(overall context - working through TaPL on my own, reimplemnting typecheckers in haskell) the type checkers all follow the same pattern, in ocaml they throw an exception when the small step fails, which may mean taking another branch in the eval, but that that sub expression has hit bottom. it is self admittedly not good ocaml, and it seems to be even worse haskell, as i try to extend the simple evaluator i have to deal with managing reporting errors. having the single small step evaluator return a Maybe is fairly close. then the evaluator above it just bottoms out when eval1 expr returns Nothing, by passing expr back up as the result. but it occurs to me that it might be better to express it as an unfold, where the result is a list with the last element as the irresucible expression. or am i insane / intoxicated ?