parentheses around a function call in a ternary choice

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

Hi respectable Monks!

trying to explain ternary operator in Perl to someone, I found this "strange" (for me) behaviour:

when I use the return value of a sub in the predicate evaluation of a ternary operation,

nothing seems to work when this evaluation happens between parentheses. Some explanation ?

The example : This code :

sub get1 {
    return 1;
}
print  get1() ?  "1: Flag is up\n"  :  "1: Flag is not up\n";
print  &get1 ?  "2: Flag is up\n"  :  "2: Flag is not up\n";

print  (get1())  ?  "3: Flag is up\n"  :  "3: Flag is not up\n";
print  (&get1 ) ?  "4: Flag is up\n"  :  "4: Flag is not up\n";
[download]

gives this output :

1: Flag is up
2: Flag is up
[download]

Where did cases 3 and 4 disappear ?

Comment on parentheses around a function call in a ternary choice Select or Download Code

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Re: parentheses around a function call in a ternary choice by moritz (Cardinal) on Jun 23, 2009 at 20:56 UTC
`print (...)` is always parsed as a function call `print( ... )` (independently of what comes afterwards), so your code parses as `( print get1() ) ? ... : ...;` [download] `use warnings` will make that line complain about useless constants in void context, which the rest of the line actually is. BTW Perl 6 uses whitespaces to disambiguate, so `print(...)` and `print (...)` are really two different things (which seems to be what new Perl programmers expect).	[reply] [d/l] [select]
Re^2: parentheses around a function call in a ternary choice by didess (Sexton) on Jun 23, 2009 at 21:11 UTC
Thank you! it's so obvious (afterwards ;-) !	[reply]
Re: parentheses around a function call in a ternary choice by FunkyMonk (Bishop) on Jun 23, 2009 at 21:01 UTC
In addition to moritz's comments, you can fix your example by using `... print +(get1()) ? "3: Flag is up\n" : "3: Flag is not up\n"; print +(&get1 ) ? "4: Flag is up\n" : "4: Flag is not up\n"` [download] which causes the output to change to... `... 3: Flag is up 4: Flag is up` [download] Unless I state otherwise, all my code runs with strict and warnings	[reply] [d/l] [select]
Re^2: parentheses around a function call in a ternary choice by didess (Sexton) on Jun 23, 2009 at 21:20 UTC
To be very honest, the warnings coming when using "warnings" do not explain very much to me ! (which constants are we warned about ?) But your explanations do explain all clearly! Thanks to all	[reply]
Re^3: parentheses around a function call in a ternary choice by FunkyMonk (Bishop) on Jun 23, 2009 at 21:40 UTC
a successful print returns true (see print). So your prints are equivalent to `1 ? "3: Flag is up\n" : "3: Flag is not up\n";` [download] and that's a constant expression that evaluates to `"3: Flag is up\n";` [download] That's a constant that isn't used for anything -- a constant in void context. When you get errors or warnings you don't understand try adding use diagnostics;. It will try to explain the error or warning given. Unless I state otherwise, all my code runs with strict and warnings	[reply] [d/l] [select]
Re: parentheses around a function call in a ternary choice by Marshall (Canon) on Jun 23, 2009 at 22:58 UTC
Other comments well taken. I wouldn't use this deprecated &get1 syntax. get1() is the preferred way.	[reply]