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