wren ng thornton wrote:
John Goerzen wrote:
Hi folks, I have uploaded a new package to Haskell: convertible. At its heart, it's a very simple typeclass that's designed to enable a reasonable default conversion between two different types without having to remember a bunch of functions.
I once again point out that realToFrac is *wrong* for converting from Float or Double.
Yes, realToFrac is just broken, or at least the Real instances of Float and Double are. Of course, with the restriction to single parameter type classes in Haskell98, it's hard to come up with anything better - we'd end up with fooToBar for all Real / Fractional pairs.
> realToFrac (1/0::Float) ::Double 3.402823669209385e38 > > realToFrac (0/0::Float) ::Double -5.104235503814077e38
> realToFrac (0/0::Double) ::Float -Infinity
[snip] GHC makes the mess just a bit messier. The following program prints different answers when compiled with -O or without: main = do print (realToFrac (1/0::Float) :: Double) print (realToFrac (0/0::Float) :: Double) print (realToFrac (0/0::Double) :: Float) Without -O: 3.402823669209385e38 -5.104235503814077e38 -Infinity With -O: Infinity NaN NaN The reason for this behaviour are rules replacing realToFrac by direct conversions from Float to Double or vice versa, where applicable. These two evils -- realToFrac and changing the behaviour with -O -- amount to something good: proper treatment of NaNs and Infinities. Your RealToFrac class is a definitive improvement over this situation. Bertram