src/Xml.hs

   1 {-# LANGUAGE FlexibleContexts #-}
   2 {-# LANGUAGE TypeFamilies #-}
   3
   4 -- | General XML stuff.
   5 --
   6 module Xml (
   7   Child(..),
   8   DtdName(..),
   9   FromXml(..),
  10   FromXmlFk(..),
  11   ToDb(..),
  12   parse_opts,
  13   pickle_unpickle,
  14   unpickleable,
  15   unsafe_read_document,
  16   unsafe_unpickle )
  17 where
  18
  19 -- System imports.
  20 import Control.Exception ( SomeException(..), catch )
  21 import Database.Groundhog.Core ( PersistEntity(..) )
  22 import Text.XML.HXT.Core (
  23   (>>>),
  24   (/>),
  25   PU,
  26   SysConfigList,
  27   XmlTree,
  28   isElem,
  29   no,
  30   readDocument,
  31   runX,
  32   withRemoveWS,
  33   withSubstDTDEntities,
  34   withValidate,
  35   xpickleVal,
  36   xunpickleDocument,
  37   xunpickleVal,
  38   yes )
  39
  40
  41
  42 -- | Common associated type shared by 'FromXml' and 'FromXmlFk'. This
  43 --   basically just forces the client to define the \"database
  44 --   version\" of his type.
  45 --
  46 class ToDb a where
  47   -- | Each instance @a@ must declare its associated database type @Db a@.
  48   type Db a :: *
  49
  50 -- | A typeclass for XML types that can be converted into an
  51 --   associated database type. The story behind this is long, but
  52 --   basically, we need to different types most XML thingies we're
  53 --   going to import: a database type and an XML type.
  54 --
  55 --   Both Groundhog and HXT are very particular about the types that
  56 --   they can use, and there's no way to reuse e.g. a type that HXT
  57 --   can pickle in Groundhog. This typeclass gives us a standard way
  58 --   to get the database type from the XML type that we have to define
  59 --   for HXT.
  60 --
  61 class (ToDb a) => FromXml a where
  62   -- | A function for getting a @Db a@ out of an @a@.
  63   from_xml :: a -> Db a
  64
  65
  66 -- | A class for XML representations which are children of other
  67 --   elements. The foal is to associate a child XML element with its
  68 --   parent element's database type. This is required to construct the
  69 --   database analogue of @a@ in 'FromXmlFk'.
  70 --
  71 class Child a where
  72   -- | The type of our parent object, i.e. to the type to whom our
  73   --   foreign key will point.
  74   type Parent a :: *
  75
  76
  77 -- | Some database types cannot be constructed from the XML type
  78 --   alone; they must be supplied a foreign key to a parent object
  79 --   first. Members of this class can be converted from an XML
  80 --   representation to a database representation in this manner.
  81 --
  82 class (Child a, ToDb a) => FromXmlFk a where
  83   -- | The function that produces a @Db a@ out of a foreign key and an
  84   --   @a@. The parameter order makes it easier to map this function
  85   --   over a bunch of things.
  86   from_xml_fk :: DefaultKey (Parent a) -> a -> Db a
  87
  88
  89
  90 -- | Represents the DTD filename (\"SYSTEM\") part of the DOCTYPE
  91 --   definition.
  92 newtype DtdName = DtdName String
  93
  94 -- | A list of options passed to 'readDocument' when we parse an XML
  95 --   document. All cosmetic whitespace should be removed, otherwise we
  96 --   would have to parse whitespace in each (un)pickler.
  97 --
  98 parse_opts :: SysConfigList
  99 parse_opts = [ withRemoveWS yes,
 100                withSubstDTDEntities no,
 101                withValidate no ]
 102
 103
 104 -- | Given an @unpickler@ and a @filepath@, attempt to unpickle the
 105 --   root element of @filepath@ using @unpickler@ and return both the
 106 --   original unpickled object and one constructed by pickling and
 107 --   unpickling that original. This is used in a number of XML tests
 108 --   which pickle/unpickle and then make sure that the output is the
 109 --   same as the input.
 110 --
 111 --   We return the object instead of an XmlTree (which would save us
 112 --   an unpickle call) because otherwise the type of @a@ in the call
 113 --   to 'xpickle' would be ambiguous. By returning some @a@s, we allow
 114 --   the caller to annotate its type.
 115 --
 116 --   Note that this will happily pickle nothing to nothing and then
 117 --   unpickle it back to more nothing. So the fact that the
 118 --   before/after results from this function agree does not mean that
 119 --   the document was successfully unpickled!
 120 --
 121 pickle_unpickle :: PU a -- ^ @unpickler@ returning an @a@
 122                 -> FilePath -- ^ Path to the document to unpickle.
 123                 -> IO ([a], [a])
 124 pickle_unpickle unpickler filepath = do
 125   -- We need to check only the root message element since
 126   -- readDocument produces a bunch of other junk.
 127     expected <- runX arr_getobj
 128     actual <- runX $ arr_getobj
 129                      >>>
 130                      xpickleVal unpickler
 131                      >>>
 132                      xunpickleVal unpickler
 133
 134     return (expected, actual)
 135   where
 136     arr_getobj = readDocument parse_opts filepath
 137                    />
 138                    isElem -- Drop the extra junk readDocument pulls in.
 139                    >>>
 140                    xunpickleVal unpickler
 141
 142
 143
 144 -- | Is the given XML file unpickleable? Unpickling will be attempted
 145 --   using the @unpickler@ argument. If we unilaterally used the
 146 --   generic 'xpickle' function for our unpickler, a type ambiguity
 147 --   would result. By taking the unpickler as an argument, we allow
 148 --   the caller to indirectly specify a concrete type.
 149 --
 150 --   Apologies the the name; unpickleable means \"we can unpickle
 151 --   it\", not \"not pickleable.\"
 152 --
 153 unpickleable :: FilePath -> PU a -> IO Bool
 154 unpickleable filepath unpickler = do
 155   xmldoc <- try_unpickle `catch` (\(SomeException _) -> return [])
 156   return $ (not . null) xmldoc
 157   where
 158     try_unpickle = runX $ readDocument parse_opts filepath
 159                           >>>
 160                           xunpickleVal unpickler
 161
 162
 163 -- | Unpickle from a 'FilePath' using the given pickler. Explode if it
 164 --   doesn't work.
 165 --
 166 unsafe_unpickle :: FilePath -> PU a -> IO a
 167 unsafe_unpickle filepath unpickler =
 168   fmap head $ runX $ xunpickleDocument unpickler parse_opts filepath
 169
 170
 171 -- | Read an XML document from a 'FilePath' into an XmlTree. Explode if it
 172 --   doesn't work.
 173 --
 174 unsafe_read_document :: FilePath -> IO XmlTree
 175 unsafe_read_document filepath =
 176   fmap head $ runX $ readDocument parse_opts filepath