In the course of updating a tutorial for this year's course, I found the provided code didn't compile. This is because a key value is "undefined" in order for the students to fill it in, but I'm curious why it can't be typechecked nonetheless. The code is: reachable :: (Ord q) => FSM q -> Set(Set(q)) -> Set(Set(q)) reachable fsm@(FSM qs as ts ss fs) supers = let new :: Set(Set(q)) -- typechecking fails without this declaration new = undefined in if null new then supers else reachable fsm (supers \/ new) The \/ in the last line is just Set.union. On the face of it, the last term directly forces "new" to have type Set(Set(q)), so why doesn't Haskell see that? Without the explicit type declaration of "new", we get: • Could not deduce (Foldable t0) arising from a use of ‘null’ from the context: Ord q bound by the type signature for: reachable :: forall q. Ord q => FSM q -> Set (Set q) -> Set (Set q) at CLTutorial9.hs:127:1-60 The type variable ‘t0’ is ambiguous These potential instances exist: instance Foldable (Either a) -- Defined in ‘Data.Foldable’ instance Foldable Set -- Defined in ‘Data.Set.Internal’ instance Foldable Maybe -- Defined in ‘Data.Foldable’ ...plus two others ...plus 27 instances involving out-of-scope types (use -fprint-potential-instances to see them all) • In the expression: null new In the expression: if null new then supers else reachable fsm (supers \/ new) In the expression: let new = undefined in if null new then supers else reachable fsm (supers \/ new) | 130 | in if null new then supers else reachable fsm (supers \/ new) | ^^^^^^^^