Re: [Haskell-cafe] Mystery of an Eq instance

2013/9/22 Mike Meyer :

On Sat, Sep 21, 2013 at 5:28 PM, Bardur Arantsson wrote: Trying to make something whose name is "Not A Number" act like a number sounds broken from the start.

The point here is that IEEE floats are actually more something like a "Maybe Float", with various "Nothing"s, i.e. the infinities and NaNs, which all propagate in a well-defined way. Basically a monad built into your CPU's FP unit. ;-)

I just went back through the thread, and the only examples I could find where that happened (as opposed to where floating point calculations or literals resulted in unexpected values) was with NaNs. Just out of curiosity, do you know of any that don't involve NaNs?

Well, with IEEE arithmetic almost nothing you learned in school about math holds anymore. Apart from rounding errors, NaNs and infinities, -0 is another "fun" part: x * (-1) is not the same as 0 - x (Hint: Try with x == 0 and use recip on the result.)

Float violates the expected behavior of instances of - well, pretty much everything it's an instance of. Even if you restrict yourself to working with integer values that can be represented as floats. If we're going to start removing it as an instance for violating instance expectations, we might as well take it out of the numeric stack (or the language) completely.

Exactly, and I am sure 99.999% of all people wouldn't like that removal. Learn IEEE arithmetic, hate it, and deal with it. Or use something different, which is probably several magnitudes slower. :-/