Re: Comma's and blocks

Why are the comma's excluded in some spots and not others?

They're not excluded... you may have completely misunderstood some of the syntactic forms.

  map {/dothis/} @variable;
  grep {/more stuff/} @more;
[download]

That's roughly equivalent to:

  map(sub {/dothis/}, @variable);
  grep(sub {/more stuff/}, @more);
[download]

except that map() and grep() play with $_ on your behalf.

As for:

  print A "hello";
  print $somevar, "can be seperated like this";
[download]

The second print statement is equivalent to

  print STDOUT $somevar, "can be seperated like this";
[download]

.. unless you've called select() to change which is the default filehandle.

How does this work with your own prototypes and subroutines?

Prototypes allow the Perl parser to apply (for your subs) the same parser tricks it employs on built-in functions (like push()).

Which perldoc covers this in depth?

perlsyn, perlsub, perlfunc should do it.

-David

Comment on Re: Comma's and blocks Select or Download Code

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Re^2: Comma's and blocks by zer (Deacon) on Oct 04, 2007 at 06:30 UTC
David, Thanks for the response! The map and grep is a lot clearer (kind of). `#ex 1 map (sub {/dothis/},@_); map {/and this/}, @_; #now doesnt compile #ex 2 print STDOUT "hello"; print <STDOUT> , "Hello"; #runs` [download] In the above example. The first line is making sense and opened my eyes to that for the map. As well the print uses a comma when a proper file handle '<' and '>' are used. Why do the use of proper terms affect the use of commas?	[reply] [d/l]
Re^3: Comma's and blocks by erroneousBollock (Curate) on Oct 04, 2007 at 06:35 UTC
print <STDOUT> , "Hello"; #runs That doesn't do what you think. <STDOUT> reads from the STDOUT filehandle (which won't do anything useful). You can't put a comma between the file-handle and the printable content, the syntax is not short for any other syntax. `print HANDLE expression` is the normal form. `print expression` is parsed by perl to mean `print LATESTHANDLE expression` where LATESTHANDLE is the last handle given to `select()` (the default is STDOUT). -David	[reply] [d/l] [select]
Re^3: Comma's and blocks by mwah (Hermit) on Oct 04, 2007 at 09:41 UTC
zer: As well the print uses a comma when a proper file handle '<' and '>' are used. Why do the use of proper terms affect the use of commas As has been said in another response, the `print <STDOUT> , "Hello";` invokes reading a filehandle in list context and printing the result afterwards - but not through your "proper file handle" ;-) It has probably also been noted elswhere that this is like `print STDOUT <STDOUT>, "Hello";` The notation `print filehandle LIST` stems (IIRC) from the "indirect object notation", like `$q = new CGI;` #(no comma) which would, in "direct" object notation read: `$q = CGI->new()` From this point of view, the file example might be equivalent to `STDOUT->print("Hello");` #, which indeed is the case. The above idioms can be used in Perl after including the IO::Handle module, then it'll look like (pseudocode): `# from http://perldoc.perl.org/IO/Handle.html $io = new IO::Handle; # indirect object notation +! if ($io->fdopen(fileno(STDOUT),"w")) { $io->print("Hello"); # direct object notation! }` [download] See: IO::Handle documentation Regards mwa	[reply] [d/l] [select]


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