failure to compile list eval in block

ed_hoch has asked for the wisdom of the Perl Monks concerning the following question:

program1:

{
  7;
}
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program2:

{
  7, 10;
}
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Program1 compiles just fine. (Define a block, evaluate the statement 7; in that block, exit the block.)

However, Program2 does not compile. I suspect this has something to do with Perl "thinking" I'm trying to evaluate an anonymous hashref expression as a statement (which I'm not.)

Does anyone know what the syntax error Perl finds here is about?

Thanks, Ed Update: Thanks for everyone's help. To clarify, I wasn't using this in a purposeful program, I was just trying to understand how Perl works, and it was frustrating me that I couldn't figure out why 2 very similar programs were being parsed so differently.

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Re: failure to compile list eval in block
by LanX (Saint) on Dec 16, 2012 at 00:22 UTC

Because of the numbers (remember => is just a fat comma) this is interpreted as a hash.

eliminating the ; helps seeing what's happening

> perl -MO=Deparse
{
  7, 10
}
__END__
+{7, 10};           # <-- a literal hash
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But what do you expect a codeblock with 7,10 to do?

EDIT:

one workaround to help the parser understand your intentions is an early semicolon

 perl -MO=Deparse
{
  ; 7,10;
}
__END__
{
    '???', '???';    # <-- constants in a codeblock
}
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Cheers Rolf

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Re: failure to compile list eval in block
by Athanasius (Archbishop) on Dec 16, 2012 at 02:39 UTC

As an alternative to LanX’s workaround, you can use parentheses:

12:32 >perl -MO=Deparse -e " { (7, 10); } "
{
    '???', '???';
}
-e syntax OK

12:32 >
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Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: failure to compile list eval in block
by ikegami (Patriarch) on Dec 16, 2012 at 13:45 UTC

{; ... }   # Block
+{ ... }   # Hash constructor
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For example,

>perl -ce"{ 1, 2; }"
syntax error at -e line 1, near "; }"
-e had compilation errors.

>perl -ce"{; 1, 2; }"
-e syntax OK

>perl -ce"map { f() }, @a"
syntax error at -e line 1, near "},"
-e had compilation errors.

>perl -ce"map +{ f() }, @a"
-e syntax OK
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Re: failure to compile list eval in block
by Anonymous Monk on Dec 16, 2012 at 01:04 UTC

B::Deparse is B::Deparse

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