harbl/src/Network/DNS/RBL/Domain/Digit.hs

   1 -- | The simplest module you'll ever see. It contains the 'Digit' type
   2 --   and a Parsec parser to parse one. We don't export its constructor
   3 --   because then you could do something dumb like stick a letter
   4 --   inside one.
   5 --
   6 --   These are defined in RFC1035, Section 2.3.1, \"Preferred name
   7 --   syntax\" <https://tools.ietf.org/html/rfc1035#section-2.3.1>:
   8 --
   9 --     <digit> ::= any one of the ten digits 0 through 9
  10 --
  11 module Network.DNS.RBL.Domain.Digit (
  12   Digit,
  13   digit )
  14 where
  15
  16 import qualified Text.Parsec as Parsec ( digit )
  17 import Text.Parsec.String ( Parser )
  18
  19 import Network.DNS.RBL.Pretty ( Pretty(..) )
  20
  21
  22 -- | A wrapper around a single digit character.
  23 --
  24 --   ==== _Examples_
  25 --
  26 --   >>> Digit '1'
  27 --   Digit '1'
  28 --
  29 --   >>> let d1 = Digit '2'
  30 --   >>> let d2 = Digit '2'
  31 --   >>> let d3 = Digit '3'
  32 --   >>> d1 == d2
  33 --   True
  34 --   >>> d1 == d3
  35 --   False
  36 --
  37 newtype Digit = Digit Char deriving (Eq, Show)
  38
  39
  40 -- | Pretty-printing for digits that we've already parsed. Just
  41 --   shows/prints the digit character.
  42 --
  43 --   ==== _Examples_
  44 --
  45 --   >>> let d = Digit '1'
  46 --   >>> pretty_print d
  47 --   1
  48 --
  49 instance Pretty Digit where pretty_show (Digit d) = [d]
  50
  51
  52 -- | Parse a single digit, but wrap it in our 'Digit' type.
  53 --
  54 --   ==== _Examples_
  55 --
  56 --   >>> import Text.Parsec ( parseTest )
  57 --
  58 --   Digits are parsed correctly:
  59 --
  60 --   >>> parseTest digit "3"
  61 --   Digit '3'
  62 --
  63 --   But letters are not:
  64 --
  65 --   >>> parseTest digit "x"
  66 --   parse error at (line 1, column 1):
  67 --   unexpected "x"
  68 --   expecting digit
  69 --
  70 digit :: Parser Digit
  71 digit = fmap Digit Parsec.digit