how to write function that returns function

[Fernando Rodríguez]

On 14 May 2002 16:03:25 -0700, spam at bugbear.com (Paul Graham) wrote:

>I am not a Python expert, and I'm hoping someone
>can tell me how in Python to write a function
>of one argument x that returns a function of one
>argument y that returns x+y.
>
>Here, in Scheme, is what I want to write:
>
>(define foo (x) (lambda (y) (+ x y)))
>
>I found on the web a page that says I could define 
>this as follows:
>
>def addn(x):
>  return lambda y,z=y: x+z

Get a recent version of Python that includes 'nested scopes' a la Lisp, and
try:

>>> from __future__ import nested_scopes  # Turn on nested scopes
>>> def f(x):
	def g(y):
		return y +x
	return g

>>> adder = f(3)
>>> adder(4)
7

BTW, this is not exactly the same behavior you find in Lisp. For example:

x = 0 #Global variable
def f():
  x = 3  # Doesnt create a dynamic binding, it creates a new local variable
  return x

The 'x = 3' doesn't "setf", it "let"s

I find this behavior disturbing, and I hope special variables in Arc will
behave as in Common Lisp.

Good luck with Arc! :-)




-----------------------
Fernando Rodriguez