Documentation

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-precision 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
   where
     go n s = case readDouble s of
                     Nothing       -> n
                     Just (k,rest) -> go (n+k) (S.tail rest)

Bare bones, unsafe wrapper for C's strtod(3). This provides a non-copying direct parsing of Double values from a ByteString. It uses strtod directly on the bytestring buffer. strtod requires the string to be null terminated, or for a guarantee that parsing will find a floating point value before the end of the string.