in my object, I have few private variables. To make sure the object is correctly initialized between each use, I'd like to initialize these private variables each time at the start of the call each time the object is called to process something.

Some of these variables represent equations, or symbols or functions. Some of these are set by the user of the object, but some are not. And I do not want to leave those that are not set, using old values from last call.

The problem  I can't use {} nor NULL to initialized these private variables, since I am telling Maple they are of type `=` or `symbol` or `function`.

To give a small example, I use a proc here to illustrate, since it the same idea, but less code.

restart;
kernelopts('assertlevel'=2):
foo:=proc(input_ode::`=`)
   local ode::`=`:={};  #i'd like to give this some initial value. But what?
   ode:=input_ode;
end proc;

now foo(diff(y(x),x)=5) gives

Error, (in foo) assertion failed in assignment, expected `=`, got {}
Same type of error happens if I use NULL.

Is there a special value or tag one can use to initialize/reset any variable, regardless of its type? I thought at first that NULL will work, but it is not.

The idea is that I'd like all my object private variable to always have known state when the same object is called each time. ie. to clean it up before the next time is made.

The above is just an example. Ofcourse I could just do this

restart;
kernelopts('assertlevel'=2):
foo:=proc(input_ode::`=`)
   local ode::`=`; #leave it un-initialized since it will be overwritten next
   ode:=input_ode;
end proc;

And now no error. 

As I am playing with Maple object to understand it better, I found strange thing.

I have simple object with 2 methods. Both are exported so they could be called from outside.

One method is called set_name and this one is called OK (I have print inside to print it is called). The next exported method is called process and when calling this method, the print never shows up. Which means it is not called. Using the same exact call.

When changing the name from process to something else, say processX now the print show, meaning the method is now called.

Is there restriction on what one can call their object method? And why would there be ?

Here is an example

restart;
module car_class()
      option object;
      local name::string:=""; 

      export process::static := proc(o::car_class)
             print("process method");             
      end proc:   

      export set_name::static := proc(o::car_class,_name::string)
             print("inside set name"); 
             o:-name := _name:
      end proc:     
end module:

my_car:=Object(car_class):  #make object

set_name(my_car,"toyota"):
process(my_car):
set_name(my_car,"toyota"):

On the screen, it only shows 

Now I changed it to 

restart;
module car_class()
      option object;
      local name::string:=""; 

      export processX::static := proc(o::car_class)
             print("process method");             
      end proc:   

      export set_name::static := proc(o::car_class,_name::string)
             print("inside set name"); 
             o:-name := _name:
      end proc:     
end module:

my_car:=Object(car_class):
set_name(my_car,"toyota"):
processX(my_car):
set_name(my_car,"toyota"):

And now on the screen it shows OK

Is there a way a way to be able to freely choose what method names to give to the object method? clearly the name "process"  is something special for Maple here for some reason.

When I do ?process, I see that Maple has some commands that have this name. But this is a name of a method inside Object class, so it should not have anything to do with any Maple own command. 

This was never an issue with module() for an example.  If one has to worry about what name to give to their methods for an object, then this will be very awkward. For example, what if one calls their method FOO and future version of Maple introduce a new command called FOO now the code will no longer work.

I wanted to create an Array() to store only objects of specific type in it.  

Array() supports the datatype=value  telling it the type of elements to store. But when I try to create an Array() using datatype of the class, it fails. I must be doing something wrong. Here is an example

restart;
module solution_class()
      option object;
      local sol::anything;
end module;
sol:=Object(solution_class); #create an object

Now the following works:

A:=Array();
A(1):=sol;

But I wanted to do

A:=Array(datatype=solution_class);
A(1) := sol;

 Error, unable to store '0' when datatype=module solution_class () local sol::anything; option object; end module
 

Help says 

And this is the case here, because type(sol,solution_class)  gives true

So 'solution_class' is valid type name.

Where is my error?

There is a whole chapter in programming guide on OO in Maple.

How much is OO actually used by Maple programmers? Is OO used much internally at Maplesoft itself in implementing internal code? I do not see questions about OO in Maple in the forum. May be it has not taken off? 

Any one knows of packages in Maple written in OO style, using the Object module? I think OO was added in Maple 16, which is 2012?

p.s. I myself like OO programming. Done alot of it in Java. I think it can be useful for large applications.

This proc is valid, yet Maple mint thinks there is syntax error

foo:= proc(x)
local z := "";
    if not x in ["A","B"] then
        return;
    fi;

    z:=cat("A
            B");    
end proc;

mint t.mpl gives

on line     7:     z:=cat("A
                          ^ Syntax error
A "then" was found without a previous matching "if".
An "end" may be needed to close the "in" construct from line 3.

But these two below procs below it gives no syntax error

foo:= proc(x)
local z := "";
    if not x in ["A","B"] then
        return;
    fi;  
end proc;

And

foo:= proc(x)
local z := "";
    z:=cat("A
            B");    
end proc;

No syntax error on either one. the syntax error only shows up when combining them. 

What does if not x in ["A","B"] then  have to do with the cat  below it? And why when they are combined, the syntax error shows?

I know of workaround, such as changing the cat to  z:=cat("A\nB");     or keep the strings on two lines, but do it like this

    z:=cat("A\n
           B");

Or write it like this

   z:=cat("A "
           "B"); 

But none of these changes were needed before adding the not x in ["A","B"]  before.

Is this a bug in mint? Is there a work around so the first example above still works using 

    z:=cat("A
            B");   

?