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
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
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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