Xmlm
Streaming XML codec.
A well-formed sequence of signals represents an XML document tree traversal in depth first order (this has nothing to do with XML well-formedness). Input pulls a well-formed sequence of signals from a data source and output pushes a well-formed sequence of signals to a data destination. Functions are provided to easily transform sequences of signals to/from arborescent data structures.
Consult the features and limitations and examples of use.
References
type encoding = [
| `UTF_8 | |
| `UTF_16 | (* Endianness determined from the BOM. *) |
| `UTF_16BE | |
| `UTF_16LE | |
| `ISO_8859_1 | |
| `US_ASCII |
]
The type for character encodings. For `UTF_16
, endianness is determined from the BOM.
The type for the optional DTD.
The type for attribute and element's expanded names (uri,local)
. An empty uri
represents a name without a namespace name, i.e. an unprefixed name that is not under the scope of a default namespace.
type attribute = name * string
The type for attributes. Name and attribute data.
The type for signals. A well-formed sequence of signals belongs to the language of the doc
grammar :
doc ::= `Dtd tree
tree ::= `El_start child `El_end
child ::= `Data trees | trees
trees ::= tree child | epsilon
The trees
production is used to expresses the fact that there will never be two consecutive `Data signals in the children of an element.
Input and output deal only with well-formed sequences or exceptions are raised. However on output consecutive `Data
signals are allowed.
Namespace name value bound to the reserved "xml"
prefix.
Namespace name value bound to the reserved "xmlns"
prefix.
val pp_dtd : Stdlib.Format.formatter -> dtd -> unit
pp_dtd ppf dtd
prints an unspecified representation of dtd
on ppf
.
val pp_name : Stdlib.Format.formatter -> name -> unit
pp_name ppf name
prints an unspecified representation of name
on ppf
.
val pp_attribute : Stdlib.Format.formatter -> attribute -> unit
pp_attribute ppf att
prints an unspecified representation of att
on ppf
.
val pp_tag : Stdlib.Format.formatter -> tag -> unit
pp_tag ppf tag
prints an unspecified representation of tag
on ppf
.
val pp_signal : Stdlib.Format.formatter -> signal -> unit
pp_signal ppf s
prints an unspecified representation of s
on ppf
.
type error = [
| `Max_buffer_size | (* Maximal buffer size exceeded ( |
| `Unexpected_eoi | (* Unexpected end of input. *) |
| `Malformed_char_stream | (* Malformed underlying character stream. *) |
| `Unknown_encoding of string | (* Unknown encoding. *) |
| `Unknown_entity_ref of string | (* Unknown entity reference, details. *) |
| `Unknown_ns_prefix of string | (* Unknown namespace prefix details *) |
| `Illegal_char_ref of string | (* Illegal character reference. *) |
| `Illegal_char_seq of string | (* Illegal character sequence. *) |
| `Expected_char_seqs of string list * string | (* Expected one of the character sequences in the list but found another. *) |
| `Expected_root_element | (* Expected the document's root element. *) |
]
The type for input errors.
val error_message : error -> string
Converts the error to an english error message.
The type for input sources. For `String
starts reading at the given integer position. For `Fun
the function must return the next byte as an int
and raise End_of_file
if there is no such byte.
val make_input : ?enc:encoding option -> ?strip:bool -> ?ns:(string -> string option) ->
?entity:(string -> string option) -> source -> input
Returns a new input abstraction reading from the given source.
enc
, character encoding of the document, details. Defaults to None
.strip
, strips whitespace in character data, details. Defaults to false
.ns
is called to bind undeclared namespace prefixes, details. Default returns always None
.entity
is called to resolve non predefined entity references, details. Default returns always None
.Inputs a signal. Repeated invocation of the function with the same input abstraction will generate a well-formed sequence of signals or an Error
is raised. Furthermore there will be no two consecutive `Data
signals in the sequence and their string is always non empty.
This behaviour is deprecated: after a well-formed sequence was input another may be input, see eoi
and details.
Raises Error
on input errors.
If the next signal is a :
`Data
signal, inputs it and invokes data
with the character data.`El_start
signal, inputs the sequence of signals until its matching `El_end
and invokes el
and data
as follows
el
, is called on each `El_end
signals with the corresponding `El_start
tag and the result of the callback invocation for the element's children.data
, is called on each `Data
signals with the character data. This function won't be called twice consecutively or with the empty string.Invalid_argument
.Raises Error
on input errors and Invalid_argument
if the next signal is not `El_start
or `Data
.
Same as input_tree
but reads a complete well-formed sequence of signals.
Raises Error
on input errors and Invalid_argument
if the next signal is not `Dtd
.
val eoi : input -> bool
The type for deconstructing data structures of type 'a
.
The type for output destinations. For `Buffer
, the buffer won't be cleared. For `Fun
the function is called with the output bytes as int
s.
val make_output : ?decl:bool -> ?nl:bool -> ?indent:int option ->
?ns_prefix:(string -> string option) -> dest -> output
Returns a new output abstraction writing to the given destination.
decl
, if true
the XML declaration is output (defaults to true
).nl
, if true
a newline is output when the root's element `El_end
signal is output. Defaults to false
.indent
, identation behaviour, see details. Defaults to None
.ns_prefix
, undeclared namespace prefix bindings, see details. Default returns always None
.Outputs a signal.
This behaviour is deprecated: after a well-formed sequence of signals was output a new well-formed sequence can be output.
Raises Invalid_argument
if the resulting signal sequence on the output abstraction is not well-formed or if a namespace name could not be bound to a prefix.
val output_depth : output -> int
output_depth o
is o
's current element nesting level (undefined before the first `El_start
and after the last `El_end
).
Outputs signals corresponding to a value by recursively applying the given value deconstructor.
Raises see output
.
Same as output_tree
but outputs a complete well-formed sequence of signals.
Raises see output
.
WARNING. The functioral interface is deprecated and will be removed.
Make
allows client to specify types for strings and internal buffers. Among other things this can be used to perform hash-consing or to process the character stream, e.g. to normalize unicode characters or to convert to a custom encoding.
module type String = sig ... end
Input signature for strings.
module type Buffer = sig ... end
Input signature for internal buffers.
module Make (String : String) (Buffer : Buffer with type string = String.t) : S with type string = String.t
Functor building streaming XML IO with the given strings and buffers.
The module assumes strings are immutable, thus strings the client gives or receives during the input and output process must not be modified.
The parser supports ASCII, US-ASCII, UTF-8, UTF-16, UTF-16LE, UTF-16BE and ISO-8559-1 (Latin-1) encoded documents. But strings returned by the library are always UTF-8 encoded.
The encoding can be specified explicitly using the optional argument enc
. Otherwise the parser uses UTF-16 or UTF-8 if there is a BOM at the beginning of the document. If there is no BOM it uses the encoding specified in the XML declaration. Finally, if there is no XML declaration UTF-8 is assumed.
The parser performs attribute data normalization on every attribute data. This means that attribute data does not have leading and trailling white space and that any white space is collapsed and transformed to a single space character (U+0020
).
White space handling of character data depends on the strip
argument. If strip
is true
, character data is treated like attribute data, white space before and after elements is removed and any white space is collapsed and transformed to a single space character (U+0020
), except if the data is under the scope of a xml:space attribute whose value is preserve. If strip
is false
all white space data is preserved as present in the document (however all kinds of line ends are translated to the newline character (U+000A
).
Xmlm's names are expanded names. The parser automatically handles the document's namespace declarations. Undeclared namespace prefixes can be bound via the callback ns
, which must return a namespace name. If ns
returns None
an `Unknown_ns_prefix
error is raised.
Attributes used for namespace declarations are preserved by the parser. They are in the ns_xmlns
namespace. Default namespace declarations made with xmlns have the attribute name (Xmlm.ns_xmlns, "xmlns")
. Prefix declarations have the prefix as the local name, for example xmlns:ex results in the attribute name (Xmlm.ns_xmlns, "ex")
.
Regarding constraints on the usage of the xml and xmlns prefixes by documents, the parser does not report errors on violations of the must constraints listed in this paragraph.
Character references and predefined entities are automatically resolved. Other entity references can be resolved by the callback entity
, which must return an UTF-8 string corresponding to the replacement character data. The replacement data is not analysed for further references, it is added to the data as such modulo white space stripping. If entity
returns None
the error `Unknown_entity_ref
is returned.
WARNING. This feature is deprecated and will be removed.
When a well-formed sequence of signals is input, no data is consumed beyond the closing '>'
of the document's root element.
If you want to parse a document as defined in the XML specification, call eoi
after a well-formed sequence of signals, it must return true
. If you expect another document on the same input abstraction a new well-formed sequence of signals can be input
. Use eoi
to check if a document follows (this may consume data).
Invoking eoi
after a well-formed sequence of signals skips whitespaces, comments and processing instructions until it gets to either an XML declaration or a DTD or the start of a new element or the end of input (in which case eoi
returns true
). If there is a new document but there is no XML declaration or the declaration specifies UTF-16, the same encoding as for the previous document is used.
':'
because of namespaces).Sys.max_string_length
. The error `Max_buffer_size
is raised if the limit is hit.Outputs only UTF-8 encoded documents. Strings given to output functions must be UTF-8 encoded, no checks are performed. Unicode characters that are not legal XML characters are replaced by the Unicode replacement character.
Xmlm's names are expanded names. Expanded names are automatically converted to qualified names by the output abstraction. There is no particular api to specify prefixes and default namespaces, the actual result depends solely on the output of attributes belonging to the ns_xmlns
namespace. For example to set the default namespace of an element to http://example.org/myns, use the following attribute :
(* xmlns='http://example.org/myns' *)
let default_ns = (Xmlm.ns_xmlns, "xmlns"), "http://example.org/myns"
To bind the prefix "ex"
to http://example.org/ex, use the following attribute :
(* xmlns:ex='http://example.org/ex' *)
let ex_ns = (Xmlm.ns_xmlns, "ex"), "http://example.org/ex"
Note that outputing input signals without touching namespace declaration attributes will preserve existing prefixes and bindings provided the same namespace name is not bound to different prefixes in a given context.
The callback ns_prefix
of an output abstraction can be used to give a prefix to a namespace name lacking a prefix binding in the current output scope. Given a namespace name the function must return the prefix to use. Note that this will not add any namespace declaration attribute to the output. If the function returns None
, output
will raise Invalid_argument
. The default function returns always None
.
Output can be indented by specifying the indent
argument when an output abstraction is created. If indent
is None
(default) signal output does not introduce any extra white space. If ident
is Some c
, each signal
is output on its own line (for empty elements `El_start
and `El_end
are collapsed on a single line) and nested elements are indented with c
space characters.
WARNING. This feature is deprecated and will be removed.
After a well-formed sequence of signals was output, the output abstraction can be reused to output a new well-formed sequence of signals.
'<'
,'>'
,'&'
, and '\"'
are automatically escaped to predefined entities.("","dip d")
will produce a non well-formed document because of the space character.strip = false
and output with indent = None
.indent = None
and suitable `Data
signalsSequential processing has the advantage that you don't need to get the whole document tree in memory to process it.
The following function reads a single document on an input channel and outputs it.
let id ic oc =
let i = Xmlm.make_input (`Channel ic) in
let o = Xmlm.make_output (`Channel oc) in
let rec pull i o depth =
Xmlm.output o (Xmlm.peek i);
match Xmlm.input i with
| `El_start _ -> pull i o (depth + 1)
| `El_end -> if depth = 1 then () else pull i o (depth - 1)
| `Data _ -> pull i o depth
| `Dtd _ -> assert false
in
Xmlm.output o (Xmlm.input i); (* `Dtd *)
pull i o 0;
if not (Xmlm.eoi i) then invalid_arg "document not well-formed"
The following function reads a sequence of documents on an input channel and outputs it.
let id_seq ic oc =
let i = Xmlm.make_input (`Channel ic) in
let o = Xmlm.make_output ~nl:true (`Channel oc) in
while not (Xmlm.eoi i) do Xmlm.output o (Xmlm.input i) done
The following function reads a sequence of documents on the input channel. In each document's tree it prunes non root elements whose name belongs to prune_list
.
let prune_docs prune_list ic oc =
let i = Xmlm.make_input (`Channel ic) in
let o = Xmlm.make_output ~nl:true (`Channel oc) in
let copy i o = Xmlm.output o (Xmlm.input i) in
let prune (name, _) = List.mem name prune_list in
let rec process i o d =
let rec skip i d = match Xmlm.input i with
| `El_start _ -> skip i (d + 1)
| `El_end -> if d = 1 then () else skip i (d - 1)
| s -> skip i d
in
match Xmlm.peek i with
| `El_start tag when prune tag -> skip i 0; process i o d
| `El_start _ -> copy i o; process i o (d + 1)
| `El_end -> copy i o; if d = 0 then () else process i o (d - 1)
| `Data _ -> copy i o; process i o d
| `Dtd _ -> assert false
in
let rec docs i o =
copy i o; (* `Dtd *)
copy i o; (* root start *)
process i o 0;
if Xmlm.eoi i then () else docs i o
in
docs i o
A document's sequence of signals can be easily converted to an arborescent data structure. Assume your trees are defined by :
type tree = E of Xmlm.tag * tree list | D of string
The following functions input/output xml documents from/to abstractions as value of type tree
.
let in_tree i =
let el tag childs = E (tag, childs) in
let data d = D d in
Xmlm.input_doc_tree ~el ~data i
let out_tree o t =
let frag = function
| E (tag, childs) -> `El (tag, childs)
| D d -> `Data d
in
Xmlm.output_doc_tree frag o t
We show how to process XML data that represents tabular data (some people like do that).
The file we need to deal with represents nominal data about W3C bureaucrats. There are no namespaces and attributes are ignored. The element structure of the document is :
<list>
<bureaucrat> represents a W3C bureaucrat (zero or more).
A bureaucrat contains the following elements, in order.
In OCaml we represent a W3C bureaucrat by this type :
type w3c_bureaucrat = {
name : string;
surname : string;
honest : bool;
obfuscation_level : float;
trs : string list; }
The following functions input and output W3C bureaucrats as lists of values of type w3c_bureaucrat
.
let in_w3c_bureaucrats src =
let i = Xmlm.make_input ~strip:true src in
let tag n = ("", n), [] in
let error () = invalid_arg "parse error" in
let accept s i = if Xmlm.input i = s then () else error () in
let rec i_seq el acc i = match Xmlm.peek i with
| `El_start _ -> i_seq el ((el i) :: acc) i
| `El_end -> List.rev acc
| _ -> error ()
in
let i_el n i =
accept (`El_start (tag n)) i;
let d = match Xmlm.peek i with
| `Data d -> ignore (Xmlm.input i); d
| `El_end -> ""
| _ -> error ()
in
accept (`El_end) i;
d
in
let i_bureaucrat i =
try
accept (`El_start (tag "bureaucrat")) i;
let name = i_el "name" i in
let surname = i_el "surname" i in
let honest = match Xmlm.peek i with
| `El_start (("", "honest"), []) -> ignore (i_el "honest" i); true
| _ -> false
in
let obf = float_of_string (i_el "obfuscation_level" i) in
let trs = i_seq (i_el "tr") [] i in
accept (`El_end) i;
{ name = name; surname = surname; honest = honest;
obfuscation_level = obf; trs = trs }
with
| Failure _ -> error () (* float_of_string *)
in
accept (`Dtd None) i;
accept (`El_start (tag "list")) i;
let bl = i_seq i_bureaucrat [] i in
accept (`El_end) i;
if not (Xmlm.eoi i) then invalid_arg "more than one document";
bl
let out_w3c_bureaucrats dst bl =
let tag n = ("", n), [] in
let o = Xmlm.make_output ~nl:true ~indent:(Some 2) dst in
let out = Xmlm.output o in
let o_el n d =
out (`El_start (tag n));
if d <> "" then out (`Data d);
out `El_end
in
let o_bureaucrat b =
out (`El_start (tag "bureaucrat"));
o_el "name" b.name;
o_el "surname" b.surname;
if b.honest then o_el "honest" "";
o_el "obfuscation_level" (string_of_float b.obfuscation_level);
List.iter (o_el "tr") b.trs;
out `El_end
in
out (`Dtd None);
out (`El_start (tag "list"));
List.iter o_bureaucrat bl;
out (`El_end)