitely doesn't optimize because if you type EOF at some  
point you get a big long stack trace... talk about turtles all the way  

However, the natural imperative way to test such a readline in Python  
would seem to me to be something like:

def imp_loop():
     while True:
         print "Say something"
         readline(lambda line: void_print("you said %s" % line))


--- nadeem

On Apr 24, 2009, at 4:31 PM, Eli Barzilay wrote:

> Yes.  I should also have added that control might go through several
> pieces of code, and most such pieces of code will do the "obvious
> thing" and end with tail calls -- so things just sort themselves out.
> (I remember that Joe Marshall phrased this nicely at some point, I
> don't remember the sentence though.)
> On Apr 24, Matthias Felleisen wrote:
>> I think Eli meant to add that the call to the callback is in TP and
>> that the call to loop in the callback is in TP and, by the TCO
>> guarantee for Scheme, all of this (well-designed) code becomes a loop
>> (from the perspective of space consumption) on the target machine --
>> Matthias
>> On Apr 24, 2009, at 3:57 PM, Eli Barzilay wrote:
>>> Here's a piece of code that I'm using now -- it's some gui thing,  
>>> and
>>> for technical reasons, I'm implementing a `read-line' method that
>>> reads a line from the gui somehow.  Since the gui is on a single
>>> thread, I made `read-line' get a callback, which is called on the  
>>> line
>>> when it has been read.  To play with my code I wrote a loop that  
>>> reads
>>> lines in a loop:
>>>         (let loop ()
>>>           (output "Say something")
>>>           (read-line (lambda (line)
>>>                        (output (format "You said: ~a" line))
>>>                        (loop))))
>>> The thing is that the loop goes through the callback, so it doesn't
>>> translate to a simple (imperative) loop.  Is there a way to do  
>>> this in
>>> python?
>>> (Looks like it shouldn't be hard, but I can't think of anything.  It
>>> might be that I'm blinded by scheme, or it might be the result of
>>> sleep deprivation...)
> -- 
>          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli  
> Barzilay:
>                  http://www.barzilay.org/                 Maze is  
> Life!
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