How to use Maxima's diff in function definition?

Question

I want to use a function's derivative in an other function. How should this be done in Maxima?

E.g:

f(x) := 2*x^4;
g(x) := diff(f(x),x)-8;

Now g(x) yields 8x^3-8 as expected, but g(0) gives an error, since diff(f(0),0) doesn't make sense. But then how should I properly define g?

Answer 1

Note that quote-quote is only understood when the code is parsed. That's OK if you only work in the interpreter but if you put stuff into scripts, it is possible to have unintended effects.

Another way to do this. It works the same in the interpreter and in a script.

define (g(x), diff (f(x), x) - 8);

See 'define'.