slurping __DATA__


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slurping DATA

by LanceDeeply (Chaplain)

on Aug 08, 2003 at 14:11 UTC ( #282185=perlquestion: print w/ replies, xml )

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

I know this code:

my $data = do { local $/; <DATA> };
[download]

slurps the __DATA__ section into my scalar $data, but what is perl doing in that do loop?

    my $data = do
    {
        local $/; # localize RS?
        <DATA>    # evaluate <DATA> in scalar context ?
    }
[download]

what's happenning in there?

Comment on slurping __DATA__ Select or Download Code
Re: slurping __DATA__ by Zeroth (Beadle) on Aug 08, 2003 at 14:17 UTC
See: `perldoc -f do` And read the part about "do BLOCK". That should explain it quite well.	[reply] [d/l]
Re: slurping __DATA__ by jeffa (Chancellor) on Aug 08, 2003 at 14:17 UTC
do is "not really a function". It is a block that has a seperate scope from wherever it was called from. This allows you to localize `$/` so that when execution leaves our "temporary" scope, `$/` will be what it was before it was changed. If you look up `$/` in perlvar, you will see You may set it to a multi-character string to match a multi-character terminator, or to "undef" to read through the end of file. Finally, `<DATA>` returns the next line from the filehandle `DATA`, and since `$/` is undefined, the entire contents of the file are returned and "caught" in a scalar. UPDATE: i should add that this is one of those few cases where local is a good choice. For example, `local $/;` [download] Is essentially the same as `local $/ = undef;` [download] Which is not the same as `$/ = undef;` [download] Even though that last snippet will be contained inside a `do` block ... `$/` will still be `undef` afterwards. Try this - make two files `foo.txt` and `bar.txt` and run this: Read more... (500 Bytes) jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l] [select]
Re: Re: slurping __DATA__ by LanceDeeply (Chaplain) on Aug 08, 2003 at 14:44 UTC
Thanks! I did read perldoc perlvar but I didnt know that `local $/;` was `undef`-ing it for the `do`'s block context. I'll have to go brush up on `local`, 'cause I never use it.	[reply] [d/l] [select]
3Re: slurping __DATA__ by jeffa (Chancellor) on Aug 08, 2003 at 14:49 UTC
For the record, tye was the first one to show me that trick. (tye++ yet again) But .... please be careful with `local` - reserve its use for temporarily resetting Perl's built-in variables. Don't use it on, well, anything else. ;) Read Anti-snippet (or, "local" considered dangerous to fools") if you haven't already. And you are most welcome. :) jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply]

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