  We systematically study two-body nonleptonic decays of light lambda hyperon
$\Lambda \to p \pi^- (n\pi^0)$ with account for both short- and long-distance
effects. The short-distance effects are induced by five topologies of external
and internal weak $W^\pm$ exchange, while long-distance effects are saturated
by an inclusion of the so-called pole diagrams with intermediate
$\frac{1}{2}^+$ and $\frac{1}{2}^-$ baryon resonances. The contributions from
$\frac12^+$ resonances are calculated straightforwardly by account for nucleon
and $\Sigma$ baryons whereas the contributions from $\frac{1}{2}^-$ resonances
are calculated by using the well-known soft-pion theorem in the current-algebra
approach. It allows one to express the parity-violating $S$-wave amplitude in
terms of parity-conserving matrix elements. From our previous analysis of heavy
baryons we know that short-distance effects induced by internal topologies are
not suppressed in comparison with external $W$-exchange diagram and must be
included for description of data. Here, in the case of $\Lambda$ decays we
found that the contribution of external and internal $W$-exchange diagrams is
sizably suppressed, e.g., by one order of magnitude in comparison with data,
which are known with quite good accuracy. Pole diagrams play the major role to
get consistency with experiment.
