For some historical background on this sort of issue, see Gabriel's 1988
article Technical Issues of Separation in Function Cells and Value
Cells<http://www.nhplace.com/kent/Papers/Technical-Issues.html>;,
which introduced the terminology Lisp-1 vs. Lisp-2.
-s
On Tue, Apr 26, 2011 at 13:08, Mark H Weaver <mhw at netris.org> wrote:
> Wilhelm Haager <wilhelm.haager at htlstp.ac.at> writes:
> > Is there any difference between subvar(A,i) and just A[i]?
>
> I know of at least one important difference, shown below:
>
> (%i2) robust_first_element(A) := subvar(A,1)$
> (%i3) brittle_first_element(A) := A[1]$
> (%i4) M[1] : 5$
> (%i5) robust_first_element(M);
> (%o5) 5
> (%i6) brittle_first_element(M);
> (%o6) 5
> (%i7) A[1] : 11$
> (%i8) robust_first_element(M);
> (%o8) 5
> (%i9) brittle_first_element(M);
> (%o9) 11
>
> As you can see here, both robust_first_element and brittle_first_element
> work the same way if there is no global array A[]. However, if A[]
> exists, then the A[1] in brittle_first_element will refer to the global
> array, and not to the local parameter A.
>
> The names of arrays, functions, and bare variables are each kept in
> their own namespace. For example, you can use a variable named "exp" to
> store an expression, and that does not conflict with the built-in exp(x)
> exponential function. You could also have an array exp[].
>
> In this case, the separation of namespaces implies that the A[1] within
> the definition of brittle_first_element does not refer to that
> function's formal parameter A, but instead the array A[].
>
> However, as a convenience feature, if the array A[] does not exist, then
> Maxima will try looking up A in the normal variable namespace.
> Unfortunately, this "convenience" feature encourages the creation of
> brittle code.
>
> subvar(A,1), on the other hand, evaluates A as a bare variable, before
> evaluating the array reference itself.
>
> Similarly, consider:
>
> (%i10) robust_assign_first_element(A, value) := arraymake(A,[1]) :: value$
> (%i11) brittle_assign_first_element(A, value) := A[1] : value$
> (%i12) A[1];
> (%o12) 11
> (%i13) M[1];
> (%o13) 5
> (%i14) robust_assign_first_element(M, 6);
> (%o14) 6
> (%i15) M[1];
> (%o15) 6
> (%i16) A[1];
> (%o16) 11
> (%i17) brittle_assign_first_element(M, 7);
> (%o17) 7
> (%i18) M[1];
> (%o18) 6
> (%i19) A[1];
> (%o19) 7
>
> Best,
> Mark
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