Re: [Haskell-cafe] Python vs Haskell in tying the knot

15 Jul 2009

      On Wed, Jul 15, 2009 at 2:18 PM, Max Rabkin wrote:
...
...
fib = 1:1:fib `plus` (tail fib) where plus = zipWith (+)
...
...
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
On Wed, Jul 15, 2009 at 7:33 PM, Cristiano
Paris wrote:
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.
Isn't tying the knot (in the way 'fib' does) straightforward with closures
a la Python/Ruby/Smalltalk (without mutation)?
Even in a syntactically clumsy language like Java, a
tying-the-knot implementation equivalent to the canonical Haskell one is
not difficult, e.g.

static L fibs = new L() {
    public int head() { return 1; }
    public L tail() {
        return  new L() {
            public int head() { return 1; }
            public L tail() {
                return new L() {
                    public int head() { return fibs.head() +
fibs.tail().head(); }
                    public L tail() { return zip(fibs.tail(),
fibs.tail().tail()); }
                };
            }
        };
    }
};

Given a definition of list L and zip...

interface L { int head(); L tail(); }
static L zip(final L l0, final L l1) {
    return new L() {
        public int head() { return l0.head() + l1.head(); }
        public L tail() { return zip(l0.tail(), l1.tail()); }
    };
}

Re: [Haskell-cafe] Python vs Haskell in tying the knot

Robert Greayer