in reply to Nested Hash Dereferencing Syntax

Perl sees %$testhash, but doesn't know of any $testhash. You want %{ $testhash{ "Tier 1" } }

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Re^2: Nested Hash Dereferencing Syntax
by jodv (Initiate) on Feb 24, 2009 at 17:31 UTC
    Nice. I tried parens, but not curlies, in my attempt to make sure Perl was evaluating the value of $testhash{ "Tier 1" } before trying to dereference.
    Do you know why curlies, and not parens, are necessary here? Just curious.
    Thanks for your help!
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      Here is an explanation of why parens cannot be used there.

      If you could write %($hash{$key}) then you should expect to be able to write %($hash{$key}){$other_key}. But perlvar tells us that $( is a built-in special variable, so correctly knowing how to parse that would be very, very hard.

      Incidentally %( is a valid Perl hash, though it is not used for anything and accessing it can get tricky. But the following is valid Perl: 

      %( = 0..9;
print keys(%();

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        I thought of that, but dismissed it. He could have named $( something different if he wanted to use parens for variables, or did "${var}" come later? I figured it had been in Perl from the start due to its parallel in bourne.
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      Do you know why curlies, and not parens, are necessary here?

      Because parentheses only ever group things, never performing any nesting or scoping or accessing.

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      No, I don't know why. Curlies are always used where are variables name is expected. 
      my $x = ${$hash_ref}{key};  # Original syntax
my $x = $hash_ref->{key};   # Newer, cleaner syntax

print($foo, "bar\n");
print("${foo}bar\n");       # same
print("$foobar\n");         # XXX

my $x = ${"varname"};       # Symbolic reference

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      etc.

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      It strikes me that curlies (or braces) are used for legacy reasons given that the provenance of the language is the *NIX shell (see, to name but one, Perl).

      As we all know, *NIX shell uses braces in references to environment variables - when and only when the variable isn't followed by a printable, non-word boundary char can the braces be omitted e.g. ${foo}_bar is the var foo with _bar appended, whereas $foo_bar refers to a variable named foo_bar.

      This (string interpolation) is true for both shell and perl, but interestingly (I really ought to get out more:-) not in make(1) scripts - which can take braces or parens as separators.

      Update:

      Thanx to tilly for the hint.

      A user level that continues to overstate my experience :-))
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        I don't understand what you mean by saying that Perl is like shell in this regard unless you are talking about string interpolation.
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