What is the adequate alternative remedy principle?

The adequate alternative remedy principle was fully discussed in Harelkin v. University of Regina, 1979 CanLII 18 (SCC), [1979] 2 S.C.R. 561, at p. 586, where Beetz J., for the majority, held at p. 576 that "even in cases involving lack of jurisdiction", the prerogative writs maintain their discretionary nature. Dickson J. (as he then was, dissenting), took a narrower view of discretion in the case of jurisdictional error (pp. 608-9). He nevertheless concluded, at p. 610, that where a jurisdictional error "derives from a misinterpretation of a statute, a statutory right of appeal may well be adequate". ...