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S04 -- closure traits clarification

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David Christensen
April 29, 2005 08:57
S04 -- closure traits clarification
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In trying to hack closure trait support into pugs, I have some 
questions about closure traits, variable with "will" traits and 
introspection.  (Apologies if some of this has been discussed on the 
list before -- I'm just going off of the synopses, which if definite 
clarification on some of these issues has been made, should probably be 
updated to reflect the decisions made.)

Firstly, it is suggested in S04 that variables indicated with a "will" 
predicate contribute to the corresponding block-level trait.  I.e., if 
we have the following bit of code:

if $dbh {
	my $sth will undo {$dbh.rollback} will keep {$dbh.commit} = FIRST 
		say "DB error!";
		say "We're good!";

Then the block has in effect 5 total traits, 2 UNDO block, 2 KEEP 
blocks and 1 FIRST blocks.  From what I understand, the blocks for each 
trait are executed in FIFO order, thus we would rollback before we 
report the error in this contrived example.


1) What type of introspection, if any, are we providing to the language 
level?  I.e., are we providing something along the lines of

     %traits = &?BLOCK.traits

where %traits is keyed on trait name (FIRST, LAST, whatever) and in 
turn is an array of closures?  This would mean, for instance that we 
could say


to get the current block's FIRST closures, if any.  When parsing the 
block for traits, coming across a new FIRST block would be akin to 

     push &?BLOCK.traits<FIRST>, {...block contents...}

Specifically, I'm looking for definition of the syntax, which is only 
alluded to in the Synopsis.

2) If we accept the introspection at the block-level above, it seems 
clear that we should also accept the same .traits method on variables.  
I.e., in the above DBI example, we should get back the closure(s) for 
undoing by referring to $sth.traits<UNDO>.  Is a variable-level trait a 
single entry, or can we have multiple "will undo {...}" predicates on a 
single variable?  (The utility of such is left as an exercise to the 

3) User-definable traits.  Now, this may be a closed domain of sorts, 
but do we need to allow for the possibility of user-defined traits?  
(I'm thinking here of variable-level "will" predicates.)  If so, do 
user-defined traits get normalized to UPPER?  It would seem like we 
would want consistency here, because if "will undo {...}" and "UNDO 
{...}" get stored in the same trait slot, we're obviously transforming 
one of the identifiers -- should this behavior be specific to our 
"built-in" ones, or to all traits?

4) Which of the closure traits are supported as "will" predicates on 
variables?  Not all of the closure traits make sense on the 
variable-level -- this information will be useful when trying to parse 
the "will" predicates.


David Christensen

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