[Haskell-beginners] "Phantom" function?

[Baa]

Hello, List. I'm trying to write function like this:

  type VerifyJson a = IO Bool

  isRight :: FromJSON a => FilePath -> VerifyJson a
  isRight testbed = do
    js <- readFile testbed
    return $ isJust (decode js) <<<<< ERROR IS HERE ! 

So I get error:

    60  23 error           error:
     • Could not deduce (FromJSON a0) arising from a use of ‘decode’
       from the context: FromJSON a
         bound by the type signature for:
                    isRight :: FromJSON a => FilePath -> VerifyJson a
         at /home/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX/.stack-work/intero/intero7476qP4.hs:57:1-46
       The type variable ‘a0’ is ambiguous
       These potential instances exist:
         instance FromJSON DotNetTime
           -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
         instance FromJSON Value
           -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
         instance (FromJSON a, FromJSON b) => FromJSON (Either a b)
           -- Defined in ‘aeson-1.0.2.1:Data.Aeson.Types.FromJSON’
         ...plus 25 others
         ...plus 52 instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘MB.isJust’, namely ‘(decode js)’
       In the second argument of ‘($)’, namely ‘MB.isJust (decode js)’
       In a stmt of a 'do' block: return $ MB.isJust (decode js) (intero)

Is it possible to write such (phantom?;)) function? Sure, I can make workaround like:

    class FromJSON a => VerifyJson a where
      isRight :: FilePath -> IO Bool
      isRight testbed = do
        js <- readFile testbed
        return $ MB.isJust (decode js::Maybe a)

but in this case I must declare instances for all verificating types (right?), like:

    instance VerifyJson MyType

But idea was to minimize `isRight` users code...

===
Best regards, Paul