Re: [Haskell-cafe] type checking failure curiosity

My guess is that you've disabled the monomorphism restriction somehow... but you need the MR to type-check this code. Without the MR, `new` gets a type `forall a. a`, which gets specialized differently in `new`'s two occurrences, meaning that the type information from the second occurrence doesn't affect the first one... which you need it to in order to type-check the `null new` call. I coincidentally ran into this same issue elsewhere today. I think we need to do a better job around error messages in this case. Richard

On Nov 14, 2021, at 11:52 AM, Julian Bradfield wrote:

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) | ^^^^^^^^ _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.