This is for imperative iterations. $\,\langle$ exp ${}_{1}$ $\rangle\,$ must be of bool type. The type of this expression is unit. This expression iterates the evaluation of $\,\langle$ exp ${}_{2}$ $\rangle\,$ while $\,\langle$ exp ${}_{1}$ $\rangle\,$ is evaluated to true. If $\,\langle$ exp ${}_{1}$ $\rangle\,$ is evaluated to false, it returns ().

19.17 While expression while ⟨exp1⟩ do ⟨exp2⟩

19.17 While expression while $\,\langle$ exp ${}_{1}$ $\rangle\,$ do $\,\langle$ exp ${}_{2}$ $\rangle\,$