Beefy Boxes and Bandwidth Generously Provided by pair Networks
Keep It Simple, Stupid
 
PerlMonks  

Re^3: ternary operator as lvalue failing to induce evaluation of right-hand expression in list context

by choroba (Abbot)
on Aug 05, 2013 at 09:36 UTC ( #1047891=note: print w/ replies, xml ) Need Help??


in reply to Re^2: ternary operator as lvalue failing to induce evaluation of right-hand expression in list context
in thread ternary operator as lvalue failing to induce evaluation of right-hand expression in list context

This is strange. The output of Deparse -p is not parsed the same way as the original:

$ perl -MO=Deparse -e 'sub Foo{};(time ? @y=() : $x) = Foo()' | per +l -e syntax OK $ perl -MO=Deparse,-p -e 'sub Foo{};(time ? @y=() : $x) = Foo()' | per +l -e syntax OK Assignment to both a list and a scalar at - line 4, near "))" Execution of - aborted due to compilation errors.

Perl 5.10.1

لսႽ ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ


Comment on Re^3: ternary operator as lvalue failing to induce evaluation of right-hand expression in list context
Download Code
Re^4: ternary operator as lvalue failing to induce evaluation of right-hand expression in list context (accumulator)
by Anonymous Monk on Aug 05, 2013 at 10:42 UTC

    Compare

    (time ? ($count=()) : $a) = Foo(); ## list+scalar (time ? @a : $a) = Foo(); ## list+scalar (time ? (@a=()) : $a) = Foo(); ## list+scalar (time ? $a=() : $a) = Foo(); ## scalar is scalar (time ? @a=() : $a) = Foo(); ## skates through as scalar $a[0]

    It appears perl doesn't recognize the array+assignment(accumulator) as a list without an extra parenthesis, so it does what it thinks should happen

    Is this a bug? Yes, in that it already protects programmers from this misunderstanding ( perldiag#Assignment to both a list and a scalar), but it missed this particular variant

    Also, Deparse isn't bulletproof :) Deparse says \my(%hash) is the same as \my %hash but it isn't

Re^4: ternary operator as lvalue failing to induce evaluation of right-hand expression in list context
by ed_hoch (Acolyte) on Aug 05, 2013 at 17:54 UTC
    Thanks everyone for all the help. Again, with: This is perl 5, version 16, subversion 2 (v5.16.2) built for x86_64-linux I do:
    ~home>perl -MO=Concise, -ce '$z = (time ? @x=() : $x) = 365' e <@> leave[1 ref] vKP/REFC ->(end) 1 <0> enter ->2 2 <;> nextstate(main 1 -e:1) v:{ ->3 d <2> sassign vKS/2 ->e b <2> sassign sKS/2 ->c 3 <$> const(IV 365) s ->4 - <1> null sKPRM*/1 ->b 5 <|> cond_expr(other->6) sKRM*/1 ->f 4 <0> time[t1] s ->5 a <2> aassign[t3] sKRMS* ->b - <1> ex-list lK ->7 6 <0> pushmark s ->7 - <0> stub lP ->- - <1> ex-list lK ->a 7 <0> pushmark s ->8 9 <1> rv2av[t2] lKRM*/1 ->a 8 <$> gv(*x) s ->9 - <1> ex-rv2sv sKRM*/1 ->- f <$> gvsv(*x) s ->b - <1> ex-rv2sv sKRM*/1 ->d c <$> gvsv(*z) s ->d -e syntax OK
    versus
    ~home>perl -MO=Concise, -ce '$z = (time ? (@x=()) : $x) = 365' Assignment to both a list and a scalar at -e line 1, at EOF -e had compilation errors. e <@> leave[1 ref] KP/REFC ->(end) 1 <0> enter ->2 2 <;> nextstate(main 1 -e:1) :{ ->3 d <2> sassign KS/2 ->e b <2> sassign KS/2 ->c 3 <$> const(IV 365) s ->4 - <1> null KP/1 ->b 5 <|> cond_expr(other->6) K/1 ->f 4 <0> time[t1] s ->5 a <2> aassign[t3] KPS ->b - <1> ex-list lK ->7 6 <0> pushmark s ->7 - <0> stub lP ->- - <1> ex-list lK ->a 7 <0> pushmark s ->8 9 <1> rv2av[t2] lKRM*/1 ->a 8 <$> gv(*x) s ->9 - <1> ex-rv2sv sK/1 ->- f <$> gvsv(*x) s ->b - <1> ex-rv2sv sK/1 ->d c <$> gvsv(*z) s ->d
    There are several differences, starting with the fact that in the first case (the case where it compiles, but I would have expected it not to), apparently the assignment to the ternary is parsed as scalar context (s), where in the second case (the case where it gives compiler errors, as I would have expected), the context isn't indicated. perlop indicates that for a conditional expression, the scalar/list context propagates down to the 2nd or 3rd argument. I suspect something wild is going on with parsing "@x=()" and "(@x=())" as scalar lvalues, where the first is legitimate, and the second isn't. This is really just a guess, though.

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://1047891]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others exploiting the Monastery: (5)
As of 2014-10-26 08:01 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    For retirement, I am banking on:










    Results (152 votes), past polls