[Haskell-cafe] type checking failure curiosity

[Brandon Allbery]

At a guess, the problem here is that you need to use
ScopedTypeVariables and `forall` to bring the right `q` into scope.
Otherwise, `q` represents a *new* type variable unrelated to the one
in the type signature for `reachable`, and of course ghc cannot
reconcile that independent type variable with the requirement that it
match the one in the earlier type signature.

On Sun, Nov 14, 2021 at 2:58 PM Julian Bradfield
<jcb+hc at julianbradfield.org> 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)
>     |         ^^^^^^^^
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com