  We show how theoretical, principally lattice, calculations of the scalar form
factors in semileptonic pseudoscalar-to-pseudoscalar decays can be used to
extract information about the corresponding elastic s-wave scattering channels.
We find values for the scattering lengths m_pi a = 0.179(17)(14), 0.26(26) and
0.29(4) for elastic s-wave isospin-1/2 K pi, B pi and D pi channels
respectively. We also determine phase shifts. For the D K channel we find hints
that there is a bound state which can be identified with the recently
discovered D_{s0}^+(2317).
