[Haskell-beginners] "Phantom" function?

[Baa]

Seems that this works:

  {-# LANGUAGE ExistentialQuantification #-}
  ...

  type VerifyJson a = IO Bool

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

but what is the difference?!?


> 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
>