src/Xml.hs

   1 {-# LANGUAGE TypeFamilies #-}
   2
   3 -- | General XML stuff.
   4 --
   5 module Xml (
   6   DtdName(..),
   7   ToFromXml(..),
   8   parse_opts,
   9   pickle_unpickle,
  10   unpickleable )
  11 where
  12
  13 import Control.Exception ( SomeException(..), catch )
  14 import Database.Groundhog ( AutoKey )
  15 import Text.XML.HXT.Core (
  16   (>>>),
  17   (/>),
  18   PU,
  19   SysConfigList,
  20   XmlPickler(..),
  21   hasName,
  22   readDocument,
  23   runX,
  24   withRemoveWS,
  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
  63 -- | Represents the DTD filename ("SYSTEM") part of the DOCTYPE
  64 --   definition.
  65 newtype DtdName = DtdName String
  66
  67 -- | A list of options passed to 'readDocument' when we parse an XML
  68 --   document. All cosmetic whitespace should be removed, otherwise we
  69 --   have to parse it in each pickler.
  70 --
  71 parse_opts :: SysConfigList
  72 parse_opts = [ withRemoveWS yes ]
  73
  74
  75 -- | Given a root element name and a file path, return both the
  76 --   original unpickled root "object" and the one that was constructed
  77 --   by pickled and unpickling the original. This is used in a number
  78 --   of XML tests which pickle/unpickle and then make sure that the
  79 --   output is the same as the input.
  80 --
  81 --   We return the object instead of an XmlTree (which would save us
  82 --   an unpickle call) because otherwise the type of @a@ in the call
  83 --   to 'xpickle' would be ambiguous. By returning some @a@s, we allow
  84 --   the caller to annotate its type.
  85 --
  86 --   Note that this will happily pickle nothing to nothing and then
  87 --   unpickle it back to more nothing. So the fact that the
  88 --   before/after results from this function agree does not mean that
  89 --   the document was successfully unpickled!
  90 --
  91 pickle_unpickle :: XmlPickler a
  92                 => String
  93                 -> FilePath
  94                 -> IO ([a], [a])
  95 pickle_unpickle root_element filepath = do
  96   -- We need to check only the root message element since
  97   -- readDocument produces a bunch of other junk.
  98     expected <- runX arr_getobj
  99     actual <- runX $ arr_getobj
 100                      >>>
 101                      xpickleVal xpickle
 102                      >>>
 103                      xunpickleVal xpickle
 104
 105     return (expected, actual)
 106   where
 107     arr_getobj = readDocument parse_opts filepath
 108                    />
 109                    hasName root_element
 110                    >>>
 111                    xunpickleVal xpickle
 112
 113
 114
 115 -- | Is the given XML file unpickleable? Unpickling will be attempted
 116 --   using the @unpickler@ argument. If we unilaterally used the
 117 --   generic 'xpickle' function for our unpickler, a type ambiguity
 118 --   would result. By taking the unpickler as an argument, we allow
 119 --   the caller to indirectly specify a concrete type.
 120 --
 121 unpickleable :: XmlPickler a => FilePath -> PU a -> IO Bool
 122 unpickleable filepath unpickler = do
 123   xmldoc <- try_unpickle `catch` (\(SomeException _) -> return [])
 124   return $ (not . null) xmldoc
 125   where
 126     try_unpickle = runX $ readDocument parse_opts filepath
 127                           >>>
 128                           xunpickleVal unpickler