what's wrong with that syntax construction?

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

I would like to generate random hash, i.e. where key is random number and value is also random number

my @x = map { rand => rand } (0 .. 5)
Compile error: syntax error at (eval 310) line 5, near "} ("
BEGIN not safe after errors--compilation aborted at (eval 310) line 5.
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Mean while, the following works:

my @x = map { rand() => rand } (0 .. 5);
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WBR, basiliscos.

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Re: what's wrong with that syntax construction?
by BrowserUk (Patriarch) on Jun 15, 2016 at 13:31 UTC

Without the parens, the first 'rand' is taken as a string; and that causes the parser to see { 'rand' => rand } as an anomymous hash.

With the parens, it parses { rand() => rand } as an anonymous block as you intend.

You can also force the parser to see the anonymous block like this: my @x = map {; rand => rand } (0 .. 5);.

The extra semicolon is legal inside an anonymous block, but not in an anonymous hash, so the parser gives the benefit of the doubt and goes the way you intend.

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Re: what's wrong with that syntax construction?
by Athanasius (Archbishop) on Jun 15, 2016 at 13:31 UTC

Hello basiliscos,

The “fat comma” operator, =>, stringifies its left-hand operand, so the Perl compiler sees a string and not a function, i.e. my @x = map { 'rand', rand } (0 .. 5). But with parentheses, the expression rand() is evaluated before the => operator, so it is the function’s return value that is stringified.

See perlop#Comma-Operator.

Hope that helps,

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

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Re: what's wrong with that syntax construction?
by Eily (Monsignor) on Jun 15, 2016 at 13:33 UTC

In this case it's a good thing you get a syntax error because if it did compile it wouldn't do what you want. The fat comma (=>) forces its left side to be interpreted as a bareword, so a string most of the time*. So the left side of rand => rand is "rand". Somehow, seeing something that can only be a bareword after the { confuses perl into parsing the construct incorrectly.

* print rand =>; is parsed as print(rand, $_);, not print(rand()); and not print("rand"); either.

Edit: as pointed out by BrowserUK and Athanasius, here the bareword is interpreted as a string, which makes the code look like an anonymous hashref.

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