I tried the following one-liner more out of curiosity than anything and was surprised that it actually worked without the %
sigil.
$ perl -E 'say for keys ::'
It works on both versions 5.8.8 and 5.16.3; though the latter version emits this warning:
Hash %:: missing the % in argument of keys() at -e line 1.
How does this even work? What is so special about %::
that allows it to run and print its keys, even without the sigil?
Note that the keys do not get printed with %main::
.
$ perl -E 'say for keys main::'
Hash main:: missing the % in argument 1 of keys() at -e line 1.
::
isn't special; prior to Perl 5.22.0, you can omit the %
and pass any identifier to keys
.
However:
keys main::
is equivalent to keys %{'main'}
or just keys %main
keys ::
is equivalent to keys %{'::'}
or just keys %::
. %main::
(but not %main
) is an alias for %::
. The relevant code is in toke.c (the following is from 5.8.8):
/* Look for a subroutine with this name in current package,
unless name is "Foo::", in which case Foo is a bearword
(and a package name). */
if (len > 2 &&
PL_tokenbuf[len - 2] == ':' && PL_tokenbuf[len - 1] == ':')
{
if (ckWARN(WARN_BAREWORD) && ! gv_fetchpv(PL_tokenbuf, FALSE, SVt_PVHV))
Perl_warner(aTHX_ packWARN(WARN_BAREWORD),
"Bareword \"%s\" refers to nonexistent package",
PL_tokenbuf);
len -= 2;
PL_tokenbuf[len] = '\0';
gv = Nullgv;
gvp = 0;
}
else {
len = 0;
if (!gv)
gv = gv_fetchpv(PL_tokenbuf, FALSE, SVt_PVCV);
}
/* if we saw a global override before, get the right name */
if (gvp) {
sv = newSVpvn("CORE::GLOBAL::",14);
sv_catpv(sv,PL_tokenbuf);
}
else {
/* If len is 0, newSVpv does strlen(), which is correct.
If len is non-zero, then it will be the true length,
and so the scalar will be created correctly. */
sv = newSVpv(PL_tokenbuf,len);
}
len
is the length of the current token.
If the token is main::
, a new scalar is created with its PV (string component) set to main
.
If the token is ::
, a typeglob is fetched with gv_fetchpv
.
gv_fetchpv
lives in gv.c and has special logic for handling ::
:
if (*namend == ':')
namend++;
namend++;
name = namend;
if (!*name)
return gv ? gv : (GV*)*hv_fetch(PL_defstash, "main::", 6, TRUE);
This fetches the typeglob stored in the default stash under key main::
(ie typeglob *main::
).
Finally, keys
expects its argument to be a hash, but if you pass it an identifier, it treats it as the name of a hash. See Perl_ck_fun
in op.c:
case OA_HVREF:
if (kid->op_type == OP_CONST &&
(kid->op_private & OPpCONST_BARE))
{
char *name = SvPVx(((SVOP*)kid)->op_sv, n_a);
OP * const newop = newHVREF(newGVOP(OP_GV, 0,
gv_fetchpv(name, TRUE, SVt_PVHV) ));
if (ckWARN2(WARN_DEPRECATED, WARN_SYNTAX))
Perl_warner(aTHX_ packWARN2(WARN_DEPRECATED, WARN_SYNTAX),
"Hash %%%s missing the %% in argument %"IVdf" of %s()",
name, (IV)numargs, PL_op_desc[type]);
op_free(kid);
kid = newop;
kid->op_sibling = sibl;
*tokid = kid;
}
else if (kid->op_type != OP_RV2HV && kid->op_type != OP_PADHV)
bad_type(numargs, "hash", PL_op_desc[type], kid);
mod(kid, type);
break;
This works for things other than ::
, too:
$ perl -e'%h = (foo => "bar"); print for keys h'
foo
( As of 5.22.0 , you're no longer allowed to omit the %
sigil.)
You can also see this with B::Concise:
$ perl -MO=Concise -e'keys main::'
Hash %main missing the % in argument 1 of keys() at -e line 1.
6 <@> leave[1 ref] vKP/REFC ->(end)
1 <0> enter ->2
2 <;> nextstate(main 1 -e:1) v:{ ->3
5 <1> keys[t2] vK/1 ->6
4 <1> rv2hv[t1] lKRM/1 ->5
3 <$> gv(*main) s ->4
-e syntax OK
$ perl -MO=Concise -e'keys ::'
Hash %:: missing the % in argument 1 of keys() at -e line 1.
6 <@> leave[1 ref] vKP/REFC ->(end)
1 <0> enter ->2
2 <;> nextstate(main 1 -e:1) v:{ ->3
5 <1> keys[t2] vK/1 ->6
4 <1> rv2hv[t1] lKRM/1 ->5
3 <$> gv(*main::) s ->4
-e syntax OK
Using:
perl -MO=Deparse -E 'say for keys ::'
Says:
use feature 'current_sub', 'evalbytes', 'fc', 'say', 'state', 'switch', 'unicode_strings', 'unicode_eval';
say $_ foreach (keys %main::);
So it treats :: as %:: in these perl versions without a strict
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