Andy Gimblett schrieb: > Hi all, > > Short version: How can I pretty print and parse values of type Double > such that those operations are each other's inverse?
Maybe you have more luck with show and read (without Parsec.Token). Your example: x = 9.91165677454629 fails because the computation performed by the parser 9.0 + 0.91165677454629 yields 9.911656774546291 Cheers Christian > Long version: I'm writing and QuickCheck-testing a parser using the > approach set out here: > > http://lstephen.wordpress.com/2007/07/29/parsec-parser-testing-with-quickcheck/ > > > That is, each syntactic category gets a pretty-printer and a parser and > an Arbitrary instance, and QuickCheck checks that (parse . prettyPrint) > == id, basically. Somewhat unsurprisingly, this sometimes fails for > floating point values (I'm using Doubles). > > Now, I know that floats are in some sense imprecise, and comparing for > equality is fraught with peril, but it seems that if x==x then it ought > to be at least _possible_ to arrange matters such that (parse . > prettyPrint x) == x as well. At worst, pretty-printing the underlying > binary representation!? So my feeling is that my parser could be improved. > > At the moment I'm working around it by defining a type class which > checks for equality within some margin of error, and using that instead > of Eq - but it's messier than I'd like, so I wondered if there was > something obvious I'm missing. > > As hpaste.org seems to be down, I'll attach a code example here instead. > > Thanks! > > -Andy > > -- > Andy Gimblett > http://gimbo.org.uk/ > > > > > ------------------------------------------------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe