[Haskell-cafe] Justification for Ord inheriting from Eq?

[Robert Dockins]

On Thursday 06 April 2006 06:44 pm, John Meacham wrote:
> On Thu, Apr 06, 2006 at 10:52:52PM +0100, Brian Hulley wrote:

[snip a question about Eq and Ord classes]

> well, there are a few reasons you would want to use inheritance in
> haskell, some good, some bad.
>
> 1. one really does logically derive from the other, Eq and Ord are like
> this, the rules of Eq says it must be an equivalance relation and that
> Ord defines a total order over that equivalance relation. this is a good
> thing, as it lets you write code that depends on these properties.

<PUBLIC SERVICE ANNOUNCEMENT>

Many of you probably know this already, but for those who might not know:

Prelude> let x = read "NaN" :: Float
Prelude> x == x
False
Prelude> x == 0
False
Prelude> 0 < x
False
Prelude> x < 0
False

Ewwwwww! Be careful how far you depend on properties of typeclasses, and make 
sure you document it when you do.


<we now return to you regularly scheduled discussion>


Rob Dockins