Re: [Haskell-cafe] Re: Wikipedia on first-class object

On 27 Dec 2007, at 10:44 AM, Achim Schneider wrote:

Wolfgang Jeltsch wrote:

...

Am Donnerstag, 27. Dezember 2007 16:34 schrieb Cristian Baboi:

...

I'll have to trust you, because I cannot test it.

let x=(1:x); y=(1:y) in x==y .

I also cannot test this:

let x=(1:x); y=1:1:y in x==y

In these examples, x and y denote the same value but the result of x == y is _|_ (undefined) in both cases. So (==) is not really equality in Haskell but a kind of weak equality: If x doesn’t equal y, x == y is False, but if x equals y, x == y might be True or undefined.

[1..] == [1..] certainly isn't undefined, it always evaluates to True,

If something happens, it does eventually happen. More importantly, we can prove that [1..] == [1..] = _|_, since [1..] == [1..] = LUB (n >= 1) [1..n] ++ _|_ == [1..n] ++ _|_ = LUB (n >= 1) _|_ = _|_ jcc