[Haskell-cafe] type checking failure curiosity

[Julian Bradfield]

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)
    |         ^^^^^^^^