Re: Two Range Operator Anomalies

Your second "anomaly" has to do with foreach loop aliasing. Particularly:

If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the foreach loop index variable is an implicit alias for each item in the list that you're looping over.

So:

my @a = (1,2,3,4);
for my $i (@a) { $i = 5 };
print "@a";  # prints: 5 5 5 5
[download]

while:

for my $i (1,2,3,4) { $i = 5 };
[download]

errors out with: Modification of a read-only value attempted ...

I think what's happening with your 3 examples of the second anomaly is this:

foreach my $letter ('a' .. 'c', 'd' .. 'f') creates a list of lvalues due to the comma operator in the middle. Perhaps this is done only once at compile time? which explains the persistence.
foreach my $letter ('a' .. 'f') has been optimized as a counting loop, so the range operator does NOT generate a list.
foreach my $letter (@letters) always has @letters redefined right before the loop, so each call to letters() yields consistent results.

Comment on Re: Two Range Operator Anomalies Select or Download Code

Replies are listed 'Best First'.
Re^2: Two Range Operator Anomalies by ikegami (Patriarch) on Mar 11, 2009 at 18:08 UTC
Perhaps this is done only once at compile time? Yes. In the past, I have confirmed this by generating long lists and taking memory measurements. It can also be seen in the opcode tree `$ perl -MO=Concise -e'@a = (1..3)' ... - <1> ex-list lK ->6 3 <0> pushmark s ->4 5 <1> rv2av lKP/1 ->6 4 <$> const[AV ] s ->5 ...` [download] compared with `$ perl -MO=Concise -e'@a = ($x..$y)' ... - <1> ex-list lK ->7 3 <0> pushmark s ->4 - <1> null lKP/1 ->- 6 <1> flop lK ->7 d <1> flip[t6] lK ->7 4 <\|> range(other->5)[t5] lK/1 ->c - <1> ex-rv2sv sK/1 ->d c <#> gvsv[x] s ->d - <1> ex-rv2sv sK/1 ->6 5 <#> gvsv[y] s ->6 ...` [download] so the range operator does NOT generate a list In fact, there is no range operator. `$ perl -MO=Concise -e'for ($x..$y) {}' ... 7 <{> enteriter(next->9 last->c redo->8) lKS ->a - <0> ex-pushmark s ->3 - <1> ex-list lK ->6 3 <0> pushmark s ->4 - <1> ex-rv2sv sK/1 ->5 4 <#> gvsv[x] s ->5 - <1> ex-rv2sv sK/1 ->- 5 <#> gvsv[y] s ->6 6 <#> gv[_] s ->7 ...` [download] compared with `$ perl -MO=Concise -e'for ((),$x..$y) {}' ... 8 <{> enteriter(next->a last->d redo->9) lK ->b - <0> ex-pushmark s ->3 - <1> ex-list lKM ->7 3 <0> pushmark sM ->4 - <0> stub lPM ->4 - <1> null lKM/1 ->- 6 <1> flop lKM ->7 g <1> flip[t4] lK ->7 4 <\|> range(other->5)[t3] lK/1 ->f - <1> ex-rv2sv sK/1 ->g f <#> gvsv[x] s ->g - <1> ex-rv2sv sK/1 ->6 5 <#> gvsv[y] s ->6 7 <#> gv[_] s ->8 ...` [download]	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re^2: Two Range Operator Anomalies
by ikegami (Patriarch) on Mar 11, 2009 at 18:08 UTC

Perhaps this is done only once at compile time?

Yes. In the past, I have confirmed this by generating long lists and taking memory measurements. It can also be seen in the opcode tree

$ perl -MO=Concise -e'@a = (1..3)'
...
-        <1> ex-list lK ->6
3           <0> pushmark s ->4
5           <1> rv2av lKP/1 ->6
4              <$> const[AV ] s ->5
...
[download]

compared with

$ perl -MO=Concise -e'@a = ($x..$y)'
...
-        <1> ex-list lK ->7
3           <0> pushmark s ->4
-           <1> null lKP/1 ->-
6              <1> flop lK ->7
d                 <1> flip[t6] lK ->7
4                    <|> range(other->5)[t5] lK/1 ->c
-                       <1> ex-rv2sv sK/1 ->d
c                          <#> gvsv[*x] s ->d
-                       <1> ex-rv2sv sK/1 ->6
5                          <#> gvsv[*y] s ->6
...
[download]

so the range operator does NOT generate a list

In fact, there is no range operator.

$ perl -MO=Concise -e'for ($x..$y) {}'
...
7        <{> enteriter(next->9 last->c redo->8) lKS ->a
-           <0> ex-pushmark s ->3
-           <1> ex-list lK ->6
3              <0> pushmark s ->4
-              <1> ex-rv2sv sK/1 ->5
4                 <#> gvsv[*x] s ->5
-              <1> ex-rv2sv sK/1 ->-
5                 <#> gvsv[*y] s ->6
6           <#> gv[*_] s ->7
...
[download]

compared with

$ perl -MO=Concise -e'for ((),$x..$y) {}'
...
8        <{> enteriter(next->a last->d redo->9) lK ->b
-           <0> ex-pushmark s ->3
-           <1> ex-list lKM ->7
3              <0> pushmark sM ->4
-              <0> stub lPM ->4
-              <1> null lKM/1 ->-
6                 <1> flop lKM ->7
g                    <1> flip[t4] lK ->7
4                       <|> range(other->5)[t3] lK/1 ->f
-                          <1> ex-rv2sv sK/1 ->g
f                             <#> gvsv[*x] s ->g
-                          <1> ex-rv2sv sK/1 ->6
5                             <#> gvsv[*y] s ->6
7           <#> gv[*_] s ->8
...
[download]

[reply]
[d/l]
[select]