Re: APL Christmas 2020 doodle: Tree

[Kacper Gutowski]

On Wed, Dec 23, 2020 at 10:46:13PM -0800, Russtopia wrote:
> [Dyalog]
> 9 2 9/↑'∘⌹∘' ' ⌹ '
> ∘∘∘∘∘∘∘∘∘⌹⌹∘∘∘∘∘∘∘∘∘
>          ⌹⌹
>
> [GNU]
> 9 2 9/↑'∘⌹∘' ' ⌹ '
> ∘∘∘∘∘∘∘∘∘⌹⌹∘∘∘∘∘∘∘∘∘
>
> Is it unexpected that GNU APL does not apply the Compress (/) across 
> multiple right-hand items?

There are some subtle differences in how / works in Dyalog and in GNU APL, 
but here the reason is much more prosaic.

By default the monadic ↑ and ⊃ are swapped in Dyalog.  It has a setting 
called ⎕ML that controls it, and if you set ⎕ML←2, it will interpret 
this expression the same way GNU APL does:

      9 2 9/⊃'∘⌹∘' ' ⌹ '
∘∘∘∘∘∘∘∘∘⌹⌹∘∘∘∘∘∘∘∘∘
         ⌹⌹         


-k