[Tutor] church numerals (fwd)

[Alan Gauld]

> From: Asrarahmed Kadri <ajkadri at googlemail.com>

>> #########################
>>>>> def iszero(n):
>> ...     def s(x):
>> ...         return False
>> ...     return n(s, True)
> 
> 
> Where is n() defined  ...?????

n is a parameter of the function.

Thus to call iszero you need to pass in another function
which in turn takes a function and a boolean as its 
arguments.

Going back to Danny's original example:

>>> def zero(s, z):
...     return z
...
>>> def one(s, z):
...     return s(z)

Both of these "numbers" match the type of fuction that 
iszero expects. So we can do:

iszero(zero)

Now n takes on the vale of zero
and the return line of iszero becomes:

return zero(s,True)

and zero returns its second argument which is True, 
so zero is zero, as expected.

For iszero(one)

the return line is

return one(s,True)

but inside one the return is now

return s(True), but the return value of s is False.
So the ultimate reurtn value of iszero(one) is False, again 
as expected.

This is fairly mind bending the first time you come across it 
so don;t panic,ust work your way through a few more examples 
as I did above.

-- 
Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld