On Sat, Feb 12, 2005 at 01:03:26AM -0600, Rod Adams wrote: > I also find the following incredibly disturbing: > >perl6 -e "$x = 'cat'|'dog'; say $x;" > dog > cat > > Getting iterated executions of a statement without explicitly iterating > it bothers me greatly. I work heavily in databases, where updating or > inserting twice with one call can be fatal to data consistency.
FWIW, I also find it incredibly disturbing. Although I don't have
to deal with it yet in the side-effect-free FP6, I think one way
to solve this is for the "say" to return a junction of IO actions.
Normally a statement-separating semicolon "launches" IO actions
obtained by evaluating the left-side statement, then moves on to
handle the right-side statement. For example, "say" will have
this signature:
multi sub say ([EMAIL PROTECTED]) returns IO of Bool { ... }
The "IO of Bool" can only be launched by a toplevel sequencing,
obtained by the destructive assignment:
my $a = say("xxx"); # output "xxx\n" on screen; $a is now a Bool
say("yyy"); # output "yyy\n" on screen; retval is discarded
but not via the nondestructive binding:
my $a := say("xxx"); # nothing outputted, $a is "IO of Bool"
$a; # this runs the action.
Your example then becomes:
$x := 'cat'|'dog'; # $x is now Junction of String
say $x; # Junction of (IO of String)
the toplevel sequencing only handles "IO of ..." types, so the junction
above will not print anything. Instead it may raise a warning of "using a
Junction in a void context", or something equally ominous.
(Haskell users will notice the equivalency of toplevel sequencing with
monadic I/O).
> So, if we are not having Sets, how exactly does one tell if what they
> are holding is a single value scalar, or a multi-value junction?
$x.isa("Junction").
> Can a junction hold values of completely different types, or just
> different values of the same type?
A junction typed as "Junction of Scalar" (as is the default) can
probably hold pretty much anything. There is also "Junction of Class"
and the amazingly weird "Junction of Any".
> If evaluation of one value of a junction causes an error, is $! now a
> junction as well?
I don't have answers for this one.
Thanks,
/Autrijus/
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