On Wed, Jul 15, 2009 at 7:33 PM, Cristiano
Paris
fib = 1:1:fib `plus` (tail fib) where plus = zipWith (+)
Of course, this was something I already encountered when exploring the Y-combinator. Anyhow, I tried to translate this implementation to Python using Iterators and this is what I wrote:
def fib(): yield 1 yield 1
p = plus(fib(),tail(fib()))
while True: yield p.next()
def plus(x,y): while True: yield x.next() + y.next()
print take(5,fib())
I'm not convinced that this is the "same" program as the Haskell version. No knot is tied in the Python version. To tie the knot, a data structure must contain or refer to itself. In the python version, the function which creates the data structure refers to itself, but many copies of the data structure are created.
So my considerations are:
1 - The Fibonacci generator is not an example of TTK at all and then it can be translated to Python.
This indicates that you think tying the knot should be impossible in Python. In my opinion this is not the case. By my definition of tying the knot, one needs *either* mutable variables or laziness (at least in simple cases). Since Python has the former, it is possible to tie the knot in Python. To me the simplest example of TTK in Python (not possible in Haskell because it has an infinite type): x = [] x.append(x) (If you try to print this list, one gets [[...]] in the standard REPL and [<Recursion on list with id=173852620>] in ipython) --Max