In the equation #2HI(l) -> H_2(g) + I_2(s)#, what are the reactants?
As written, hydroiodic acid is the reactant.
Of course, this is an equilibrium reaction:
I could equally write,
in which we would regard the elements as the reactants.
Which side you designate as reactants depends on whether you are interested the DECOMPOSITION of hydrogen iodide, or the FORMATION of hydrogen iodide from its elements.