harbl/src/Network/DNS/RBL/Weight.hs

   1 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
   2
   3 -- | The 'Weight' type, its instances, and a Parsec parser to parse
   4 --   one off the end of a 'Site'.
   5 --
   6 --   This is a simple newtype wrapper around an 'Int', meant to be
   7 --   used wherever a weight or an RBL score is intended (to prevent
   8 --   integer mixups). For example, the RBLs are all weighted with
   9 --   'Weight's, and when a user supplied a \"badness\" threshold, it
  10 --   will be as a 'Weight' as well. The two are then comparable but
  11 --   not with other 'Int's.
  12 --
  13 module Network.DNS.RBL.Weight (
  14   Weight(..),
  15   weight )
  16 where
  17
  18 import Text.Parsec (
  19   (<|>),
  20   char,
  21   digit,
  22   many1,
  23   option,
  24   try,
  25   unexpected )
  26 import Text.Parsec.String ( Parser )
  27 import Text.Read ( readMaybe )
  28
  29 import Network.DNS.RBL.Pretty ( Pretty(..) )
  30
  31
  32 -- | The 'Weight' wrapper around an 'Int. We use
  33 --   GeneralizedNewtypeDeriving to derive num automatically (so that we
  34 --   can sum these things).
  35 --
  36 --   ==== _Examples_
  37 --
  38 --   >>> let w1 = Weight 1
  39 --   >>> w1
  40 --   Weight 1
  41 --   >>> let w2 = Weight 1
  42 --   >>> w1 == w2
  43 --   True
  44 --   >>> let w3 = Weight 2
  45 --   >>> w1 == w3
  46 --   False
  47 --   >>> sum [w1, w2, w3]
  48 --   Weight 4
  49 --
  50 newtype Weight = Weight Int deriving (Eq, Num, Show)
  51
  52
  53 -- | Pretty-print a 'Weight'. This just shows/prints the underlying 'Int'.
  54 --
  55 --   ==== _Examples_
  56 --
  57 --   >>> pretty_print $ Weight 17
  58 --   17
  59 --
  60 instance Pretty Weight where
  61   pretty_show (Weight w) = show w
  62
  63
  64 -- | Parse the weight multiplier off the end of an input 'Site'. This
  65 --   expects there to be a \"multiplier\" character (an asterisk)
  66 --   before the integral weight.
  67 --
  68 --   ==== _Examples_
  69 --
  70 --   >>> import Text.Parsec ( parseTest )
  71 --
  72 --   Negative, zero, and positive integers are all supported:
  73 --
  74 --   >>> parseTest weight "*-5"
  75 --   Weight (-5)
  76 --
  77 --   >>> parseTest weight "*0"
  78 --   Weight 0
  79 --
  80 --   >>> parseTest weight "*17"
  81 --   Weight 17
  82 --
  83 --   If the weight is empty, it defaults to @1@:
  84 --
  85 --   >>> parseTest weight ""
  86 --   Weight 1
  87 --
  88 --   The default is used whenever parsing fails:
  89 --
  90 --   >>> parseTest weight "*hello"
  91 --   Weight 1
  92 --
  93 --   The 'Pretty' instance works as intended:
  94 --
  95 --   >>> import Text.Parsec ( parse )
  96 --   >>> pretty_print $ parse weight "" "*3"
  97 --   3
  98 --
  99 weight :: Parser Weight
 100 weight = try parse_weight <|> return (Weight 1)
 101   where
 102     parse_weight = do
 103       _ <- char '*'
 104       sign <- (char '-') <|> (option '+' (char '+'))
 105       w <- many1 digit
 106       case ( readMaybe w :: Maybe Int ) of
 107         -- If "many1 digit" gives us a list of digits, we should be able
 108         -- to convert that to an Int! It will overflow rather than fail
 109         -- if the input is too big/small, so it should really always
 110         -- succeed.
 111         Nothing -> unexpected "weight: readMaybe failed on a sequence of digits!"
 112         Just k  -> return $ Weight (if sign == '-' then negate k else k)