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Re: Transparent / Opaque references

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From:
Juerd
Date:
May 27, 2005 16:15
Subject:
Re: Transparent / Opaque references
Message ID:
20050527231504.GA31996@c4.convolution.nl
Luke Palmer skribis 2005-05-27 20:59 (+0000):
> Opaque references always need to be explicitly dereferenced (except
> for binding an array to an array reference, etc.).  Transparent
> references always automatically dereference.  The decision of what
> type of dereferencing will go on is left up to the reference taker.

The way I see things, an opaque reference is a value, while a
transparent reference is a name. And thus we already have both.

The name $foo points to a container, and so does \bar. However, to get
at the container using $foo, you use simply $foo. To get at the
container using the reference to bar, you have to dereference explicitly
with another $.

To bind a name (and thus have a transparent reference), we can either
declare the name, creating the variable at that point, or use some other
means of creating the variable, and use := to bind it later.

There are named arrays, @foo, and anonymous arrays, [].

There are named hashes, %foo, and anonymous hashes, {}.

There are only anonymous pairs. You can't dereference a pair, or bind a
name to it. 

> What I can't decide is which one \ will create, and how you will
> create the other kind.  Also, I can't decide how to one-level
> dereference the transparent references so that you can change them.

The only real problem with having only infix := for binding, is that you
can't easily use an alias (aka transparent reference) in a list. You can
have an array of aliases, but it's harder to have an array or hash in
which one element is an alias. Binding can be done explicitly:

    %hash = { key => undef, foo => 'bar' };
    %hash<key> := $variable;
    %hash<key> = 5;  # $variable is now 5 too

But there is no way to set the transparent reference (aka alias)
initially, because we lack a \-like syntax.

So I propose that := becomes a prefix operator as well as an infix one,
allowing:

    %hash = { key => := $variable, foo => 'bar' };
    %hash<key> = 5;  # $variable is now 5 too

(This almost makes \ want to be \=, and $foo \= $bar to be what we now
write as $foo = \$bar, and $foo = := $bar to be the same as $foo :=
$bar, and stacked :=s to be irrelevant... But let's not think about this
too much...)


Juerd
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