{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}
-- | General XML stuff.
--
module Xml (
Child(..),
DtdName(..),
FromXml(..),
FromXmlFk(..),
ToDb(..),
parse_opts,
parse_opts_novalidate,
pickle_unpickle,
unpickleable,
unsafe_read_document,
unsafe_read_invalid_document,
unsafe_unpickle )
where
-- System imports.
import Control.Exception ( SomeException(..), catch )
import Database.Groundhog.Core ( PersistEntity(..) )
import Text.XML.HXT.Core (
(>>>),
(/>),
PU,
SysConfigList,
XmlTree,
isElem,
no,
readDocument,
runX,
withRemoveWS,
withSubstDTDEntities,
withValidate,
xpickleVal,
xunpickleDocument,
xunpickleVal,
yes )
-- | Common associated type shared by 'FromXml' and 'FromXmlFk'. This
-- basically just forces the client to define the \"database
-- version\" of his type.
--
class ToDb a where
-- | Each instance @a@ must declare its associated database type @Db a@.
type Db a :: *
-- | A typeclass for XML types that can be converted into an
-- associated database type. The story behind this is long, but
-- basically, we need to different types most XML thingies we're
-- going to import: a database type and an XML type.
--
-- Both Groundhog and HXT are very particular about the types that
-- they can use, and there's no way to reuse e.g. a type that HXT
-- can pickle in Groundhog. This typeclass gives us a standard way
-- to get the database type from the XML type that we have to define
-- for HXT.
--
class (ToDb a) => FromXml a where
-- | A function for getting a @Db a@ out of an @a@.
from_xml :: a -> Db a
-- | A class for XML representations which are children of other
-- elements. The foal is to associate a child XML element with its
-- parent element's database type. This is required to construct the
-- database analogue of @a@ in 'FromXmlFk'.
--
class Child a where
-- | The type of our parent object, i.e. to the type to whom our
-- foreign key will point.
type Parent a :: *
-- | Some database types cannot be constructed from the XML type
-- alone; they must be supplied a foreign key to a parent object
-- first. Members of this class can be converted from an XML
-- representation to a database representation in this manner.
--
class (Child a, ToDb a) => FromXmlFk a where
-- | The function that produces a @Db a@ out of a foreign key and an
-- @a@. The parameter order makes it easier to map this function
-- over a bunch of things.
from_xml_fk :: DefaultKey (Parent a) -> a -> Db a
-- | Represents the DTD filename (\"SYSTEM\") part of the DOCTYPE
-- definition.
newtype DtdName = DtdName String
-- | A list of options passed to 'readDocument' when we parse an XML
-- document. All cosmetic whitespace should be removed, otherwise we
-- would have to parse whitespace in each (un)pickler.
--
parse_opts :: SysConfigList
parse_opts = [ withRemoveWS yes,
withSubstDTDEntities no ]
-- | Like 'parse_opts' except we don't validate the document against
-- its DTD. This is useful when we need to parse a document that we
-- /know/ is invalid so that we can deliver a better error message.
--
parse_opts_novalidate :: SysConfigList
parse_opts_novalidate = (withValidate no) : parse_opts
-- | Given an @unpickler@ and a @filepath@, attempt to unpickle the
-- root element of @filepath@ using @unpickler@ and return both the
-- original unpickled object and one constructed by pickling and
-- unpickling that original. This is used in a number of XML tests
-- which pickle/unpickle and then make sure that the output is the
-- same as the input.
--
-- We return the object instead of an XmlTree (which would save us
-- an unpickle call) because otherwise the type of @a@ in the call
-- to 'xpickle' would be ambiguous. By returning some @a@s, we allow
-- the caller to annotate its type.
--
-- Note that this will happily pickle nothing to nothing and then
-- unpickle it back to more nothing. So the fact that the
-- before/after results from this function agree does not mean that
-- the document was successfully unpickled!
--
pickle_unpickle :: PU a -- ^ @unpickler@ returning an @a@
-> FilePath -- ^ Path to the document to unpickle.
-> IO ([a], [a])
pickle_unpickle unpickler filepath = do
-- We need to check only the root message element since
-- readDocument produces a bunch of other junk.
expected <- runX arr_getobj
actual <- runX $ arr_getobj
>>>
xpickleVal unpickler
>>>
xunpickleVal unpickler
return (expected, actual)
where
arr_getobj = readDocument parse_opts filepath
/>
isElem -- Drop the extra junk readDocument pulls in.
>>>
xunpickleVal unpickler
-- | Is the given XML file unpickleable? Unpickling will be attempted
-- using the @unpickler@ argument. If we unilaterally used the
-- generic 'xpickle' function for our unpickler, a type ambiguity
-- would result. By taking the unpickler as an argument, we allow
-- the caller to indirectly specify a concrete type.
--
-- Apologies the the name; unpickleable means \"we can unpickle
-- it\", not \"not pickleable.\"
--
unpickleable :: FilePath -> PU a -> IO Bool
unpickleable filepath unpickler = do
xmldoc <- try_unpickle `catch` (\(SomeException _) -> return [])
return $ (not . null) xmldoc
where
try_unpickle = runX $ readDocument parse_opts filepath
>>>
xunpickleVal unpickler
-- | Unpickle from a 'FilePath' using the given pickler. Explode if it
-- doesn't work.
--
unsafe_unpickle :: FilePath -> PU a -> IO a
unsafe_unpickle filepath unpickler =
fmap head $ runX $ xunpickleDocument unpickler parse_opts filepath
-- | Read an XML document from a 'FilePath' into an XmlTree. Explode if it
-- doesn't work.
--
unsafe_read_document :: FilePath -> IO XmlTree
unsafe_read_document filepath =
fmap head $ runX $ readDocument parse_opts filepath
-- | The same as 'unsafe_read_document', except it allows you to read
-- documents which don't validate against their DTDs.
--
unsafe_read_invalid_document :: FilePath -> IO XmlTree
unsafe_read_invalid_document filepath =
fmap head $ runX $ readDocument parse_opts_novalidate filepath