[FWP] thoughts on while(defined(EXP warnings

["David L. Nicol" <david@kasey.umkc.edu>]

perl -w warns me that is is safer to write constructions
of the form

while($line=<HANDLE>){

as

while(defined($line=<HANDLE>)){

and I surely agree.


"defined" is implemented as a function call in these
cases, and profiling indicates that a performance hit is taken,
contrary to the performance improvement  I had hoped for, since
not as much is needed to be checked:  when evaluating definedness,
we only are concerned with one of the several aspects of "truth"
as it is normally defined, and we need not bother to check the rest
of them.

I wonder how complex it would be to make "defined," when it is
used as the outer function call in any execution path construction,
(i.e. anywhere the warning would be triggered), not be a function
call but instead modify what truth means for the purposes of the
truth-interpreting construct.

Right now truth is set as a language-wide constant.  What if
"defined" stopped being a function but instead became a pragma
restricting "truth" in the context in which it appears?  It seems
this could be implemented in a variety of ways :)

One would be to have a variable "truth mask" which would have bits
within it on or off depending on what truth means at the moment, which
would need to be referred to constantly.

Another would be to have an alternate form of boolean evaluator
primitive
which expressions of the form (defined(A)?B:C) would use instead, by
a mechanism surely about as complex as the one that currently warns
about
dangerous constructions.

This collapse could take place in any and all circumstances where
the "defined" function is called in a boolean context.

Determining a "boolean" as opposed to "scalar" context I guess would be
the
real extension to perl internals, wouldn't it.



 
___________________________________________________________________
                           David Nicol 816.235.1187 nicold@umkc.edu
                                     armchair perl internals hacker

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