Moggi proposed a monadic account of computational effects.
He also presented the computational lambda-calculus,
$\lambda_c$, a core call-by-value functional programming
language for effects; the effects themselves are
created and handled with the aid of additional {\em operations}.
The question arises as to whether one can give a corresponding general
treatment of operational semantics.
We do this in the case of {\em algebraic} operations,
which may be infinitary (an alternative is to use {\em generic
effects}). Algebraic operations seem to be precisely those which create
effects.
We consider call-by-value PCF with--and without--recursion,
an extension of $\lambda_c$ with arithmetic.
We prove general adequacy theorems, and illustrate these with
examples: nondeterminism, probabilistic nondeterminism,
interactive input/ouput, (global) state, and exceptions;
certain difficulties remain in the last two cases.