Data.XML.Pickle

This module provides XML picklers that plug into the XML tree of the xml-types package. This module was "inspired" by hexpat-pickle.

The API differences between hexpat-pickle and this module include:

• When unpickling, picklers will consume matching elmements so that they will be ignored by sucessive picklers. To circumvent this behaviour, use xpPeek
• Wrappers like xpWrap are uncurried
• There are no lazy unpicklers
• Most unpicklers will produce an error when their child unpicklers fail to consume all elements. Use xpClean to discard those elements

The data type PU t a represents both a pickler (converting Haskell data to XML) and an unpickler (XML to Haskell data), so your code only needs to be written once for both serialization and deserialization. The PU primitives, such as xpElem for XML elements, may be composed into complex arrangements using xpPair and other combinators.

Most picklers will try to find the first match rather than failing when the first element doesn't match. This is why the target type often is a list. To prevent this behaviour and commit the pickler to the first element available, use xpIsolate.

The top level of the document does not follow this rule, because it is a single node type. xpRoot is needed to adapt this to type [Node] for your pickler to use. You would typically define a pickler for a whole document with xpElem, then pickle it to a single Node with pickleTree (xpRoot myDocPickler) value.

NB: Unresolved entities are considered an error and will trigger an exception

When unpickling, the following invariant regarding the list of remaining elements should be observed:

• The returned list should be a subset of or the initial list itself, that is, no elements should be added or changed
• The relative order of elements should be preserved
• Elements may, however, be removed from anywhere in the list

Here is a simple example to get you started:

{-# LANGUAGE OverloadedStrings #-}
import Data.Text
import Data.XML.Types
import Data.XML.Pickle

-- Person name, age and description
data Person = Person Text Int Text

xpPerson :: PU [Node] Person
xpPerson =
    -- How to wrap and unwrap a Person
    xpWrap (\((name, age), descr) -> Person name age descr)
           (\(Person name age descr) -> ((name, age), descr)) $
    xpElem "person"
        (xpPair
            (xpAttr "name" xpId)
            (xpAttr "age" xpPrim))
        (xpContent xpId)

people = [
    Person "Dave" 27 "A fat thin man with long short hair",
    Person "Jane" 21 "Lives in a white house with green windows"]

main = do
    print $ pickle (xpRoot $ xpElemNodes "people" $ xpAll xpPerson) people

Program output would be an xml-value equivalent to:

<people>
  <person name="Dave" age="27">
    A fat thin man with long short hair
  </person>
  <person name="Jane" age="21">
    Lives in a white house with green windows
 </person>
</people>

Functions marked with compat are included for compatibility with hexpat-pickle.

Types

data PU t a

Constructors

PU

Fields

unpickleTree :: t -> UnpickleResult t a

pickleTree :: a -> t

Instances

Category * PU

type Attribute = (Name, [Content])

data UnpickleResult t a

Constructors

UnpickleError UnpickleError

NoResult Text

Not found, description of element

Result a (Maybe t)

Result and remainder. The remainder is wrapped in Maybe to avoid a Monoid constraint on t

Invariant: When t is a Monoid, the empty remainder should always be Nothing instead of Just mempty

Instances

Monad (UnpickleResult t)

Functor (UnpickleResult t)

Applicative (UnpickleResult t)

(Show t, Show a) => Show (UnpickleResult t a)

Pickler invocation

pickle :: PU t a -> a -> t

Pickle a tree.

unpickle :: PU t a -> t -> Either UnpickleError a

Unpickle a tree.

Primitive picklers

xpUnit :: PU [a] ()

Doesn't create or consume anything, always succeeds.

xpZero :: PU [t] a

The zero pickler.

Encodes nothing, always fails during unpickling. (Same as xpThrow "got xpZero").

xpThrow

Arguments

:: Monoid m

=> String

Error message

-> PU m a

No output when pickling, always generates an error with the specified message on unpickling.

xpIso :: (a -> b) -> (b -> a) -> PU a b

Isomorphic pickler.

xpPartial :: (a -> Either Text b) -> (b -> a) -> PU a b

Value-preserving picklers

xpId :: PU a a

Returns everything (remaining), untouched.

xpFst :: Monoid b => PU t (a, b) -> PU t a

xpSnd :: Monoid a => PU t (a, b) -> PU t b

xpTrees :: PU a a

xpId. (compat)

xpHead :: PU [a] a

Return one element, untouched.

xpTree :: PU [a] a

xpHead. (compat)

xpText0 :: PU Text Text

Specialised version of xpId. (compat)

xpText :: PU Text Text

Like xpText0, but fails on non-empty input.

xpString :: PU Text String

Convert Text to/from String.

xpRoot :: PU [a] b -> PU a b

Transforms a pickler on lists to a pickler on single elements.

NB: Will error when the given pickler doesn't produce exactly one element.

xpPrim :: (Show a, Read a) => PU Text a

Convert text to/from any type that implements Read and Show. Fails on unpickle if read fails.

XML specific picklers

Attributes

xpAttribute :: Name -> PU Text a -> PU [Attribute] a

Pickle to/from attribute.

xpAttribute' :: Name -> PU Text a -> PU [Attribute] (Maybe a)

Pickle attribute if Just is given, on unpickling return Just val when the attribute is found, Nothing otherwise.

xpAttribute_ :: Name -> Text -> PU [Attribute] ()

Pickle an attribute with the specified name and value, fail if the same attribute is not present on unpickle.

xpAttr :: Name -> PU Text a -> PU [Attribute] a

xpAttribute (compat)

xpAttrImplied :: Name -> PU Text a -> PU [Attribute] (Maybe a)

xpAttrFixed :: Name -> Text -> PU [Attribute] ()

xpAttribute_ (compat)

xpAddFixedAttr :: Name -> Text -> PU [Attribute] b -> PU [Attribute] b

Add an attribute with a fixed value.

Elements

xpElem

Arguments

:: Name

Name of the Element

-> PU [Attribute] a

Pickler for attributes

-> PU [Node] n

Pickler for child nodes

-> PU [Node] (a, n)

When unpickling, tries to find the first element with the supplied name. Once such an element is found, it will commit to it and fail if any of the picklers don't match.

xpElemWithName

Arguments

:: PU [Attribute] a

Pickler for attributes

-> PU [Node] n

Pickler for child nodes

-> PU [Node] (Name, a, n)

Pickle Element without a restriction on the name. The name as taken / returned as the first element of the triple.

xpElemByNamespace

Arguments

:: Text

Namespace

-> PU Text name

Pickler for the local name

-> PU [Attribute] a

Pickler for attributes

-> PU [Node] n

Pickler for child nodes

-> PU [Node] (name, a, n)

Find element by name space, prefixes are ignored.

xpElemVerbatim :: PU [Node] Element

Pickler returns the first found Element untouched.

Unpickler wraps element in NodeElement.

xpElemAttrs :: Name -> PU [Attribute] b -> PU [Node] b

A helper variant of xpElem for elements that contain attributes but no child tags.

xpElemNodes :: Name -> PU [Node] b -> PU [Node] b

A helper variant of xpElem for elements that contain child nodes but no attributes.

xpElemText :: Name -> PU [Node] Text

A helper variant of xpElem for elements that contain only character data.

xpElemBlank :: Name -> PU [Node] ()

Helper for Element values that don't contain anything.

xpElemExists :: Name -> PU [Node] Bool

When pickling, creates an empty element iff parameter is True.

When unpickling, checks whether element exists. Generates an error when the element is not empty.

xpElems

Arguments

:: Name

Name of the Element values

-> PU [Attribute] a

Pickler for attributes

-> PU [Node] n

Pickler for child nodes

-> PU [Node] [(a, n)]

Handle all elements with a given name. The unpickler will fail when any of the elements fails to unpickle.

Character Content

xpContent :: PU Text a -> PU [Node] a

Get the first non-element NodeContent from a node.

xpBool :: PU Text Bool

Converts Booleans to XML boolean values

• "true" and "1" are read as True
• "false" and "0" are read as False
• All other values generate an unpickle error

Will always generate "true" or "false" (not "0" or "1") when pickling.

Pickler combinators

Choice

xpOption :: PU [t] a -> PU [t] (Maybe a)

Lift a pickler. Nothing is returned when the given pickler doesn't return a value (e.g. the element isn't found). Does not affect unpickling errors. Nothing is pickled to mempty.

A typical example is:

xpElemAttributes "score" $ xpOption $ xpAttribute "value" xpPrim

in which Just 5 is encoded as <score value="5"/> and Nothing as <score/>.

xpDefault :: Eq a => a -> PU [t] a -> PU [t] a

Optional conversion with default value.

Unlike xpWithDefault the default value is not encoded in the XML document, during unpickling the default value is inserted if the pickler doesn't return a value.

xpWithDefault :: a -> PU t a -> PU t a

Attempt to use a pickler. Return a default value when the pickler doesn't return anything (doesn't touch on UnpickleError).

Unlike xpDefault, the default value is encoded in the XML document.

xpMap

Arguments

:: Ord k

=> Name

Element name (elt)

-> Name

Attribute name (attr)

-> PU Text k

Pickler for keys (key)

-> PU [Node] a

Pickler for values (value)

-> PU [Node] (Map k a)

Standard pickler for maps

This pickler converts a map into a list of pairs of the form

<elt attr="key">value</elt>

xpAlt

Arguments

:: (a -> Int)

Selector function

-> [PU t a]

List of picklers

-> PU t a

Execute one of a list of picklers. The selector function is used during pickling, and the integer returned is taken as a 0-based index to select a pickler from pickler options. Unpickling is done by trying each list element in order until one returns a Result (the selector is not used).

This is typically used to handle each constructor of a data type. However, it can be used wherever multiple serialization strategies apply to a single type.

NB: This function will ignore all errors as long as one of the branches returns a result. Also, it will produce an error when all branches return NoResult. Use xpChoice for a saner version of this function.

xpChoice

Arguments

:: (a -> Int)

Selector function

-> [PU t a]

List of picklers

-> PU t a

Execute one of a list of picklers. The selector function is used during pickling, and the integer returned is taken as a 0-based index to select a pickler from pickler options. Unpickling is done by trying each list element in order until one returns a Result or an Error.

This is typically used to handle each constructor of a data type. However, it can be used wherever multiple serialization strategies apply to a single type.

This function is similar to xpAlt but it will stop unpickling on the first error. It will return NoResult iff all of the picklers return NoResult (or the list of picklers is empty).

xpEither :: PU n t1 -> PU n t2 -> PU n (Either t1 t2)

Try the left pickler first and if that doesn't produce anything the right one. Wrapping the result in Left or Right, respectively.

Not to be confused with xpWrapEither.

xpTryCatch :: PU t a -> PU t a -> PU t a

Pickler that during pickling always uses the first pickler, and during unpickling tries the first, and on failure then tries the second.

Sequencing

NB: The sequencing operations do not enforce any order on the matched elements unless stated otherwise, but you can commit individial picklers to the next available element with xpIsolate. Applying xpIsolate on all nested Picklers will in effect enforce order.

Howver, once a pickler consumes an element it will not be available to following picklers. You can circumvent this behaviour with xpPeek.

If you want ensure that all elements are consumed after the last pickler is run you may want to use xpClean.

• ** Lists | The List pickler combinators will pickle lists in the given order without any special treatment and unpickle as stated.

xpFindMatches :: PU [b] a -> PU [b] [a]

When unpickling, tries to apply the pickler to all elements returning and consuming only matched elements.

xpFindFirst :: (t -> Bool) -> PU [t] a -> PU [t] a

Select a single element from the list and apply unpickler to it.

Returns no value when no element matches the predicate.

Fails when the unpickler doesn't return a value.

When pickling, this is a noop.

xpAll :: PU [a] b -> PU [a] [b]

Tries to apply the pickler to all the remaining elements; fails if any of them don't match.

xpSubsetAll

Arguments

:: (a -> Bool)

Predicate to select the subset

-> PU [a] b

Pickler to apply on the subset

-> PU [a] [b]

For unpickling, apply the given pickler to a subset of the elements determined by a given predicate.

Pickles like xpAll.

xpAllByNamespace :: Text -> PU [Node] b -> PU [Node] [b]

Apply unpickler to all elements with the given namespace.

Pickles like xpAll.

xpList0 :: PU [a] b -> PU [a] [b]

xpAll. (compat)

xpSeqWhile :: PU [a] b -> PU [a] [b]

When unpickling, successively applies pickler to single elements until it doesn't return anything; returns all matched elements.

xpList :: PU [a] b -> PU [a] [b]

xpSeqWhile. (compat)

xpListMinLen :: Int -> PU [a] b -> PU [a] [b]

Like xpList, but only succeed during unpickling if at least a minimum number of elements are unpickled.

Tuples

Tuple combinators apply their picklers from left to right. They will succeed when all their constituents produce a value.

xp2Tuple :: PU [a] b1 -> PU [a] b2 -> PU [a] (b1, b2)

Combines 2 picklers.

xpPair :: PU [a] b1 -> PU [a] b2 -> PU [a] (b1, b2)

xp2Tuple. (compat)

(<#>) :: PU [a] b1 -> PU [a] b2 -> PU [a] (b1, b2)

xp2Tuple.

xp3Tuple :: PU [a] a1 -> PU [a] a2 -> PU [a] a3 -> PU [a] (a1, a2, a3)

Combines 3 picklers.

xpTriple :: PU [a] a1 -> PU [a] a2 -> PU [a] a3 -> PU [a] (a1, a2, a3)

xp3Tuple. (compat)

xp4Tuple :: PU [a] a1 -> PU [a] a2 -> PU [a] a3 -> PU [a] a4 -> PU [a] (a1, a2, a3, a4)

Combines 4 picklers.

xp5Tuple :: PU [a] a1 -> PU [a] a2 -> PU [a] a3 -> PU [a] a4 -> PU [a] a5 -> PU [a] (a1, a2, a3, a4, a5)

Combines 5 picklers.

xp6Tuple :: PU [a] a1 -> PU [a] a2 -> PU [a] a3 -> PU [a] a4 -> PU [a] a5 -> PU [a] a6 -> PU [a] (a1, a2, a3, a4, a5, a6)

You guessed it ... Combines 6 picklers.

Wrappers

Value wrappers

xpWrap :: (a -> b) -> (b -> a) -> PU t a -> PU t b

Apply a bijection before pickling / after unpickling.

xpConst :: a -> PU t () -> PU t a

Ignore input/output and replace with constant values.

xpWrapEither :: Show e => (a -> Either e b) -> (b -> a) -> PU t a -> PU t b

Like xpWrap, except it strips Right (and treats Left as a failure) during unpickling.

Not to be confused with xpEither.

xpWrapMaybe :: (a -> Maybe b) -> (b -> a) -> PU t a -> PU t b

Like xpWrap, but strips Just (and treats Nothing as a failure) during unpickling.

xpWrapMaybe_ :: String -> (a -> Maybe b) -> (b -> a) -> PU t a -> PU t b

Like xpWrap, but strips Just (and treats Nothing as a failure) during unpickling, with specified error message for Nothing value.

xpAssert :: Text -> (a -> Bool) -> PU t a -> PU t a

Test predicate when unpickling. Fails with given error message when the predicate return false.

NB: The predicate will only be tested while unpickling. When pickling, this is a noop.

xpMayFail :: PU t a -> PU t a

Instead of failing the pickler will return no result.

xpUnliftElems :: PU [Node] a -> PU [Element] a

Unlift a pickler on Node values to a pickler on Element values. Nodes generated during pickling that are not Element values will be silently discarded.

Bookkeeping

Change the semantics of picklers

xpIsolate :: PU [t] a -> PU [t] a

Noop when pickling.

When unpickling, only give access to the first element.

xpPeek :: PU t a -> PU t a

When unpickling, don't consume the matched element(s), noop when pickling.

Cleannes

Picklers keep track of elements left over after unpickling, so the may be [clean] an unpickling is considered clean when it doesn't leave any remainng elements.

xpClean :: PU t a -> PU t a

Run unpickler and consume and discard remaining elements.

When pickling, this is a noop.

Error handling

data UnpickleError

Constructors

ErrorMessage Text

TraceStep (Text, Text) UnpickleError

Variants [UnpickleError]

Instances

Show UnpickleError

Exception UnpickleError

Typeable * UnpickleError

ppUnpickleError :: UnpickleError -> String

(<++>) :: (Text, Text) -> UnpickleError -> UnpickleError infixl 6

(<?+>) :: (Text, Text) -> PU t a -> PU t a infixr 1

Add a back trace level to the error report.

(<?>) :: (Text, Text) -> PU t a -> PU t a infixr 0

Override the last backtrace level in the error report.

(<??>) :: Text -> PU t a -> PU t a

data UnresolvedEntityException

Constructors

UnresolvedEntityException

Instances

Show UnresolvedEntityException

Exception UnresolvedEntityException

Typeable * UnresolvedEntityException

Helper functions

flattenContent :: [Node] -> [Node]

Merge successive NodeContent values.

tErr :: Text -> UnpickleResult t a -> UnpickleResult t a

getRest :: UnpickleResult [a] b -> UnpickleResult [a] (b, [a])

Try to extract the remainig elements, fail if there are none.