single element lists
Jonathan Scott Duff
We're discussing the proper semantics of (1)[0] on #perl6. Here's
where we're at so far:
1. specialise ()[] to parse as (,)[]
2. scalars are singleton lists, so ()[] naturally
3. make (1)[0] die horribly.
We all seem to agree that #3 is least useful and probably wrong. But
there's a divide between whether #1 or #2 is the "right" behavior.
#2 implies that (1)[0][0][0][0] == 1
#1 means that (1)[0] == 1 and (1)[0][0] is an error
FWIW, I'm in favor of #1
What does p6l think? (What does @Larry think?)
-Scott
--
Jonathan Scott Duff
du...@pobox.com
Juerd
> 1. specialise ()[] to parse as (,)[]
> 2. scalars are singleton lists, so ()[] naturally
> 3. make (1)[0] die horribly.
> #1 means that (1)[0] == 1 and (1)[0][0] is an error
#1 also means that ($aref)[0] is $aref, rather than $aref[0].
I pick #2, also because I think being able to pass scalars where arrays
are expected simplifies other parts of the language as well.
Juerd
--
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Thomas Sandlaß
> What does p6l think? (What does @Larry think?)
I favor #3 as syntax error.
But note $TSa == all( none(@Larry), one($p6l) ) or so :)
--
TSa (Thomas Sandlaß)
Aaron Sherman
> We're discussing the proper semantics of (1)[0] on #perl6. Here's
> where we're at so far:
>
> 1. specialise ()[] to parse as (,)[]
> 2. scalars are singleton lists, so ()[] naturally
> 3. make (1)[0] die horribly.
It may or may not help, but I direct your attention to:
--
Aaron Sherman <a...@ajs.com>
Senior Systems Engineer and Toolsmith
"It's the sound of a satellite saying, 'get me down!'" -Shriekback
Craig DeForest
to 1. The artificial distinction between a scalar and an array of
length 1 (in each dimension) is the source of endless hassles, and it's
a pretty simple DWIM to allow indexing of element 0 of any unused
dimension. That makes it much, much easier to write code that
generalizes, because you don't have to check for the scalar case -- you
can always assume that any given dimensional axis will always have one
value along it (*), and loop/thread/whatever along that dimension.
(*) Of course, that's assuming that there's at least one value in the
list as a whole -- PDL does, and perl 6 should, support zero-element
lists; any looping or threading construct on a zero-element list is a no-op.
Rob Kinyon
> Jonathan Scott Duff skribis 2005-05-11 11:45 (-0500):
> > 1. specialise ()[] to parse as (,)[]
> > 2. scalars are singleton lists, so ()[] naturally
> > 3. make (1)[0] die horribly.
> > #2 implies that (1)[0][0][0][0] == 1
> > #1 means that (1)[0] == 1 and (1)[0][0] is an error
>
> #1 also means that ($aref)[0] is $aref, rather than $aref[0].
>
> I pick #2, also because I think being able to pass scalars where arrays
> are expected simplifies other parts of the language as well.
#2 is also more intuitive and comfortable. () in P5 constructs a list,
which is why (split ' ', $foo)[0] works so nicely.
Autrijus Tang
> On Wed, May 11, 2005 at 11:45:12AM -0500, Jonathan Scott Duff wrote:
> :
> : We're discussing the proper semantics of (1)[0] on #perl6. Here's
> : where we're at so far:
> :
> : 1. specialise ()[] to parse as (,)[]
> : 2. scalars are singleton lists, so ()[] naturally
> : 3. make (1)[0] die horribly.
> I think it has to be #1. We can't have (@foo)[0] working at two different
> indirection levels depending on whether @foo == 1.
Hm? Under #2, no matter whether @foo is (1) or (1,2), the construct
(@foo)[0] would always means @foo.[0]. Not sure how the length of @foo
matters here.
Thanks,
/Autrijus/
Larry Wall
:
I think it has to be #1. We can't have (@foo)[0] working at two different
indirection levels depending on whether @foo == 1.
Larry
rottcodd
postcircumfix[] ? Does ($f)[0] == $f or ($f)[0] == $f[0] ? What about
($f1 + $f2)[0] ? (Assuming the vector-like class also overloads +) If
($f1 + $f2)[0] == $f1 + $f2, how would you get the other meaning?
Larry Wall
: Hm? Under #2, no matter whether @foo is (1) or (1,2), the construct
: (@foo)[0] would always means @foo.[0]. Not sure how the length of @foo
: matters here.
Tell you what, let's require P5's (...)[] to be translated to [...][],
so (...)[] should assume scalar context that will return some kind of
array reference. (What Luke said about (1,(2,3),4)[] still holds, though.
Commas create lists, and lists by default impose list context, and
parens are only for grouping in lists, not scalarifiying.)
Larry
Autrijus Tang
Sure (and done). Now that #1 is eliminated, the question is now
whether a simple scalar can be treated as a small (one-element) array
reference, much like a simple pair can be treated as a small
(one-element) hash reference.
1.[0]; # evaluates to 1?
If yes, then (1)[0] means the same as 1.[0] and 1.[0][0][0]. If no,
(1)[0] is a runtime error just like 1.[0] -- i.e. unable to find the
matching .[] multisub under Int or its superclasses.
Thanks,
/Autrijus/
Larry Wall
: Sure (and done). Now that #1 is eliminated, the question is now
: whether a simple scalar can be treated as a small (one-element) array
: reference, much like a simple pair can be treated as a small
: (one-element) hash reference.
:
: 1.[0]; # evaluates to 1?
:
: If yes, then (1)[0] means the same as 1.[0] and 1.[0][0][0]. If no,
: (1)[0] is a runtime error just like 1.[0] -- i.e. unable to find the
: matching .[] multisub under Int or its superclasses.
Maybe we should just let someone poke a Subscriptable role into some
class or other to determine the behavior if they care.
Larry
Wolverian
> where we're at so far:
>
> 1. specialise ()[] to parse as (,)[]
> 2. scalars are singleton lists, so ()[] naturally
> 3. make (1)[0] die horribly.
(1)[0] means 1[0], which is probably undefined, so it dies. That could
be detected at compile time. (my $foo = 1)[0] means $foo[0], which would
die at runtime, unless there's type inference going on. In any case, I
don't see a List in ()[] without a list-creating expression in the ().
> -Scott
--
wolverian
James Mastros
> : If yes, then (1)[0] means the same as 1.[0] and 1.[0][0][0]. If no,
> : (1)[0] is a runtime error just like 1.[0] -- i.e. unable to find the
> : matching .[] multisub under Int or its superclasses.
>
> Maybe we should just let someone poke a Subscriptable role into some
> class or other to determine the behavior if they care.
]»(Whatever $self: Int $index) ? (I'd hope the error message is a bit
more newbie-friendly, but that's the only special-casing I see it
needing...)
-=- James Mastros
Larry Wall
Different abstraction level. Maybe defining the postcircumfix is
exactly what the role does.
Larry