Re: for all(@foo) {...}

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Larry Wall

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Apr 24, 2005, 1:29:20 AM4/24/05
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On Sun, Apr 24, 2005 at 03:02:16PM +1000, Brad Bowman wrote:
: Hi,
:
: I'm trying to understand the following section in S03:
:
: S03/"Junctive operators"
:
: Junctions are specifically unordered. So if you say
: for all(@foo) {...}
: it indicates to the compiler that there is no coupling between loop
: iterations and they can be run in any order or even in parallel.
:
: Is this a "for" on a one element list, which happens to
: be a junction, or does the all() flatten?

No, S03 is probably just wrong there. Junctions are scalar values, and
don't flatten in list context. Maybe we need something like:

for =all(@foo) {...}

to iterate the junction.

: Is the whole block run once with 1,2 and 3, or does the
: junction go into the block and autothread each operation?

I expect =all(@foo) would do the former, while all(@foo) would do
the latter, in which case you might as well have used "given" instead.

: for all(1,2,3) {
: next if $_ < 2; # testing 1 or all(1,2,3) ?
: %got{$_} = 1;
: }
: say %got.perl; # "(('2', 1), ('3', 1))" or "()" ?

Well, { 2 => 1, 3 => 1 } is the more likely notation.

: The "no coupling" in s03 suggests to me that the right
: answer is "(('2', 1), ('3', 1))", but I'm just guessing.

I think =all(@foo) should do what you expect there. Without the =
it should return { 1 => 1, 2 => 1, 3 => 1 } since there's only one
loop iteration, and it is *not* true that all(1,2,3) < 2. If you'd
said

for any(1,2,3) {...}

then it would have done the "next", because 1 < 2.

I should say that I don't see that =all() is different from =any().
They each just produce a list in "random" order. Though I suppose,
if we say that =one(1,2,3) should randomly pick one value, then
=any(1,2,3) should pick anywhere from 1 to 3 values. And, of course,
=none(1,2,3) should return a list of all the things that aren't 1, 2,
or 3 in random order. Maybe a lazy implementation will be beneficial
at that point. :-)

Larry

Larry Wall

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Apr 24, 2005, 1:44:47 AM4/24/05
to perl6-l...@perl.org
On Sat, Apr 23, 2005 at 10:29:20PM -0700, Larry Wall wrote:

: On Sun, Apr 24, 2005 at 03:02:16PM +1000, Brad Bowman wrote:
: : Hi,
: :
: : I'm trying to understand the following section in S03:
: :
: : S03/"Junctive operators"
: :
: : Junctions are specifically unordered. So if you say
: : for all(@foo) {...}
: : it indicates to the compiler that there is no coupling between loop
: : iterations and they can be run in any order or even in parallel.
: :
: : Is this a "for" on a one element list, which happens to
: : be a junction, or does the all() flatten?
:
: No, S03 is probably just wrong there. Junctions are scalar values, and
: don't flatten in list context. Maybe we need something like:
:
: for =all(@foo) {...}
:
: to iterate the junction.

For the purposes of S03 it would have been better to use an example
without list context like:

-> $x {...}(all(@foo))

or maybe

all(@foo).each:{...}

I think that "given" specifically does not autothread it's block, so

given any(1,2,3) {...}

matches each case against any(1,2,3). That is, there is an MMD variant
of "given" that accepts a Junction, which disables autothreading.

Or maybe S03 really just wants to say that

if all(@foo).each:{...} {...}

is allowed to stop evaluating cases in "random" order when it gets
the first false value back from .each, while

if any(@foo).each:{...} {...}

is allowed to stop as soon as it gets a true value.

Larry

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