Calling Conventions and Pass By Reference
nothingmuch at woobling.org
Tue Sep 2 19:43:32 BST 2008
On Tue, Sep 02, 2008 at 19:18:38 +0100, Simon Wistow wrote:
> A thought - what would the advantages and disadvantages of having only
> references in a language.
I'm going to assume you mean low level referencing semantics (value
> The downside is that, of course, you can spooky actions at a distance -
> "I passed in this url to this library and it came back relative rather
> than absolute, wtf?".
The workaround for that is to have parameters be immutable aliases
by default, and only allow writing if explicitly asked for.
Similarly all dereferencing of a readonly value returns a read only
alias to its innards.
It's the symbol itself that is readonly, and not the data.
> I suppose then you'd have to have really good, COWed deep cloning
> available which is not a hugely difficult goal (In general, I'm not
> talking about whether or not it's hard to retrofit into an existing
I don't think so... The diff is usually that
my $x = $foo
becomes a binding, not an assignment. This is actually less work for
the compiler. (in perl every assignment involves a copy of some
structure into a new container)
Thins like this:
$foo = $foo . "bar";
no longer work in the same way.
For 90% of the cases this makes no difference, but does affect
my $x = "foo";
my $ref = \$x;
$x = $x . "bar";
$$ref; # still "foo"
but conversly you have:
my $x = 3;
my $y = $x;
$y; # 4
IIRC python works like that.
Yuval Kogman <nothingmuch at woobling.org>
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