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-quickche...
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/
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