Andy Gimblett schrieb:
Hi Christian, [...]
It may make sense to use something like readMaybe (which is missing in the Prelude) instead of "read" to allow the parser to fail more nicely.
It seems to be kicking up reasonable errors as it is, e.g.:
*Main> parse aFloat "" "2e-h" Left (line 1, column 4): unexpected "h" expecting digit
yes, this is fine, because you reject "-h", but suppose it was passed to read (due to a programming error).
I haven't seen any uncaught exceptions propagating, if that's what you're worried about...?
Yes, "read" will always work for you. But you could use a parser *almost* as simple as many1 $ oneOf "NaInfity+-.eE0123456789" and ask "readMaybe" if the parsed String can be read as Double.
So here's what I have now:
float' :: TokenParser st -> GenParser Char st Double float' t = do n <- maybeChar "-" spaces fs <- choice [symbol t "NaN", symbol t "Infinity", do whole <- many1 digit frac <- option "" $ do char '.' ds <- many1 digit return $ '.' : ds ex <- option "" $ do choice [char 'e', char 'E'] s <- maybeChar "+-" ds <- many1 digit return $ concat ["e", s, ds] return $ concat [whole, frac, ex] ] whiteSpace t return $ read $ n ++ fs where maybeChar :: String -> GenParser Char st String maybeChar as = option "" (choice (map char as) >>= \a -> return [a])
I would omit handling of spaces (that's a separate lexing step). It's enough to be able to parse those numbers, that are possible results of "show" (for round-trip). "symbol t" could be replaced by "(try . string)" in order to get rid of the TokenParser (that I don't like). Spaces following an initial minus sign are quite unusual and rather indicate that the sign does not belong to number, but that the sign is a separate operation.
You can also break it immediately before do, which I think is sometimes more clear.
If not an extra space is added following "do" this leads to an "odd" indentation of at least one line.
I'm curious: which line in the above is indented oddly? Oh, wait: you don't mean odd as in "strange", do you? You mean odd as in "not even"? So, e.g. the "spaces" line starts at column 5? What's wrong with that?
Right, again a matter of taste. Cheers Christian P.S. below is my parser for tptp numbers (as comparison). It rejects leading zeros, but allows an initial + sign. "fmap read real" would work if the input does not start with + (or is NaN or Infinity). -- | does not allow leading zeros natural :: Parser String natural = string "0" <|> many1 digit decimal :: Parser String decimal = do s <- option "" $ string "+" <|> string "-" ds <- natural return $ s ++ ds real :: Parser String real = do d <- decimal f <- option "" $ do p <- char '.' n <- many1 digit return $ p : n e <- option "" $ do x <- char 'e' <|> char 'E' g <- decimal return $ x : g return $ d ++ f ++ e