Re: [Bug-apl] First function with axis specification

[Frederick Pitts]

Hello Jürgen,

Why does your justification for not reporting AXIS ERROR's apply to monadic functions like sign(+), first (↑), factorial (!),

not(∼), etc, but not their dyadic partners, add, pick, binomial and with-out? For example,

      a ← 3 3 ⍴ ⍳ 9

      b ← 2 + a

      +[1] a

1 2 3

4 5 6

7 8 9

      a +[1] b

AXIS ERROR

      a+[1]b

      ^    ^

Not that I am suggesting you do it, the axis specification could be just as easily ignored for the add function as the sign function. To the contrary, I believe sign, first, factorial, not, and other monadic functions that do not support axis specifications should also report AXIS ERROR's. I'm confident doing so would benefit anyone trying to learn APL and those of us who occasionally get confused about what is and is not supported by the language.

Per your comments in item 3) of your email, I was able make monadic primitives that do not support axis specifications report SYNTAX_ERROR's if an axis-specification is applied to them. The only wrinkle was that I had to add methods eval_XB, eval_ALXb, eval_LRXB and eval_ALRXB to eval_XXX group of methods modified. And for consistency with how the dyadic function axis specification errors are reported, AXIS_ERROR should be used rather than SYNTAX_ERROR.

Regards,

On Wed, 2017-07-05 at 00:01 +0200, Juergen Sauermann wrote:

Hi Fred,

this is actually a language extension which has a deeper background.

I define a language extension to be a (different) behaviour of the interpreter when
the standard or the IBM language reference return an error. Sometimes extensions
are introduced or adopted from other interpreters (for example ⍳1 2 3 taken from
Dyalog) or because the language becomes more general.

So what you see is a language extension and the reason is this.

1. GNU APL supports axis arguments for defined functions. This is so that you can
write proper wrapper functions around APL primitives. Since defined functions can
not be overloaded you need a way to write a wrapper that can emulate the optional
axis argument of the wrapped apl primitive. I was always annoyed by not being able
to write such wrappers, in particular for benchmarking purposes.

2. Since the axis argument is optional in all apl primitives that can have one, the axis
is also optional in GNU APL defined functions. You can check for its presence with
⎕NC just like for optional left arguments.

3. Another concept in GNU APL is that there shall be as few differences as possible
between primitive functions (including ⎕-functions) and defined functions. A consequence
of that (which simplifies the parser a lot) is that also all primitive function can have
an axis argument. The default behaviour in both cases is to ignore the axis argument.
That is what you see for ↑[] but also for all other primitives that have no axis variant.
You can change that by changing e.g.

   virtual Token eval_AXB(Value_P A, Value_P X, Value_P B)
      { return eval_AB(A, B); }

to:

   virtual Token eval_AXB(Value_P A, Value_P X, Value_P B)
      { SYNTAX_ERROR; }


in all eval_XXX() functions declared in Function.hh that have an X (for axis) in their name. Since all functions are derived from this class it will change all primitives, but
also all defined functions and operators. I personally would not do it, though.

This language extension has been in GNU APL since almost day one. Therefore I
cannot predict how often it is used (you can also do some dirty tricks with it). For
that reason I believe it is better not to change it.

Best Regards,
Jürgen Sauermann

On 07/04/2017 07:42 PM, Frederick Pitts wrote:

Hello Jürgen,

	The IBM 'APL2 Programming: Language Reference' suggests that
axis specification is not allowed with the first function in that
'Figure 7. Functions and Operators That Allow Axis Specification', page
  45, does not list 'First' by the '↑' symbol, whereas ravel, catenate
and laminate are listed by the ',' symbol. The text under Figure 7 says
an AXIS ERROR occurs if a function or operator is not in the list.

	ISO/IEC 13751:2000(E)' contains 'Table 4: The Form Table',
pages 71-74, which claims to list all syntactic-units for which
evaluation sequences exist.  The first function with axis specification
 does not appear in that table.

	Yet Gnu-APL allows

	↑[ 2 ] 3 3 3 ⍳ 27
1

which is the first scalar in the ravel of the argument and the axis
specification is being ignored.

	Neither IBM or ISO specifications say that the first function
with axis specification is part of the language.  I respectfully submit
that Gnu-APL should report an attempt to use first function with axis
specification as a syntax error.

Regards,

Fred