Am Dienstag 23 Februar 2010 14:44:50 schrieb Andy Gimblett:
Hi all,
Short version: How can I pretty print and parse values of type Double such that those operations are each other's inverse?
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-quic kcheck/
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.
Parse it as a Rational, then convert with fromRational. I don't know whether that will always have parse . prettyPrint == id, but it'll be much closer. The naturalOrFloat default implementation uses fraction = do{ char '.' ; digits <- many1 digit <?> "fraction" ; return (foldr op 0.0 digits) } <?> "fraction" where op d f = (f + fromIntegral (digitToInt d))/10.0 and division by 10 isn't exact with a binary representation.
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/