Re: [Bug-apl] Dyadic / (replicate) does not work with ¨ (each)

[Juergen Sauermann]

Hi,

I have changed the parser to degrade / and friends from operator to 
function in the examples below.

One remaining case is (with / any of / ⌿ \ or ⍀):

S//B

where the left / is a function when S is a value and and an operator if 
S is a
function (which is not known at parse time). The binding rules do not 
help in this
case because there are different bindings when / is a function and when
/ is an operator. Both cases are valid. To deal with this special case,
one can put S in parentheses:

(S)//B

In that case S is assumed to be a value (and the left / is function 
compress).

See SVN 133.

/// Jürgen


On 02/15/2014 07:49 AM, Nick Lobachevsky wrote:
> The / and \ symbols (and their first coordinate versions ⌿ and ⍀) were
> overloaded from the start.  Not enough characters on the 2741 typeball
> or something.  Symbol pressure.
> First, they are the compression and expansion functions.
> 1 0 1 / 1 2 3
> Second, they are the reduction and scan operators.
> +/1 2 3
> This was well and good until the introduction of APL2 language
> extensions such as allowing user functions and functions other than
> the 21 scalar dyadic functions as the function argument to scan and
> reduction (plus/1 2 3).  Now use of / in the context of an operator
> argument results in a syntactic ambiguity.  For more info, check APL2
> binding strength.
>
> http://www.scribd.com/doc/124733639/APL2-Language-Reference-Manual
>
> There were some papers from Phil Benkhard at IBM.
>
> Normally, the workaround to this problem was for the programmer to
> supply a cover function for reduction, exactly as in your example.
> IBM APL2 and APL2000 would need this.  Dyalog, however, has got this
> to work, but not without consequences.  Consider the following:
>
>        //(1 0 1) (1 2 3)
> 1 3
>
> This would read as "compress reduction".  (In C++ and Java etc. it
> would be a comment)
>
> Dyalog also has a system function called []VFI, a kind of portmaneu of
> []VI and []FI, introduced by STSC in the 1960s, available today with
> APL2000.  []VI '1 2 3 cat' would return 1 1 1 0, the 1s marking valid
> numbers, while []FI '1 2 3 cat' would return 1 2 3 0, converting valid
> strings to numbers and invalid ones to zero.  A typical usage might be
> ([]VI x)/[]FI x.  Dyalog combined them together, such that []VFI '1 2
> 3 cat' would return a nested vector (1 1 1 0).  But I digress, but to
> show there actually exists a practical application.
>
>        //⎕VFI '1 2 3 cat'
> 1 2 3
>
> But now try
>
>        +///⎕VFI '1 2 3 cat'
> LENGTH ERROR
>
>        +/⊃(//)⎕VFI '1 2 3 cat'
>   1 2 3
>
> This would be a "plus reduction compress reduction", which also does
> not return the expected answer of 6.
>
>        +/⊃//⎕VFI '1 2 3 cat'
> LENGTH ERROR
>
>        +/⊃(//)⎕VFI '1 2 3 cat'
> 6
>
> This works in Dyalog, possibly some others.  Plus reduction reduce,
> and this works because of the (relatively) new dyadic reduction, 2+/1
> 2 3 4 5.
>
>        +//(2) (1 2 3 4 5)
>   3 5 7 9
>
> In short, to avoid this problem altogether, possibly to avoid
> introducing problems in the language parser, some implementations may
> disallow / and \ functions as operator arguments.