Derivative as a function



[Robert Dodier <robert_dodier@yahoo.com>, Thu, 21 Oct 2004 22:34:17 -0700 (PDT)]:
>  (%i1) f(x) := x^2$
>  
>  (%i2) df(x) := ''(diff(f(x), x));
...
> Note that df(x) := ''diff(f(x),x)$ doesn't work.

I find this extremely surprising.  Can someone explain why $ does not
work, whereas ; does?  So far I thought that the only difference was
whether the result of the evaluation is displayed or not.

TIA,

Albert.