#421 substpart changes its argument in cmucl maxima

[Anonymous]

Paolo Nason, INFN, sez. di Milano
Universita' di Milano-Bicocca,
Dip. di Fisica "G. Occhialini",
Piazza della Scienza, 3
20126 Milano
Paolo.Nason@mib.infn.it

In cmucl maxima substpart has side effects:

(C26) lll:[1,2,3];
(D26) [1,2,3]

(C27) substpart(5,lll,2);
(D27) [1,5,3]

(C28) lll;
(D28) [1,5,3]

lll has been modified.

In gcl maxima:

(C1) lll:[1,2,3];
(D1) [1, 2, 3]

(C2) substpart(5,lll,2);
(D2) [1, 5, 3]

(C3) lll;
(D3) [1, 2, 3]
lll remains the same, as it should.

[Wolfgang Jenkner]

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Thanks for the bug report.

The problem seems to be that MPART (in comm.lisp) contains the form

(SETQ EXP (copy-all-levels EXP))

and COPY-ALL-LEVELS is defined (in maxmac.lisp) as

(DEFMACRO COPY-ALL-LEVELS (LIST)
#+(or cl NIL) `(COPY-TREE ,LIST)
#-(or lispm NIL) `(SUBST NIL NIL ,LIST))

So the form above does the same thing as

(setq exp (subst nil nil exp)). Now the CLHS says

If no changes are made [by SUBST or friends], the original tree
may be returned.

This explains why GCL behaves in a different way from CMUCL (SBCL and
CLISP behave like CMUCL).

Since COPY-ALL-LEVELS occurs only twice in the whole of Maxima and we
support only ANSI CL anyway, I think that we don't need such an
abstraction and we could just replace it by COPY-TREE. Objections?

[Wolfgang Jenkner]

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Patch committed.

[Raymond Toy]

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This doesn't happen in CVS.