Xml.hs

   1 {-# LANGUAGE TypeFamilies #-}
   2
   3 -- | General XML stuff.
   4 --
   5 module Xml (
   6   ToFromXml(..),
   7   parse_opts,
   8   pickle_unpickle )
   9 where
  10
  11 import Database.Groundhog ( AutoKey )
  12 import Text.XML.HXT.Core (
  13   (>>>),
  14   (/>),
  15   SysConfigList,
  16   XmlPickler(..),
  17   hasName,
  18   no,
  19   readDocument,
  20   runX,
  21   withPreserveComment,
  22   withRemoveWS,
  23   withSubstDTDEntities,
  24   withValidate,
  25   xpickleVal,
  26   xunpickleVal,
  27   yes )
  28
  29
  30 -- | A typeclass for types which can be converted into an associated
  31 --   XML type. The story behind this is long, but basically, we need
  32 --   to different types for each XML thingie we're going to import: a
  33 --   database type and an XML type. Both Groundhog and HXT are very
  34 --   particular about the types that they can use, and there's no way
  35 --   to reuse e.g. a type that HXT can pickle in Groundhog. So this
  36 --   typeclass gives us a way to get the XML type from the Groundhog
  37 --   type.
  38 --
  39 --   At first there appears to be an equally-valid approach, getting the
  40 --   Groundhog type from the XML one. But Groundhog won't use type family
  41 --   instances, so here we are.
  42 --
  43 class ToFromXml a where
  44   -- | Each instance a must declare its associated XML type (Xml a)
  45   type Xml a :: *
  46   type Container a :: *
  47
  48   -- | And provide a function for getting an (Xml a) out of an "a."
  49   to_xml :: a -> Xml a
  50
  51   -- | And provide a function for getting an "a" out of an (Xml a).
  52   from_xml :: Xml a -> a
  53
  54   -- | Often we need to provide a key to use as a foreign key into
  55   --   some container. If the instance "belongs" to some other object,
  56   --   then it might need to be passed a key before it can un-XML
  57   --   itself. For example, the XML version of 'NewsTeam' doesn't
  58   --   contain a message ID which is part of its database type.
  59   from_xml_fk :: AutoKey (Container a) -> Xml a -> a
  60   from_xml_fk _ = from_xml
  61
  62 -- | A list of options passed to 'readDocument' when we parse an XML
  63 --   document. We don't validate because the DTDs from TSN are
  64 --   wrong. As a result, we don't want to keep useless DTDs
  65 --   areound. Thus we disable 'withSubstDTDEntities' which, when
  66 --   combined with "withValidate no", prevents HXT from trying to read
  67 --   the DTD at all.
  68 --
  69 parse_opts :: SysConfigList
  70 parse_opts =
  71   [ withPreserveComment no,
  72     withRemoveWS yes,
  73     withSubstDTDEntities no,
  74     withValidate no ]
  75
  76
  77 -- | Given a root element name and a file path, return both the
  78 --   original unpickled root "object" and the one that was constructed
  79 --   by pickled and unpickling the original. This is used in a number
  80 --   of XML tests which pickle/unpickle and then make sure that the
  81 --   output is the same as the input.
  82 --
  83 --   We return the object instead of an XmlTree (which would save us
  84 --   an unpickle call) because otherwise the type of @a@ in the call
  85 --   to 'xpickle' would be ambiguous. By returning some @a@s, we allow
  86 --   the caller to annotate its type.
  87 --
  88 pickle_unpickle :: XmlPickler a
  89                 => String
  90                 -> FilePath
  91                 -> IO ([a], [a])
  92 pickle_unpickle root_element filepath = do
  93   -- We need to check only the root message element since
  94   -- readDocument produces a bunch of other junk.
  95     expected <- runX $ arr_getobj
  96     actual <- runX $ arr_getobj
  97                      >>>
  98                      xpickleVal xpickle
  99                      >>>
 100                      xunpickleVal xpickle
 101
 102     return (expected, actual)
 103   where
 104     arr_getobj = readDocument parse_opts filepath
 105                    />
 106                    hasName root_element
 107                    >>>
 108                    xunpickleVal xpickle