Question: Why doesn't the following simple code work with implicit procedure?

I'm not sure why I get an error in the following simple maple code:

test:=s->fsolve(t^3=s,t);     % Simplified implicit expression for illustration.      

test(8);  % (this yields the answer 2 - good)

plot(test(s),s=0..8);

Error, (in fsolve) s is in the equation, and is not solved for

Why do I receive this error?

Same error occurs with:

fdiff(test(s),s=1);

I'm a casual user of Maple and don't see why these commands are giving these errors. If I change fsolve to solve in the test procedure, test(8) yields the 3 cube roots, and the plot command gives a different error:

Error, (in plot) unexpected options: [-(1/2)*s^(1/3)+((1/2)*I)*3^(1/2)*s^(1/3), -(1/2)*s^(1/3)-((1/2)*I)*3^(1/2)*s^(1/3), s = 0 .. 8]

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