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Re: === and array-refs

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From:
Audrey Tang
Date:
August 17, 2006 20:53
Subject:
Re: === and array-refs
Message ID:
6FEE40C3-E86C-4364-A99F-4E002896BB47@audreyt.org

在 2006/8/18 上午 3:31 時，Ben Morrow 寫到：
> Just to make sure I've got all this straight:
>
>     =:= compares names
>     === compares containers
>     eqv compares values

=:= evaluates both sides as lvalue -- that's VAR() -- and compare  
them with ===.
=== evaluates both sides as rvalue and, for container (mutable)  
types, compares their pointer address.
eqv evaluates both sides as rvalue and, for container (mutable)  
types, compares their content values.

None of the three coerces their arguments to concatenated List (aka  
list flattening, aka slurpy context).

> So given an array @W,
>
>     my @X := @W;            # @X =:= @W
>     my @Y =  @W;            # @Y === @W but @Y !=:= @W
>     my @Z =  @W.clone;      # @Z eqv @W but @Z !=== @W

Your Array example would be correct with the $ sigil:

     my $w = [1,2,3,4]; # Scalar containing Array
     my $x := $w;       # $x =:= $w
     my $y =  $w;       # $y === $w but $y !=:= $w
     my $z =  $w.clone; # $z eqv $w but $z !=== $w

However, &infix:<=> when its left-hand side is an array, has an  
different signature
(and indeed, different precedence) than when its left-hand side is a  
scalar:

     my $y = $w;       # does not flatten $w
     my @Y = @W;       # does flatten @w, essentially doing a .clone
     my @Z = @W.clone; # same as "my @Z = @W" really.

So the two assignments (@Y and @Z) above turns out to be the same  
thing, and neither
will make "===" hold afterwards.

Cheers,
Audrey
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