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Re^2: How to interpolate CONSTANTS in Here docs?

by CoVAX (Beadle)
on Feb 11, 2015 at 22:32 UTC ( #1116403=note: print w/replies, xml ) Need Help??


in reply to Re: How to interpolate CONSTANTS in Here docs?
in thread How to interpolate CONSTANTS in Here docs?

Thank you for your answer.

I'm not understanding the @{[]} syntax, although I do recognize the array index [], the hash {}, and the array @ -- can you explain what is going on, please?

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  • Comment on Re^2: How to interpolate CONSTANTS in Here docs?

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Re^3: How to interpolate CONSTANTS in Here docs?
by BrowserUk (Pope) on Feb 11, 2015 at 23:16 UTC

    Anonymonk has explained the syntax; but I thought I'd add that I'm not keen on doing it that way. Personally I prefer:

    #! perl -slw use strict; use constant { OUT_DIR => 'out', OUT_FIELDSEP => '=', OUT_FILENAME => 'ks', OUT_SUFXSTOR => 'storable', VAL_MIN => 15, VAL_MAX => 3600, }; printf <<USAGE, OUT_DIR, OUT_FILENAME, OUT_SUFXSTOR, OUT_FIELDSEP, VAL +_MIN, VAL_MAX; This program does this and that with the %s. The filename is %s and the file suffix is %s. Fields are separated with the %s character. Enter a value between %s and %s. USAGE

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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Re^3: How to interpolate CONSTANTS in Here docs?
by Anonymous Monk on Feb 11, 2015 at 22:49 UTC

    [...] builds a reference to an anonymous array, which is in turn de-referenced by @{...}. It's roundabout, but necessary in this case because the constants built by constant are actually functions, like sub OUT_DIR () { 'out' }, and a function call like OUT_DIR() (or OUT_DIR) isn't interpolated in strings, while scalars ($foo) and arrays (@bar) are, including dereferenced arrayrefs (@{$arrayref} - see e.g. Using References).

Re^3: How to interpolate CONSTANTS in Here docs?
by shmem (Chancellor) on Feb 14, 2015 at 22:09 UTC
    I'm not understanding the @{[]} syntax, although I do recognize the array index [], the hash {}, and the array @ -- can you explain what is going on, please?

    The array index isn't an array index, the hash isn't a hash.
    @{ } dereferences an array reference. ${ } dereferences a scalar reference. [ ] constructs an array refrence, \ constructs a scalar reference. So, instead of writing

    This program does this and that with the @{[OUT_DIR]}. The filename is @{[OUT_FILENAME]} and the file suffix is @{[OUT_SUFXSTOR]}. Fields are separated with the @{[OUT_FIELDSEP]} character. Enter a val +ue between @{[VAL_MIN]} and @{[VAL_MAX]}.

    as in BrowserUk's example, you could also write

    This program does this and that with the ${\OUT_DIR}. The filename is ${\OUT_FILENAME} and the file suffix is ${\OUT_SUFXSTOR}. Fields are separated with the ${\OUT_FIELDSEP} character. Enter a valu +e between ${\VAL_MIN} and ${\VAL_MAX}.

    The reference implementation in perl is the author's way of introducing ásbestos gélōs into the language, great fun and a good opportunity to distinguish yourself with expertise, once you grok cause and syntax.

    Quick, what does ~@~ mean?

    perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'
Re^3: How to interpolate CONSTANTS in Here docs?
by ikegami (Pope) on Feb 16, 2015 at 02:15 UTC

    If you spread it out,

    "foo @{[ F ]} bar"

    you get

    my @anon_array = F; my $anon_ref = \@anon_array; "foo @{ $anon_ref } bar"

    As you can see,

    • @BLOCK (@{ STATEMENTS }) is an array dereference, not "a hash".

    • [ LIST ] is an array constructor, not "an array index". It creates an array, assigns the result of LIST, and returns a reference to that array.

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