The three-item form is used if the associated function is ambivalent (applied
to the P-cells of ⍵ if monadic, applied to corresponding Q-cells of ⍺ and
R-cells of ⍵ if dyadic). I don't believe it is possible to define ambivalent
functions in ISO APL however, so it is kind of redundant. It is probably left
over from Sharp APL.
As to the negative rank, I believe that it is an obvious flaw in the standard.
Being able to apply a function to the items of the argument is incredibly
useful. Of course this is achievable like so:
{⍺ (f ⍤ (¯1+≢⍴⍺⍵)) ⍵} ⍝ without ⍺ for the monadic form
Now that I've checked, I'm pretty sure that GNU APL does support rank (negative
or not), but not with all primitives. Try it out with ⊖ or +⌿ on a rank-3 or
above array, and then with ]BOXING and ⊂ on the same array… looks like a bug to
me.
The rank operator isn't easy to grasp, and it's surely harder to implement.
Good luck,
Louis
> On 27 Apr 2016, at 13:14, Jay Foad <jay.f...@gmail.com> wrote:
>
> Incidentally, it works like this in Dyalog and NARS2000 too, though
> the Dyalog documentation doesn't mention the 3-item form.
>
> Jay.
>
> On 27 April 2016 at 09:02, Jay Foad <jay.f...@gmail.com> wrote:
>> Given g ← f⍤P Q R:
>> P is the monadic rank
>> Q is the left rank
>> R is the right rank
>>
>> So:
>> g Y applies g to the P-cells of Y
>> X g Y applies g to the Q-cells of X and the R-cells of Y
>>
>> The ⌽3⍴⌽y1 stuff is just a too-cute way of saying that you can specify
>> fewer than 3 values in the right operand, and:
>> R is shorthand for R R R
>> Q R is shorthand for R Q R
>
