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Extend AUTOLOAD functionality to AUTOGLOB
Maintainer: David Cantrell <david@cantrell.org.uk> Date: 26 Sep 2000 Last Modified: 30 Sep 2000 Mailing List: perl6-language-subs@perl.org Number: 324 Version: 2 Status: Developing Original co-author: David H. Adler <dha@panix.com>
The functionality of the existing AUTOLOAD should be available for all datatypes.
Comments received have been that it would be better to use AUTOSCALAR, AUTOHASH etc instead of AUTOGLOB in the interests of encapsulation. The argument being that if you only want to implement AUTO scalars in your program, then doing:
if($type_of_thing eq 'scalar') {
# do some stuff
} else {
SUPER::...
# use our parent's AUTOGLOB
}
breaks encapsulation. However, these sort of shenanigans are common in other OO languages such as Java and C++, and in any case, we still need to 'break encapsulation' like this if RFC 8 is implemented. If programmers really want to use AUTOHASH, AUTOSCALAR etc then they can put their own suitably constructed AUTOGLOB subroutine in the UNIVERSAL class which will do all the appropriate magic.
There was also a desire to 'control what was auto-globbable' (Nate Wiger). Whilst I can understand this, I feel that allowing this for some datatypes and not others is an arbitrary restriction and goes against the general perl ethos. By making all autoglobbing be off by default and having quite fine-grained control over what can be autoglobbed and in what scope, I feel that we give advanced programmers the power that they need whilst also protecting novices from shooting themselves in the foot. Well, protect them from blowing their foot completely off anyway.
Leon Brocard commented that the ability to refuse a request a la RFC 8 would be really useful. On consideration, I agree, and have incorporated that into this revised version. I have also removed the 'special case' of an AUTOGLOBbed sub not returning a reference unlike all other types. This is also in line with RFC 8. See also RFC 190 for a proposal for re-dispatching methods which may be preferable to using SUPER like in the example above.
Finally, also in line with RFC 8 I have removed the special $AUTOGLOB variable, instead passing the name of the item in with AUTOGLOB's arguments.
Whilst the current AUTOLOAD is very useful, we do not feel that it goes far enough. It only works for subroutines. We believe this sort of functionality should be available for anything in the symbol table. A sensible name would be AUTOGLOB.
For example: one current use for AUTOLOAD is in the Sub::Approx module. It is not, however, possible to write Scalar::Approx, Hash::Approx, Array::Approx, or Filehandle::Approx. This would allow for catching common errors such as mis-typing of class fields as well as mis-typing of class methods.
In the interests of backward compatibility, the existing AUTOLOAD should be kept temporarily. It's use should be deprecated and should generate a suitable warning under -w. It would not be upgraded in line with other RFCs submitted (notably 8, 190 and 232) but the changes proposed would go in to the AUTOGLOB melting pot. AUTOLOAD should be removed from the development codebase after the first stable release of perl 6.
The AUTOGLOB subroutine should expect to take three parameters, the invocant, a scalar parameter specifying what type of item is being AUTOGLOBbed, and another scalar with the name of the item being AUTOGLOBbed; followed by - in the case of a sub - the sub's arguments. We suggest that the 'type' scalar should take values like 'scalar' for an AUTOGLOBbed scalar, 'array' for an AUTOGLOBbed array, and so on.
We suggest that the AUTOGLOB subroutine should return a reference to an item of the appropriate type. That will then be dereferenced and used as normal in the calling code. If it returns anything other than a reference this should be a fatal error, unless it returns undef (which, as in RFC 8 should be taken to mean "this isn't an error but I'm declining to handle this request"), in which case the AUTOGLOB should be suitably delegated as in RFC 8 (see also RFC 190).
As AUTOGLOBbing is expected to be a relatively expensive and dangerous operation, it should be off by default (with the exception of subs), and should be turned on and off via a new lexically scoped autoglob pragma:
use autoglob; # turn on all AUTOGLOBbing
use autoglob qw(scalar hash); # turn it on for scalars and hashes
no autoglob qw(filehandle scalar); # turn it off for ...
Changes from the semantics of AUTOLOAD mean that existing AUTOLOADs will almost certainly break badly if simply renamed. Therefore it would be wise to keep the existing AUTOLOAD (although deprecated) for the first stable release of perl6, but remove it in later releases. This will give people sufficient time to heed the warnings (we do heed warnings, right?) and update their code.
RFC 8: The AUTOLOAD subroutine should be able to decline a request
RFC 190: Objects : NEXT pseudoclass for method redispatch
RFC 232: Replace AUTOLOAD by a more flexible mechanism