bytestring-lexing- Parse literals efficiently from bytestrings

MaintainerDon Stewart <>






readDouble :: ByteString -> Maybe (Double, ByteString)Source

Parse the initial portion of the ByteString as a Double precision floating point value. The expected form of the numeric literal is given by:

  • An optional + or - sign
  • Decimal digits, OR
  • 0 [oO] and a sequence of octal digits, OR
  • 0 [xX] and a sequence of hexadecimal digits, OR
  • An optional decimal point, followed by a sequence of decimal digits,
  • And an optional exponent

The result is returned as a pair of a double-precisoin floating point value, and the remaining input, or Nothing, should no parse be found.

For example, to sum a file of floating point numbers, one per line,

 import qualified Data.ByteString.Char8  as S
 import qualified Data.ByteString.Unsafe as S
 import Data.ByteString.Lex.Double
 main = print . go 0 =<< S.getContents
     go n s = case readDouble s of
                     Nothing       -> n
                     Just (k,rest) -> go (n+k) (S.tail rest)