[Haskell-cafe] Re: Proper round-trip HughesPJ/Parsec for Doubles?
haskell at gimbo.org.uk
Tue Feb 23 11:08:29 EST 2010
>> 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
That seems to do the trick! Below, for the record, the code I've come
up with (I threw away the Either Integer Double part so it's a bit
simpler, also). I'm sure it can be improved, but this is passing all
tests reliably, it seems.
Many thanks, Christian and Daniel, for your help!
parseDouble :: Parser Double
parseDouble = try $ do (symbol toks) "-"
n <- floater
return $ negate n
where toks = makeTokenParser emptyDef
-- This could definitely be improved, but it's working. :-)
floater :: Parser Double
floater = do w <- many1 digit
f <- many1 digit
e <- optionMaybe $ do char 'e' -- Optional exponent part
n <- option "" (char '-' >> return
"-") -- Optional negation in exponent
m <- many1 digit
return $ n ++ m
case e of Nothing -> return $ read $ w ++ "." ++ f
Just e' -> return $ read $ w ++ "." ++ f ++
"e" ++ e'
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