Re: Modification of a read-only value attempted?!?

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Re^2: Modification of a read-only value attempted?!? by kikuchiyo (Hermit) on Jan 07, 2024 at 21:07 UTC
Yes, yes, the loop variable in `for(each)` aliases elements of the list. But why is the loop variable read-only if there is only a single numeric (or string) literal in the list? AFAICT the program works as expected in all other cases. perl -MDevel::Peek -le 'for my $x ([1]->[0]) {Dump $x; $x = pack "I", +$x; Dump $x}' # OK perl -MDevel::Peek -le 'my $y=1; for my $x ($y) {Dump $x; $x = pack "I +", $x; Dump $x}' # OK perl -MDevel::Peek -le 'sub one(){1} for my $x (one) {Dump $x; $x = pa +ck "I", $x; Dump $x}' # OK perl -MDevel::Peek -le 'for my $x (0+1) {Dump $x; $x = pack "I", $x; D +ump $x}' # OK!!! perl -MDevel::Peek -le 'for my $x ("1"."1") {Dump $x; $x = pack "I", $ +x; Dump $x}' # OK perl -MDevel::Peek -le 'for my $x ("1") {Dump $x; $x = pack "I", $x; D +ump $x}' # Error, read-only perl -MDevel::Peek -le 'for my $x (1) {Dump $x; $x = pack "I", $x; Dum +p $x}' # Error, read-only [download] Experimenting on perlbanjo.com, it seems that this behavior goes back to at least Perl 5.8.	[reply] [d/l] [select]
Re^3: Modification of a read-only value attempted?!? by haj (Vicar) on Jan 07, 2024 at 21:27 UTC
The difference is that in `for my $x (1) {...}` the `1` is a literal constant. `(0..1)` on the other hand is an expression evaluating to the list (0,1). The loop is aliasing to the anonymous elements of the list, not to the literal boundaries in `(0..1)`. Perhaps it is helpful to consider that `for my $x (0,1) {...}` also dies with `Modification of a read-only value attempted at -e line 1.` `0+1` is an expression: ok! `"1"` is a literal constant: not ok! `(0..1)` is an expression: ok! `(0,1)` is a literal list: not ok!	[reply] [d/l] [select]
Re^4: Modification of a read-only value attempted?!? by LanX (Saint) on Jan 07, 2024 at 22:10 UTC
`*x = \1` also used to be a common way to define "constants". In the OP's case the aliased $x and 1 should have the same reference. But there is no point in making temporarily generated values read only, almost all code would break then. Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply] [d/l]
Re^2: Modification of a read-only value attempted?!? by jo37 (Curate) on Jan 07, 2024 at 20:57 UTC
I don't see an explanation for the difference between `for my $x (1)...` and `for my $x (0 .. 1)...` here. Update: haj explained it perfectly in Re^3: Modification of a read-only value attempted?!? Greetings, -jo `$gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$`	[reply] [d/l] [select]
Re^3: Modification of a read-only value attempted?!? by ikegami (Patriarch) on Jan 09, 2024 at 19:39 UTC
While `0` and `1` evaluate to a constant, `0..1` evaluates to two scalars which aren't constants. That said, `for (0..1)` doesn't actually use the range operator as an optimization. But the result is the same.	[reply] [d/l] [select]