Question of example from Camel Book

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

Monks,

I'm stumped on a code example from the Camel book (Section 2.5.1, 2nd Ed.):

print($foo, exit);  # Obviously not what you want.
[download]

The 'exit' function is being called from within a print function's list of arguments. How/Why does a function such as 'exit' even execute in this context? Why is it executing before $foo gets printed? Obviously, I don't understand the Camel's explanation.

As always, any insight is much appreciated.

$PM = "Perl Monk's";
$MCF = "Most Clueless ~~Friar~~ Abbot";
$nysus = $PM . $MCF;

Comment on Question of example from Camel Book Download Code

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Re: Question of example from Camel Book by myocom (Deacon) on Jun 20, 2001 at 09:34 UTC
In order to print something, it needs to evaluate the values of what it's trying to print. For example, `print ($foo, length($foo));` evaluates what `$foo`'s value is and what `length($foo)`'s value is, then prints them. In the Camel's example, `exit` gets called, with the intention of determining its value, but when you call `exit`, you do just that: exit.	[reply] [d/l] [select]
Re: Question of example from Camel Book by clemburg (Curate) on Jun 20, 2001 at 13:16 UTC
`print($foo, exit); # Obviously not what you want.` [download] The print function is called with two arguments, `$foo`, and `exit`. Both will be evaluated to get at their values, and these values will then be sent to the print function to print out. That is, `$foo` evaluates to whatever is stored in it (say, the string `"hi"`), and then `exit` is evaluated by calling the `exit` function, which aborts the program. So `print` never gets a chance to print anything. This is rather basic. More interesting is why this prints something: `print ($foo), exit;` [download] Here the expression consists of a call to the `print` function with one argument, and then a comma, and then a call to the `exit` function. As a comma operator evaluates its left argument, throws the return value away, and the evaluates its right argument, first the call to `print($foo)` is evaluated, resulting in a call to `print`, this in turn will evaluate `$foo` and send its value to the `print` function (making it print out something like `"hi"`), and then second, the call to `exit` is evaluated, aborting the program. You should read Structure and Interpretation of Computer Programs. Christian Lemburg Brainbench MVP for Perl http://www.brainbench.com	[reply] [d/l] [select]
Re: Question of example from Camel Book by lemming (Priest) on Jun 20, 2001 at 09:38 UTC
print needs to evaluate the function to see what is returned to be printed Not this fragment is useful, but you can see why one would want the function to return a value first. `use strict; use warnings; sub yaya { return " ha \n"; } my $foo = "bar"; print ($foo, yaya);` [download] Update:Ok. Took a look at that section of the Camel. It's all in the precedence of arguments and functions can be arguments. parens can modify this, but you may need a unary plus to avoid some mistakes It might be worth reading that section a few more times.	[reply] [d/l]