Re: [Maxima-discuss] Deleting hash arrays

[Michel T. <ta...@lp...>]

Le 25/01/2026 à 15:07, Barton Willis via Maxima-discuss a écrit :
> About when 'local' is needed,  maybe somebody else can help with this.
>
>  The user documentation for 'local'  waffles and  says "|:=|, |array|, 
> |dependencies|, |atvalue|, |matchdeclare|, |atomgrad|, |constant|, 
> |nonscalar|, |assume|, and some others" (the "some others" being 
> waffle-like).
>

I think you have to contrast the documentation for block and the one for 
local.  For block you have:

      If these variables are
      already bound 'block' saves the current values of the variables
      <v_1>, ..., <v_m> (if any) upon entry to the block, then unbinds
      the variables so that they evaluate to themselves; The local
      variables may be bound to arbitrary values within the block but
      when the block is exited the saved values are restored, and the
      values assigned within the block are lost.


The important word here is *values*.  So you see that in the declaration 
block([v],....) the symbol v

is untouched but only the value associated to this symbol is 
manipulated. But for local you get:


      Saves the properties associated with the symbols <v_1>, ..., <v_n>,
      removes any properties before evaluating other expressions, and
      restores any saved properties on exit from the block or other
      compound expression in which 'local' appears.


Here the important word is *properties*. Of course one needs to 
understand that a symbol has several

things attached to it, particularly a value, a function slot and a 
property list. Here local manipulates the property list.

The first example given is the one of lisp function definition, obtained 
by :=   Example:

(%i1) f(x):=x^2;
                                            2
(%o1)                             f(x) := x
(%i2) :lisp(symbol-plist '$f)

(MPROPS (NIL MEXPR ((LAMBDA) ((MLIST) $X) ((MEXPT) $X 2))))


As you can see the expression for the function f appears in the 
property-list of f, but not in the value or the function slot of f. If 
you compile f then the resulting function object appears in the function 
slot.


(%i2) compile(f);
(%o2)                                 [f]
(%i3) :lisp(symbol-function '$f)

#<FUNCTION $F>

Finally block([...] ...) isolates values inside the block, but not 
functions, while local() isolates maxima

functions inside the block, but not compiled functions. All other 
declarations which occur through the property list

behave the same as function definitions. I don't know the complete list 
of such situations, the doc above gives a partial list. Example:


(%i4) matchdeclare(b,any);
(%o4)                                done
(%i5) :lisp(symbol-plist '$b)

(MPROPS (NIL MATCHDECLARE ($ANY)))


There are other "properties" however that are not included in the 
property list:


(%i3) tellrat(a^2-2);
                                      2
(%o3)                              [a  - 2]
(%i4) :lisp(symbol-plist '$a)

NIL


So one has to do some tests to be sure.




-- 
Michel Talon