stef@zweig.sun (Stephane Payrard) (10/11/90)
I would expect a expression like ${$a} to evaluate from
inside out:
It is not the case as demonstrated by the following
test.
% perl -e '$b=2;$a="b";print ${$a};'
I would expect ${$a} to evaluate to 1 the following way:
${$a} -> ${"b"} -> $b -> 1
What is the canonic way to write an expression which
behaves like I exprected (I guess it imply to use `eval').
This lead to RFE:
supporting this kind of multilevel evalation of string:
-If an expression contains the construct ${expression},
this construct is evaluated and replaced by its value
before the string contained it is evaluated.
stef
--
Stephane Payrard -- stef@eng.sun.com -- (415) 336 3726
SMI 2550 Garcia Avenue M/S 10-09 Mountain View CA 94043
lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall) (10/12/90)
In article <STEF.90Oct11015128@zweig.sun> stef@zweig.sun (Stephane Payrard) writes:
:
: I would expect a expression like ${$a} to evaluate from
: inside out:
: It is not the case as demonstrated by the following
: test.
:
:
: % perl -e '$b=2;$a="b";print ${$a};'
:
: I would expect ${$a} to evaluate to 1 the following way:
:
: ${$a} -> ${"b"} -> $b -> 1
:
:
: What is the canonic way to write an expression which
: behaves like I exprected (I guess it imply to use `eval').
:
: This lead to RFE:
: supporting this kind of multilevel evalation of string:
: -If an expression contains the construct ${expression},
: this construct is evaluated and replaced by its value
: before the string contained it is evaluated.
Multi-level evaluation is one of the things that makes shell programming
such a pain. I'm not gonna do that to Perl. You'll note "text`command`text"
doesn't work either. I'd rather force you to make a few extra temporary
variables than have people writing "Perl scripts out of hell". They're bad
enough coming out of purgatory.
In the ${...} construct, the {} characters only perform grouping and quoting.
No evaluation of anything is implied other than a possible subscript, e.g.
inside ${foo[$bar]} or ${foo{$bar}}. In particular, in ${$a}, the second
$ is only allowed accidentally because ${$} must be allowed as a synonym
for $$, so it doesn't check the first character to make sure it's alphanumeric
like it does the rest of the characters. Thus, the way it currently stands,
you can say ${^Afoo} (where ^A is a real control/a, but you can't say ${foo^A}.
I wouldn't, of course, depend on this for anything if I were you, since
it's undocumented and may change. In fact, once upon a time, *any* sequence
of characters was allowed inside the {}, but it was tightened up some when
we instituted ${foo[$bar]}.
Larry