[Haskell-cafe] instance Enum Double considered not entirely great?

[Daniel Fischer]

On Wednesday 21 September 2011, 00:20:09, Casey McCann wrote:
> This plays havoc with the search tree used internally by Set and Map,
> the result being that if you have any NaN values in the data
> structure, you may not be able to find other values anymore. Because
> NaN values never compare equal to themselves, I'm not sure if it's
> even possible to remove them from the structure,

filter (not . isNaN)

resp.

filterWithKey (\k _ -> not $ isNaN k)

> and because of tree
> rebalancing I'm not sure how to predict what the impact of one or more
> NaNs would be over multiple operations on the data structure.

Yuck. Don't even try to predict that (unless you absolutely have to).

> 
> In short: Using Doubles in a Set, or as the key to a Map, should be
> regarded as a bug until proven otherwise (i.e., proving that NaN will
> never be inserted).
> 
> If you'd like to see an explicit demonstration (which you can try in
> GHCi yourself!) see here:
> http://stackoverflow.com/questions/6399648/what-happens-to-you-if-you-br
> eak-the-monad-laws/6399798#6399798 where I use it as an example of why
> it's important for type class instances to obey the relevant laws.

Nice and short.