"Phantom" function?

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"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

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Re: "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
>

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Re: "Phantom" function?

Baa
unfortunately call of new version:

  ..
  (isRight "data/a.json" :: VerifyJson RESTResp) `shouldReturn` True
  ..

leads to new errors:

       • Ambiguous type variable ‘a0’ arising from a use of ‘isRight’
          prevents the constraint ‘(aeson-1.0.2.1:Data.Aeson.Types.FromJSON.FromJSON
                                      a0)’ from being solved.
          Probable fix: use a type annotation to specify what ‘a0’ should be.
          These potential instances exist:
       .....
        • In the first argument of ‘shouldReturn’, namely
            ‘(isRight "data/a.json" :: VerifyJson RESTResp)’
          In a stmt of a 'do' block:
            (isRight "data/a.json" :: VerifyJson RESTResp) `shouldReturn` True
          In the second argument of ‘($)’, namely
            ‘do { (isRight "data/a.json" :: VerifyJson RESTResp)
                  `shouldReturn` True }’

I will be grateful for any help.

===
Best regards, Paul


> 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
> >  
>

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