#4567 gnuplot gives obscure Lisp error when gnuplot_command is not a string

[Wolfgang Dautermann]

Dear all,
When one outputs a variable, there is no 'type' printed - is there a (native) function to query the type? Yes, e.g numberp(), for numbers, but in general?

E.g. I query gnuplot_command, then I set it to the same value:

(%i1) gnuplot_command;
(%o1)                               gnuplot
(%i2) gnuplot_command:gnuplot;
(%o2)                               gnuplot
(%i3) plot2d(x,[x,1,2]);

Maxima encountered a Lisp error:

 The value
   $GNUPLOT
 is not of type
   (OR STRING PATHNAME SYNONYM-STREAM FILE-STREAM)
 when binding PATHNAME

Automatically continuing.
To enable the Lisp debugger set *debugger-hook* to nil.

That did not work, the variable is now of type symbol, not longer a string. But if I output the variable, I get not idea, if it is a string or symbol. It must be set enclosed in quotation marks, then it works.

(%i4) gnuplot_command:"gnuplot";
(%o4)                               gnuplot
(%i5) plot2d(x,[x,1,2]); /* works ok */

Is there any (native) function or output (typeof() or something else?) where one can get an idea, whats wrong?
Because the OUTPUT of gnuplot_command is the same in both cases. (Maybe it should be "gnuplot" in the string case)?

I found :lisp (type-of $a) but there is no Maxima equivalent?
Best regards, Wolfgang

[Stavros Macrakis]

The error you're getting is not about the variable gnuplot, but about the value of gnuplot_command. Neither Maxima nor Lisp supports the notion of a variable type.

The bug here is that Maxima is giving an obscure internal Lisp error, rather than a meaningful Maxima error. Since it is a Lisp error, it doesn't look at stringdisp when displaying the value.

A reasonable Maxima error message would be something like: "gnuplot_command is 'gnuplot', which is not a valid pathname string" or something.

[Robert Dodier]

Couple of things here. (1) plot2d/plot3d should try harder to get a string -- probably just call $string (via mfuncall since it's a Maxima defmspec) from the value of gnuplot_command.

(2) plot2d/plot3d should, of course, catch any Lisp errors and create a Maxima error instead.

[Stavros Macrakis]

I disagree that we should call $string on its arguments.
At most, accept either a string or a symbol.

Behavior like this helps no one (and should be fixed in load, too):

load(a*b);

Maxima encountered a Lisp error:

In function STRING, the value of the first argument is
((MTIMES SIMP (0 "stdin" SRC)) $A $B)
which is not of the expected type STRING

Tested in
Maxima on Android 3.2.1 September 7, 2018
Maxima 5.41.0 http://maxima.sourceforge.net
using Lisp ECL 16.1.3

[Raymond Toy]

5.41 is really old, isn't it? With current(ish) HEAD, I get:
(%i8) load(a*b); load: argument must be a string, symbol, or pathname; found: a b -- an error. To debug this try: debugmode(true);
Hmm. How do you get a Lisp pathname in Maxima? Curiously $load does actually check if the arg is pathnamep.

[Stavros Macrakis]

Agreed that 5.41 is carbon dated (from 2018!), but it's the latest version of Maxima On Android as far as I can tell.

Does anyone know how to build Maxima on Android with a more recent version?

[Jaime E. Villate]

On 17/06/25 07:22, Stavros Macrakis wrote:

Agreed that 5.41 is carbon dated (from 2018!)

More precisely, the release date of 5.41.0 was 2017-10-03.

Jaime

[Stavros Macrakis]

I was misled by the startup banner, which reads

Maxima on Android 3.2.1 September 7, 2018
Maxima 5.41.0 http://maxima.sourceforge.net

I guess September 7, 2018 is the build date of MaximaOnAndroid.

Which brings up back to the original question: does anyone know how to build MoA from Maxima source?

[Raymond Toy]

I don't know how to build it, but this appears to be the repo for it: https://github.com/YasuakiHonda/Maxima-on-Android-AS

I would certainly like an update as well. Even though I rarely use it, it comes in very handy sometimes!

[Raymond Toy]

Having stringdisp set to true makes the output from gnuplot_command clearer that a string is expected.

We could also change the definition of $gnuplot_command from a defvar to a defmvar and use the :setting-predicate option to make sure it only accepts a string.

If we do this, we should also do the same for other user-visible variables like $mgnuplot_command, $geomview_command, $xmaxima_plot_command at the very least. I see there are other user-variables that should probably be a string.

[Raymond Toy]

Unless there are objections, I'm going to make $gnuploat_command and related variables only accept strings. Everything else will produce a Maxima error.

[Raymond Toy]

Fixed by commit [c2af73] which forces $gnuplot_command t o be a string.

[Wolfgang Dautermann]

Thank you!