[Maxima] Maxima: lambdas and sublist_indices behaving strangely when acting on symbols

[Robert Dodier]

On 2012-05-14, Andy L <buggi22 at hotmail.com> wrote:

> I'm having trouble using sublist_indices on lists of symbols.
> Basically, I'm trying to implement a function that will find the
> indices at which a given symbol appears within a list of symbols.

> Can anyone explain why the following code should yield [1] instead of []?
>   f: lambda([l,x], sublist_indices(l, lambda([z], z = x)))$  f([x,y,z], a);  /* this yields [1] */

Well, I was going to go on about lexical versus dynamic scope and
Maxima's evaluation policy, but I think the origin of this problem is
that there is a bug in a function named 'maybe', which is called by
sublist_indices to evaluate the predicate. 'maybe' explicitly evaluates
its argument (incidentally contravening Maxima's general one-time
evaluation policy) and that causes the confusion you observed.
Fixing 'maybe' isn't so easy because (short answer) logical predicates
are not simplifying functions.

A simple workaround in this case is, I think, to name the argument x% or
something else which is unlikely to cause a name collision. Yes, that is
a fragile solution.

> The same function behaves as expected when the arguments are strings:
>   f(["x","y","z"], "a");  /* this yields [], as expected */

It works as expected with strings since a string can only evaluate to itself.

HTH

Robert Dodier