I am getting confused between the type of mesomeric effect (+M or -M) shown by compounds containing $\ce-COX$ where $\ce X$ is an electronegative element. Examples: $\ce -CONHCH_3$, $\ce -COOCH_3$, $\ce -COOH$. According to me, all these compounds must show both +M as well as -M effect. Since the group (eg $\ce -COOH$) can accept as well as donate $\pi$ electrons (as in $\ce CH2=CH-COOH$). However, the book says these compounds show only -M effect. Please explain me the reason behind it.