Re: [Chicken-users] Chicken for Python Programmers

[Tobia Conforto]

John Cowan wrote:
> "In Scheme all functions are lambdas": but they don't have the  
> restrictions of Python lambdas.

Maybe one could say:

In Scheme all functions are lambdas and all lambdas are functions.   
They have the power of regular Python functions and the usefulness of  
Python lambdas, at the same time.

> SRFI-1 provides append!, but note that you must capture the return  
> values of destructive (!) list functions; you can't just rely on the  
> side effects.  There is no guarantee that append! will actually side- 
> effect its arguments; you are just permitting it to do so.

Here I would add a generic explanation on the conceptual difference  
between Python's and Scheme's "destructive operations", more than a  
list of behaviours.  Let's see if I can come up with something:

A Python list is a single object. Its destructive or in-place  
operations (such as sort or append) simply operate on the whole object  
and modify it in-place.  Scheme "values" that are also single objects  
(such as strings, pairs, or hash tables) behave in the same way.

A Scheme list, on the other hand, is a chain of simpler objects  
(pairs) so you typically don't have operations that modify the "whole"  
list in-place.  Both Scheme's "destructive" and "non-destructive" list  
operations return a new list with the requested changes.  This new  
list (the return value of the operation) will have to be used  
somewhere to be of use, even if just by re-assigning it to the same  
variable:

(set! some-list (reverse! some-list))   ;same as: some_list.reverse()

The difference between non-destructive and destructive operations is  
that the destructive sort may build the new list by recycling the  
pairs that make up the old chain.  A destructive operation is often  
more optimized, but you should only use it when: 1. you are  
reassigning the return value to the same variable, see above, or 2.  
you won't use that variable anymore, after the operation.

This strange paradigm comes from Scheme's emphasis on functional  
programming.  A Scheme programmer doesn't expect a variable (such as  
"some-list" above) to change its immediate, direct value, unless he  
explicitly "set!"s it to a new value.  The functional programming  
style completely avoids "set!", or variable assignment, and that is  
why this paradigm makes sense.


-Tobia