Re: [Haskell-cafe] Re: A question about "monad laws"

On 11 Feb 2008, at 7:52 AM, Arnar Birgisson wrote:

Hi all,

On Feb 11, 2008 3:14 PM, apfelmus wrote:

...

I will be mean by asking the following counter question:

x + (y + z) = (x + y) + z

is a mathematical identity. If it is a mathematical identity, a programmer need not care about this law to implement addition + . Can anyone give me an example implementation of addition that violates this law?

Depends on what you mean by "addition". In general, algebraists call any associative and commutative operation on a set "addition", and nothing else. From that POV, there is by definition no "addition" that violates this law.

I agree. The Num Double instance should be expelled from the Prelude immediately. jcc (What? Haskell has a Float type?)