NAME

Encode::Encoding - Encode Implementation Base Class

SYNOPSIS

  package Encode::MyEncoding;
  use base qw(Encode::Encoding);

  __PACKAGE__->Define(qw(myCanonical myAlias));

As mentioned in Encode, encodings are (in the current implementation at least) defined by objects. The mapping of encoding name to object is via the %encodings hash.

The values of the hash can currently be either strings or objects. The string form may go away in the future. The string form occurs when encodings() has scanned @INC for loadable encodings but has not actually loaded the encoding in question. This is because the current "loading" process is all Perl and a bit slow.

Once an encoding is loaded then value of the hash is object which implements the encoding. The object should provide the following interface:

->name

Should return the string representing the canonical name of the encoding.

->new_sequence

This is a placeholder for encodings with state. It should return an object which implements this interface, all current implementations return the original object.

->encode($string,$check)

Should return the octet sequence representing $string. If $check is true it should modify $string in place to remove the converted part (i.e. the whole string unless there is an error). If an error occurs it should return the octet sequence for the fragment of string that has been converted, and modify $string in-place to remove the converted part leaving it starting with the problem fragment.

If check is is false then encode should make a "best effort" to convert the string - for example by using a replacement character.

->decode($octets,$check)

Should return the string that $octets represents. If $check is true it should modify $octets in place to remove the converted part (i.e. the whole sequence unless there is an error). If an error occurs it should return the fragment of string that has been converted, and modify $octets in-place to remove the converted part leaving it starting with the problem fragment.

If check is is false then decode should make a "best effort" to convert the string - for example by using Unicode's "\x{FFFD}" as a replacement character.

It should be noted that the check behaviour is different from the outer public API. The logic is that the "unchecked" case is useful when encoding is part of a stream which may be reporting errors (e.g. STDERR). In such cases it is desirable to get everything through somehow without causing additional errors which obscure the original one. Also the encoding is best placed to know what the correct replacement character is, so if that is the desired behaviour then letting low level code do it is the most efficient.

In contrast if check is true, the scheme above allows the encoding to do as much as it can and tell layer above how much that was. What is lacking at present is a mechanism to report what went wrong. The most likely interface will be an additional method call to the object, or perhaps (to avoid forcing per-stream objects on otherwise stateless encodings) and additional parameter.

It is also highly desirable that encoding classes inherit from Encode::Encoding as a base class. This allows that class to define additional behaviour for all encoding objects. For example built in Unicode, UCS-2 and UTF-8 classes use :

  package Encode::MyEncoding;
  use base qw(Encode::Encoding);

  __PACKAGE__->Define(qw(myCanonical myAlias));

To create an object with bless {Name => ...},$class, and call define_encoding. They inherit their name method from Encode::Encoding.

Compiled Encodings

Encode.xs provides a class Encode::XS which provides the interface described above. It calls a generic octet-sequence to octet-sequence "engine" that is driven by tables (defined in encengine.c). The same engine is used for both encode and decode. Encode:XS's encode forces Perl's characters to their UTF-8 form and then treats them as just another multibyte encoding. Encode:XS's decode transforms the sequence and then turns the UTF-8-ness flag as that is the form that the tables are defined to produce. For details of the engine see the comments in encengine.c.

The tables are produced by the Perl script compile (the name needs to change so we can eventually install it somewhere). compile can currently read two formats:

*.enc

This is a coined format used by Tcl. It is documented in Encode/EncodeFormat.pod.

*.ucm

This is the semi-standard format used by IBM's ICU package.

compile can write the following forms:

*.ucm

See above - the Encode/*.ucm files provided with the distribution have been created from the original Tcl .enc files using this approach.

*.c

Produces tables as C data structures - this is used to build in encodings into Encode.so/Encode.dll.

*.xs

In theory this allows encodings to be stand-alone loadable Perl extensions. The process has not yet been tested. The plan is to use this approach for large East Asian encodings.

The set of encodings built-in to Encode.so/Encode.dll is determined by Makefile.PL. The current set is as follows:

ascii and iso-8859-*

That is all the common 8-bit "western" encodings.

IBM-1047 and two other variants of EBCDIC.

These are the same variants that are supported by EBCDIC Perl as "native" encodings. They are included to prove "reversibility" of some constructs in EBCDIC Perl.

symbol and dingbats as used by Tk on X11.

(The reason Encode got started was to support Perl/Tk.)

That set is rather ad hoc and has been driven by the needs of the tests rather than the needs of typical applications. It is likely to be rationalized.