I was recently looking at the various functions the RealFrac type class provides and noticed, that Double isn't listed as an instance of it; and neither are Num, Real and Fractional mentioned as implemented instances in the documentation for Double.

Now, somebody who is familiar with the hierarchy of the numeric type classes could of course rather easily deduct, that, since Double is an instance of RealFloat (this is mentioned in the documentation on Hackage), it is also required to implement all of the above. And (surely enough) the Haskell 98 Report lists all the relevant implementations and so does a quick :info Double in GHCi.

However I'd (assume and thus) argue, that most beginners who google (or especially hoogle) anything about Haskell, are often directed to the Hackage documentation itself - and any missing information there could lead to quite a bit of a headache; even more so, if this concerns something as important as the Prelude.

Therefore I'd like to ask, whether this is a known issue and independently propose, that (in the meantime) an official recommendation somewhere to do a quick :info <type class> or :info <type> in GHCi yourself, as to find out, which type class instances are currently available, might help to alleviate some confusion.

Benjamin