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Re^5: Perl Style: ... (TRUE, FALSE and FAILED)

by LanX (Saint)
on Aug 23, 2010 at 10:08 UTC ( [id://856671]=note: print w/replies, xml ) Need Help??


in reply to Re^4: Perl Style: Is initializing variables considered taboo?
in thread Perl Style: Is initializing variables considered taboo?

> So what exactly do you mean by not returning undef to "explicitly" return false.

maybe a little example will make it clearer

DB<1> sub tst {return undef} DB<2> if ($a=tst()) {print "TRUE"} else {print "FALSE" } FALSE DB<3> if (@a=tst()) {print "TRUE"} else {print "FALSE" } TRUE DB<4> sub tst { return (); } DB<5> if ($a=tst()) {print "TRUE"} else {print "FALSE" } FALSE DB<6> if (@a=tst()) {print "TRUE"} else {print "FALSE" } FALSE

line 4 is of course redundant, return; and  return (); do exactly the same thing.

> I have found it wise to always return an explicit value,

OK, if you wanna code more "explicitly", you should better define constants for TRUE, FALSE and FAILED.

DB<9> use constant FAILED => (); DB<10> use constant FALSE => !1; DB<11> use constant TRUE => 1; DB<12> sub t_FALSE { return FALSE; } DB<13> sub t_TRUE { return TRUE; } DB<14> sub t_FAILED { return FAILED; } DB<15> if (@a=t_FAILED) {print "TRUE"} else {print "FALSE" } FALSE DB<16> if (@a=t_FALSE) {print "TRUE"} else {print "FALSE" } TRUE DB<17> if ($a=t_FALSE) {print "TRUE"} else {print "FALSE" } FALSE DB<18> if ($a=t_FAILED) {print "TRUE"} else {print "FALSE" } FALSE

If you wonder about my definition of FALSE, see Truth and Falsehood in perlsyn:

Negation of a true value by "!" or "not" returns a special fals +e value. When evaluated as a string it is treated as '', but as a number +, it is treated as 0.

UPDATE:

please note: FALSE is defined!

DB<27> p !defined (FAILED) 1 DB<28> p defined (FALSE) 1

i.e. FALSE acts like a defined value like in most other languages.

UPDATE:

On second thought FAILED should better be named EMPTY or NOTHING. Failure is just an interpretation of returning nothing.

Cheers Rolf

Replies are listed 'Best First'.
Re^6: Perl Style: ... (TRUE, FALSE and FAILED)
by ait (Hermit) on Aug 25, 2010 at 00:32 UTC
    If you wonder about my definition of FALSE, see Truth and Falsehood in perlsyn:

    From the pod you are reffering to:

    The number 0, the strings '0' and '' , the empty list () , and undef are all false in a boolean context. All other values are true. Negation of a true value by ! or not returns a special false value. When evaluated as a string it is treated as '' , but as a number, it is treated as 0.

    I still believe it's more customary to use undef as the ultimate null and false value in Perl. For example in DBI, DBIx::Class and alike, if you set a field to undef it will store in the database as an actual NULL whereas if you use '' (the empty string) you will get a blank string in the database, probably not what you'd want. I have a feeling that the explicit returning of undef is a quite popular convention :)

    This "ultimate null/false discussion" is probably steering OT and should move to a new thread IMO, though I would suspect that it has been discussed ad nauseam here in PM.

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