how to write function that returns function

[Bengt Richter]

On 15 May 2002 16:04:05 -0700, spam at bugbear.com (Paul Graham) wrote:

>> You can't directly translate that.  Access to variable of enclosing
>> scopes is read only (except in the case of global statements, but that
>> doesn't help here).
>
>It looks as if the closest thing would be something like this,
>which a Python expert sent me:
>
>def foo(n):
>  s = [n]
>  def bar(i):
>    s[0] += i
>    return s[0]
>  return bar
>
>Although you can't modify variables from outer scopes, you can
>modify *parts* of them (which to me seems like the same thing...)
>
>The reason this problem seems kind of artificial is that I don't
>need to solve this actual problem.  I heard that recent Python
>versions had added more support for lexical closures, and was 
>curious how well Python works for the type of programming in 
>Structure and Interpretation of Computer Programs; this is kind 
>of a hello_world of that genre.  (It's not a homework problem, 
>I swear.)
>
>Incidentally, here is Perl 5 code someone sent me for this case:
>
>sub foo {
>  my ($n) = @_;
>  return sub {return $n += $_[0]}}
>
<flame shields up>
 >>> foo = lambda y:([x for x in [[y]]], (lambda: (x.append(x.pop()+1),x[0])[1]))[1]
 >>> f=foo(20)
 >>> f()
 21
 >>> f()
 22
 >>> f=foo(-4)
 >>> [f() for x in 'x'*7]
 [-3, -2, -1, 0, 1, 2, 3]
</flame shields up>

;-)


Regards,
Bengt Richter