X-Git-Url: http://gitweb.michael.orlitzky.com/?p=dead%2Fharbl.git;a=blobdiff_plain;f=src%2FNetwork%2FDNS%2FRBL%2FDomain.hs;fp=src%2FNetwork%2FDNS%2FRBL%2FDomain.hs;h=0000000000000000000000000000000000000000;hp=73a69884f826945c1aacca3b118c6c7a389e6693;hb=b55e5db2a68be5d69b970bbe4b5ad447881abd3d;hpb=c4d41b93ec02ff4dc762163441ebefb0324e6f07

diff --git a/src/Network/DNS/RBL/Domain.hs b/src/Network/DNS/RBL/Domain.hs
deleted file mode 100644
index 73a6988..0000000
--- a/src/Network/DNS/RBL/Domain.hs
+++ /dev/null
@@ -1,794 +0,0 @@
-{-# LANGUAGE DoAndIfThenElse #-}
-
--- | The 'Domain' data type and its parser. A 'Domain' represents a
---   name in the domain name system (DNS) as described by
---   RFC1035. In particular, we enforce the restrictions from Section
---   2.3.1 \"Preferred name syntax\". See for example,
---
---     <https://tools.ietf.org/html/rfc1035#section-2.3.1>
---
---   We basically work with strings and characters everywhere, even
---   though this isn't really correct. The length specifications in
---   the RFCs are all in terms of octets, so really a ByteString.Char8
---   would be more appropriate. With strings, for example, we could
---   have a unicode mumbo jumbo character that takes up two bytes
---   (octets).
---
-module Network.DNS.RBL.Domain (
-  UserDomain(..),
-  user_domain )
-where
-
-import Data.Char ( toLower )
-import Text.Parsec (
-  (<|>),
-  char,
-  optionMaybe,
-  string,
-  try )
-import qualified Text.Parsec as Parsec ( digit, letter)
-import Text.Parsec.String ( Parser )
-
-import Network.DNS.RBL.Pretty ( Pretty(..) )
-
--- * Digits
-
--- | A wrapper around a digit character.
---
-newtype Digit = Digit Char deriving (Eq, Show)
-instance Pretty Digit where pretty_show (Digit d) = [d]
-
--- | Parse a single digit, but wrap it in our 'Digit' type.
---
-digit :: Parser Digit
-digit = fmap Digit Parsec.digit
-
-
--- * Letters
-
--- | A wrapper around a letter character.
---
-newtype Letter = Letter Char deriving (Show)
-instance Pretty Letter where pretty_show (Letter l) = [l]
-
-
--- | Parse a single letter, but wrap it in our 'Letter' type.
---
-letter :: Parser Letter
-letter = fmap Letter Parsec.letter
-
--- | The derived instance of 'Eq' for letters is incorrect. All
---   comparisons should be made case-insensitively. The following
---   is an excerpt from RFC1035:
---
---     2.3.3. Character Case
---
---     For all parts of the DNS that are part of the official
---     protocol, all comparisons between character strings (e.g.,
---     labels, domain names, etc.)  are done in a case-insensitive
---     manner...
---
---   Since each part of DNS name is composed of our custom types, it
---   suffices to munge the equality for 'Letter'. RFC4343
---   <https://tools.ietf.org/html/rfc4343> clarifies the
---   case-insensitivity rules, but the fact that we're treating DNS
---   names as strings makes most of those problems go away (in
---   exchange for new ones).
---
-instance Eq Letter where
-  (Letter l1) == (Letter l2) = (toLower l1) == (toLower l2)
-
--- * Letters/Digits
-
--- | A sum type representing either a letter or a digit.
---
-data LetDig =
-  LetDigLetter Letter |
-  LetDigDigit  Digit
-  deriving (Eq, Show)
-
-instance Pretty LetDig where
-  pretty_show (LetDigLetter l) = pretty_show l
-  pretty_show (LetDigDigit d) = pretty_show d
-
--- | Parse a letter or a digit and wrap it in our 'LetDig' type.
---
-let_dig :: Parser LetDig
-let_dig = (fmap LetDigLetter letter) <|> (fmap LetDigDigit digit)
-
-
--- * Hyphens
-
--- | A wrapper around a single hyphen character.
---
-newtype Hyphen = Hyphen Char deriving (Eq, Show)
-instance Pretty Hyphen where pretty_show (Hyphen h) = [h]
-
--- | Parse a single hyphen and wrap it in our 'Hyphen' type.
---
-hyphen :: Parser Hyphen
-hyphen = fmap Hyphen (char '-')
-
-
--- * Letter, Digit, or Hyphen.
-
--- | A sum type representing a letter, digit, or hyphen.
---
-data LetDigHyp =
-  LetDigHypLetDig LetDig |
-  LetDigHypHyphen Hyphen
-  deriving (Eq, Show)
-
-instance Pretty LetDigHyp where
-  pretty_show (LetDigHypLetDig ld) = pretty_show ld
-  pretty_show (LetDigHypHyphen h) = pretty_show h
-
-
--- | The following is the simplest type in the domain grammar that
---   isn't already implemented for us.
---
---     <let-dig> ::= <letter> | <digit>
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parseTest )
---
---   Letters, digits, and hyphens are all parsed:
---
---   >>> parseTest let_dig_hyp "a"
---   LetDigHypLetDig (LetDigLetter (Letter 'a'))
---
---   >>> parseTest let_dig_hyp "7"
---   LetDigHypLetDig (LetDigDigit (Digit '7'))
---
---   >>> parseTest let_dig_hyp "-"
---   LetDigHypHyphen (Hyphen '-')
---
---   However, an underscore (for example) is not:
---
---   >>> parseTest let_dig_hyp "_"
---   parse error at (line 1, column 1):
---   unexpected "_"
---   expecting letter, digit or "-"
---
-let_dig_hyp :: Parser LetDigHyp
-let_dig_hyp =
-  parse_letdig <|> parse_hyphen
-  where
-    parse_letdig :: Parser LetDigHyp
-    parse_letdig = fmap LetDigHypLetDig let_dig
-
-    parse_hyphen :: Parser LetDigHyp
-    parse_hyphen = fmap LetDigHypHyphen hyphen
-
-
--- * Letter/Digit/Hyphen strings
-
--- | A string of letters, digits, and hyphens from the RFC1035 grammar:
---
---     <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
---
---   These are represented as either a single instance of a
---   'LetDigHyp', or a string of them (recursive).
---
-data LdhStr =
-  LdhStrSingleLdh LetDigHyp |
-  LdhStrMultipleLdh LetDigHyp LdhStr
-  deriving (Eq, Show)
-
-instance Pretty LdhStr where
-  pretty_show (LdhStrSingleLdh ldh) = pretty_show ldh
-  pretty_show (LdhStrMultipleLdh ldh s) = (pretty_show ldh) ++ (pretty_show s)
-
--- | Parse a string of letters, digits, and hyphens (an 'LdhStr').
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parseTest )
---
---   Single letters, digits, and hyphens are parsed:
---
---   >>> parseTest ldh_str "a"
---   LdhStrSingleLdh (LetDigHypLetDig (LetDigLetter (Letter 'a')))
---
---   >>> parseTest ldh_str "0"
---   LdhStrSingleLdh (LetDigHypLetDig (LetDigDigit (Digit '0')))
---
---   >>> parseTest ldh_str "-"
---   LdhStrSingleLdh (LetDigHypHyphen (Hyphen '-'))
---
---   As well as strings of them:
---
---   >>> import Text.Parsec ( parse )
---   >>> pretty_print $ parse ldh_str "" "a0-b"
---   a0-b
---
-ldh_str :: Parser LdhStr
-ldh_str = try both <|> just_one
-  where
-    both :: Parser LdhStr
-    both = do
-      ldh1 <- let_dig_hyp
-      ldh_tail <- ldh_str
-      return $ LdhStrMultipleLdh ldh1 ldh_tail
-
-    just_one :: Parser LdhStr
-    just_one = fmap LdhStrSingleLdh let_dig_hyp
-
-
-
--- | A version of 'last' that works on a 'LdhStr' rather than a
---   list. That is, it returns the last 'LetDigHyp' in the
---   string. Since 'LdhStr' contains at least one character, there's
---   no \"nil\" case here.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse )
---
---   >>> let (Right r) = parse ldh_str "" "a"
---   >>> last_ldh_str r
---   LetDigHypLetDig (LetDigLetter (Letter 'a'))
---
---   >>> let (Right r) = parse ldh_str "" "abc-def"
---   >>> last_ldh_str r
---   LetDigHypLetDig (LetDigLetter (Letter 'f'))
---
-last_ldh_str :: LdhStr -> LetDigHyp
-last_ldh_str (LdhStrSingleLdh x) = x
-last_ldh_str (LdhStrMultipleLdh _ x) = last_ldh_str x
-
-
--- | A version of 'init' that works on a 'LdhStr' rather than a
---   list. That is, it returns everything /except/ the last character in
---   the string.
---
---   Since an 'LdhStr' must contain at least one character, this might
---   not be opssible (when the input is of length one). So, we return
---   a 'Maybe' value.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse )
---
---   >>> let (Right r) = parse ldh_str "" "a"
---   >>> init_ldh_str r
---   Nothing
---
---   >>> let (Right r) = parse ldh_str "" "ab"
---   >>> init_ldh_str r
---   Just (LdhStrSingleLdh (LetDigHypLetDig (LetDigLetter (Letter 'a'))))
---
---   >>> let (Right r) = parse ldh_str "" "abc-def"
---   >>> init_ldh_str r
---   Just (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'a'))) (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'b'))) (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'c'))) (LdhStrMultipleLdh (LetDigHypHyphen (Hyphen '-')) (LdhStrMultipleLdh (LetDigHypLetDig (LetDigLetter (Letter 'd'))) (LdhStrSingleLdh (LetDigHypLetDig (LetDigLetter (Letter 'e')))))))))
---
-init_ldh_str :: LdhStr -> Maybe LdhStr
-init_ldh_str (LdhStrSingleLdh _) = Nothing
-init_ldh_str (LdhStrMultipleLdh h t) =
-  Just $ case (init_ldh_str t) of
-           -- We just got the second-to-last character, we're done.
-           Nothing   -> LdhStrSingleLdh h
-
-           -- There's still more stuff. Recurse.
-           Just rest -> LdhStrMultipleLdh h rest
-
-
--- | Compute the length of an 'LdhStr'. It will be at least one, since
---   'LdhStr's are non-empty. And if there's something other than the
---   first character present, we simply recurse.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse )
---
---   >>> let (Right r) = parse ldh_str "" "a"
---   >>> length_ldh_str r
---   1
---
---   >>> let (Right r) = parse ldh_str "" "abc-def"
---   >>> length_ldh_str r
---   7
---
-length_ldh_str :: LdhStr -> Int
-length_ldh_str (LdhStrSingleLdh _) = 1
-length_ldh_str (LdhStrMultipleLdh _ t) = 1 + (length_ldh_str t)
-
--- * Letter/Digit/Hyphen string followed by a trailing Letter/Digit
-
--- | This type isn't explicitly part of the grammar, but it's what
---   shows up in the square brackets of,
---
---     <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
---
---   The ldh-str is optional, but if one is present, we must also have
---   a trailing let-dig to prevent the name from ending with a
---   hyphen. This can be represented with a @Maybe LdhStrLetDig@,
---   which is why we're about to define it.
---
-data LdhStrLetDig = LdhStrLetDig (Maybe LdhStr) LetDig
-  deriving (Eq, Show)
-
-instance Pretty LdhStrLetDig where
-  pretty_show (LdhStrLetDig Nothing ld) = pretty_show ld
-  pretty_show (LdhStrLetDig (Just s) ld) = (pretty_show s) ++ (pretty_show ld)
-
--- | Parse an 'LdhStrLetDig'. This isn't in the grammar, but we might
---   as well define the parser for it independently since we gave it
---   its own data type.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse, parseTest )
---
---   Make sure we can parse a single character:
---
---   >>> parseTest ldh_str_let_dig "a"
---   LdhStrLetDig Nothing (LetDigLetter (Letter 'a'))
---
---   And longer strings:
---
---   >>> pretty_print $ parse ldh_str_let_dig "" "ab"
---   ab
---
---   >>> pretty_print $ parse ldh_str_let_dig "" "-b"
---   -b
---
---   >>> parseTest ldh_str_let_dig "b-"
---   parse error at (line 1, column 3):
---   label cannot end with a hyphen
---
-ldh_str_let_dig :: Parser LdhStrLetDig
-ldh_str_let_dig = do
-  -- This will happily eat up the trailing let-dig...
-  full_ldh <- ldh_str
-
-  -- So we have to go back and see what happened.
-  case (last_ldh_str full_ldh) of
-    (LetDigHypHyphen _) -> fail "label cannot end with a hyphen"
-    (LetDigHypLetDig ld) ->
-      -- Ok, the label didn't end with a hyphen; now we need to split
-      -- off the last letter/digit so we can pack it into our return
-      -- type separately.
-      return $ case (init_ldh_str full_ldh) of
-                 -- We only parsed one letter/digit. This can happen
-                 -- if the label contains two characters. For example,
-                 -- if we try to parse the label "ab", then the "a"
-                 -- will be eaten by the label parser, and this
-                 -- function will be left with only "b".
-                 Nothing -> LdhStrLetDig Nothing ld
-
-                 -- Usual case: there's was some leading let-dig-hyp junk,
-                 -- return it too.
-                 leading_ldhs -> LdhStrLetDig leading_ldhs ld
-
-
-
--- | Compute the length of a 'LdhStrLetDig'. It's at least one, since
---   the let-dig at the end is always there. And when there's an
---   ldh-str too, we add its length to one.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse )
---
---   >>> let (Right r) = parse ldh_str_let_dig "" "a"
---   >>> length_ldh_str_let_dig r
---   1
---
---   >>> let (Right r) = parse ldh_str_let_dig "" "abc-def"
---   >>> length_ldh_str_let_dig r
---   7
---
-length_ldh_str_let_dig :: LdhStrLetDig -> Int
-length_ldh_str_let_dig (LdhStrLetDig Nothing _) = 1
-length_ldh_str_let_dig (LdhStrLetDig (Just ldhstring) _) =
-  1 + (length_ldh_str ldhstring)
-
-
--- * Labels
-
--- | The label type from the RFC1035 grammar:
---
---     <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
---
---   We allow the slightly more general syntax from RFC1123, Section 2.1:
---
---     The syntax of a legal Internet host name was specified in RFC-952
---     [DNS:4].  One aspect of host name syntax is hereby changed: the
---     restriction on the first character is relaxed to allow either a
---     letter or a digit.  Host software MUST support this more liberal
---     syntax.
---
-data Label = Label LetDig (Maybe LdhStrLetDig)
-  deriving (Eq, Show)
-
-instance Pretty Label where
-  pretty_show (Label l Nothing) = pretty_show l
-  pretty_show (Label l (Just s)) = (pretty_show l) ++ (pretty_show s)
-
--- | Parse a 'Label'.
---
---   In addition to the grammar, there's another restriction on
---   labels: their length must be 63 characters or less. Quoting
---   Section 2.3.1, \"Preferred name syntax\", of RFC1035:
---
---     The labels must follow the rules for ARPANET host names.  They
---     must start with a letter, end with a letter or digit, and have
---     as interior characters only letters, digits, and hyphen.  There
---     are also some restrictions on the length.  Labels must be 63
---     characters or less.
---
---   We check this only after we have successfully parsed a label.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse, parseTest )
---
---   Make sure we can parse a single character:
---
---   >>> parseTest label "a"
---   Label (LetDigLetter (Letter 'a')) Nothing
---
---   And longer strings:
---
---   >>> pretty_print $ parse label "" "abc-def"
---   abc-def
---
---   But not anything ending in a hyphen:
---
---   >>> parseTest label "abc-"
---   parse error at (line 1, column 5):
---   label cannot end with a hyphen
---
---   Or anything over 63 characters:
---
---   >>> parseTest label (['a'..'z'] ++ ['a'..'z'] ++ ['a'..'z'])
---   parse error at (line 1, column 79):
---   labels must be 63 or fewer characters
---
---   However, /exactly/ 63 characters is acceptable:
---
---   >>> pretty_print $ parse label "" (replicate 63 'x')
---   xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
---   Ensure that a label can begin with a digit:
---
---   >>> pretty_print $ parse label "" "3com"
---   3com
---
-label :: Parser Label
-label = do
-  l <- let_dig -- Guaranteed to be there
-  maybe_s <- optionMaybe ldh_str_let_dig -- Might not be there
-  case maybe_s of
-    -- It can only be one character long, from the letter...
-    Nothing -> return $ Label l maybe_s
-
-    -- The letter gives us one character, so we check that the rest is
-    -- less than 62 characters long. But in the error message we need
-    -- to report 63.
-    Just s  -> if (length_ldh_str_let_dig s) <= 62
-              then return $ Label l maybe_s
-              else fail "labels must be 63 or fewer characters"
-
-
-
--- * Subdomains
-
-
--- | The data type representing a \"subdomain\" from RFC1035:
---
---     <subdomain> ::= <label> | <subdomain> "." <label>
---
---   We have reversed the order of the subdomain and label in the
---   second option, however. This is explained in 'subdomain'.
---
-data Subdomain =
-  SubdomainSingleLabel Label |
-  SubdomainMultipleLabel Label Subdomain
-  deriving (Eq, Show)
-
-
-
-instance Pretty Subdomain where
-  pretty_show (SubdomainSingleLabel l) = pretty_show l
-  pretty_show (SubdomainMultipleLabel l s) =
-    (pretty_show l) ++ "." ++ (pretty_show s)
-
--- | Parse an RFC1035 \"subdomain\". The given grammar is,
---
---     <subdomain> ::= <label> | <subdomain> "." <label>
---
---   However, we have reversed the order of the subdomain and label to
---   prevent infinite recursion. The second option (subdomain + label)
---   is obviously more specific, we we need to try it first. This
---   presents a problem: we're trying to parse a subdomain in terms of
---   a subdomain! The given grammar represents subdomains how we like
---   to think of them; from right to left. But it's better to parse
---   from left to right, so we pick off the leading label and then
---   recurse into the definition of subdomain.
---
---   According to RFC1034, Section 3.1, two neighboring labels in a
---   DNS name cannot be equal:
---
---     Each node has a label, which is zero to 63 octets in length.  Brother
---     nodes may not have the same label, although the same label can be used
---     for nodes which are not brothers.  One label is reserved, and that is
---     the null (i.e., zero length) label used for the root.
---
---   We enforce this restriction, but the result is usually that we
---   only parse the part of the subdomain leading up to the repeated
---   label.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse, parseTest )
---
---   Make sure we can parse a single character:
---
---   >>> parseTest subdomain "a"
---   SubdomainSingleLabel (Label (LetDigLetter (Letter 'a')) Nothing)
---
---   >>> pretty_print $ parse subdomain "" "example.com"
---   example.com
---
---   >>> pretty_print $ parse subdomain "" "www.example.com"
---   www.example.com
---
---   We reject a subdomain with equal neighbors, but this leads to
---   only the single first label being parsed instead:
---
---   >>> pretty_print $ parse subdomain "" "www.www.example.com"
---   www
---
---   But not one with a repeated but non-neighboring label:
---
---   >>> pretty_print $ parse subdomain "" "www.example.www.com"
---   www.example.www.com
---
-subdomain :: Parser Subdomain
-subdomain = try both <|> just_one
-  where
-    both :: Parser Subdomain
-    both = do
-      l <- label
-      _ <- char '.'
-      s <- subdomain
-      let result = SubdomainMultipleLabel l s
-      if (subdomain_has_equal_neighbors result)
-      then fail "subdomain cannot have equal neighboring labels"
-      else return result
-
-    just_one :: Parser Subdomain
-    just_one = fmap SubdomainSingleLabel label
-
-
-
--- | Retrieve a list of labels contained in a 'Subdomain'.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse )
---
---   >>> let (Right r) = parse subdomain "" "a"
---   >>> pretty_print $ subdomain_labels r
---   ["a"]
---
---   >>> let (Right r) = parse subdomain "" "example.com"
---   >>> pretty_print $ subdomain_labels r
---   ["example","com"]
---
---   >>> let (Right r) = parse subdomain "" "www.example.com"
---   >>> pretty_print $ subdomain_labels r
---   ["www","example","com"]
---
-subdomain_labels :: Subdomain -> [Label]
-subdomain_labels (SubdomainSingleLabel l) = [l]
-subdomain_labels (SubdomainMultipleLabel l s) = l : (subdomain_labels s)
-
-
--- | Return a list of pairs of neighboring labels in a subdomain.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse )
---   >>> let (Right r) = parse subdomain "" "www.example.com"
---   >>> pretty_print $ subdomain_label_neighbors r
---   ["(\"www\",\"example\")","(\"example\",\"com\")"]
---
-subdomain_label_neighbors :: Subdomain -> [(Label,Label)]
-subdomain_label_neighbors s =
-  zip ls (tail ls)
-  where
-    ls = subdomain_labels s
-
-
--- | Return @True@ if the subdomain has any two equal neighboring
---   labels, and @False@ otherwise.
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse )
---
---   >>> let (Right r) = parse subdomain "" "www.example.com"
---   >>> subdomain_has_equal_neighbors r
---   False
---
---   >>> let (Right l) = parse label "" "www"
---   >>> let (Right s) = parse subdomain "" "www.example.com"
---   >>> let bad_subdomain = SubdomainMultipleLabel l s
---   >>> subdomain_has_equal_neighbors bad_subdomain
---   True
---
-subdomain_has_equal_neighbors :: Subdomain -> Bool
-subdomain_has_equal_neighbors s =
-  or [ x == y | (x,y) <- subdomain_label_neighbors s ]
-
-
-
--- * Domains
-
--- | An RFC1035 domain. According to RFC1035 a domain can be either a
--- subdomain or \" \", which according to RFC2181
--- <https://tools.ietf.org/html/rfc2181#section-11> means the root:
---
---      The zero length full name is defined as representing the root
---      of the DNS tree, and is typically written and displayed as
---      \".\".
---
---   We let the 'Domain' type remain true to those RFCs, even though
---   they don't support an absolute domain name of e.g. a single dot.
---   We have one more data type 'UserDomain' which handles the possibility
---   of an absolute path.
---
-data Domain =
-  DomainName Subdomain |
-  DomainRoot
-  deriving (Eq, Show)
-
-instance Pretty Domain where
-  pretty_show DomainRoot = ""
-  pretty_show (DomainName s) = pretty_show s
-
--- | Parse an RFC1035 \"domain\"
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse, parseTest )
---
---   Make sure we can parse a single character:
---
---   >>> pretty_print $ parse domain "" "a"
---   a
---
---   And the empty domain:
---
---   >>> parseTest domain ""
---   DomainRoot
---
---   We will in fact parse the \"empty\" domain off the front of
---   pretty much anything:
---
---   >>> parseTest domain "!8===D"
---   DomainRoot
---
---   Equality of domains is case-insensitive:
---
---   >>> let (Right r1) = parse domain "" "example.com"
---   >>> let (Right r2) = parse domain "" "ExaMPle.coM"
---   >>> r1 == r2
---   True
---
---   A single dot IS parsed as the root, but the dot isn't consumed:
---
---   >>> parseTest domain "."
---   DomainRoot
---
---   Anything over 255 characters is an error, so the root will be
---   parsed:
---
---   >>> let big_l1 = replicate 63 'x'
---   >>> let big_l2 = replicate 63 'y' -- Avoid equal neighboring labels!
---   >>> let big_labels = big_l1 ++ "." ++ big_l2 ++ "."
---   >>> let big_subdomain = concat $ replicate 3 big_labels
---   >>> parseTest domain big_subdomain
---   DomainRoot
---
---   But exactly 255 is allowed:
---
---   >>> import Data.List ( intercalate )
---   >>> let l1 = replicate 63 'w'
---   >>> let l2 = replicate 63 'x'
---   >>> let l3 = replicate 63 'y'
---   >>> let l4 = replicate 63 'z'
---   >>> let big_subdomain = intercalate "." [l1,l2,l3,l4]
---   >>> let (Right r) = parse domain "" big_subdomain
---   >>> length (pretty_show r)
---   255
---
-domain :: Parser Domain
-domain = try parse_subdomain <|> parse_empty
-  where
-    parse_subdomain :: Parser Domain
-    parse_subdomain = do
-      s <- subdomain
-      if length (pretty_show s) <= 255
-      then return $ DomainName s
-      else fail "subdomains can be at most 255 characters"
-
-    parse_empty :: Parser Domain
-    parse_empty = string "" >> return DomainRoot
-
-
-
--- * User domains
-
--- | This type helps clarify some murkiness in the DNS \"domain\" name
---   specification. In RFC1034, it is acknowledged that a domain name
---   input with a trailing \".\" will represent an absolute domain
---   name (i.e. with respect to the DNS root). However, the grammar in
---   RFC1035 disallows a trailing dot.
---
---   This makes some sense: within the DNS, everything knows its
---   position in the tree. The relative/absolute distinction only
---   makes sense on the client side, where a user's resolver might
---   decide to append some suffix to a relative
---   request. Unfortunately, that's where we live. So we have to deal
---   with the possibility of having a trailing dot at the end of any
---   domain name.
---
-data UserDomain =
-  UserDomainRelative Domain |
-  UserDomainAbsolute Domain
-  deriving (Eq, Show)
-
-instance Pretty UserDomain where
-  pretty_show (UserDomainRelative d) = pretty_show d
-  pretty_show (UserDomainAbsolute d) = (pretty_show d) ++ "."
-
-
--- | Parse a 'UserDomain'. This is what we'll be using to read user
---   input, since it supports both relative and absolute domain names
---   (unlike the implicitly-absolute 'Domain').
---
---   ==== _Examples_
---
---   >>> import Text.Parsec ( parse, parseTest )
---
---   We can really parse the root now!
---
---   >>> parseTest user_domain "."
---   UserDomainAbsolute DomainRoot
---
---   But multiple dots aren't (only the first):
---
---   >>> pretty_print $ parse user_domain "" ".."
---   .
---
---   We can also optionally have a trailing dot at the end of a
---   non-empty name:
---
---   >>>  pretty_print $ parse user_domain "" "www.example.com"
---   www.example.com
---
---   >>>  pretty_print $ parse user_domain "" "www.example.com."
---   www.example.com.
---
---   A \"relative root\" can also be parsed, letting the user's
---   resolver deal with it:
---
---   >>> parseTest user_domain ""
---   UserDomainRelative DomainRoot
---
-user_domain :: Parser UserDomain
-user_domain = try absolute <|> relative
-  where
-    absolute :: Parser UserDomain
-    absolute = do
-      d <- domain
-      _ <- char '.'
-      return $ UserDomainAbsolute d
-
-    relative :: Parser UserDomain
-    relative = fmap UserDomainRelative domain