Я использую GHCJSi версии 0.2.0-7.10.3: http://www.github.com/ghcjs/ghcjs/ и библиотеку reflex-dom версии 0-4 с https://github.com/reflex-frp/reflex-dom. Я не использую reflex-dom-0.3 от Hackage.
Следующая программа на Haskell не компилируется с reflex-dom-0.4:
{-# LANGUAGE RecursiveDo, ScopedTypeVariables, DeriveGeneric, OverloadedStrings #-}
import Reflex
import Reflex.Dom
import Data.Aeson
import GHC.Generics
import qualified Data.Text as T
data Apod = Apod { copyright :: T.Text
, date :: T.Text
, explanation :: T.Text
, hdurl :: T.Text
, media_type :: T.Text
, service_version :: T.Text
, title :: T.Text
, url :: T.Text
} deriving (Generic, Show)
instance FromJSON Apod
main :: IO ()
main = do
mainWidget $ el "div" $ do
buttonEvent <- button "GET"
let url = "https://api.nasa.gov/planetary/apod?api_key=DEMO_KEY"
let defaultReq = xhrRequest "GET" url def
asyncEvent <- performRequestAsync (tag (constant defaultReq) buttonEvent)
let rspApod = fmapMaybe (\r -> decodeXhrResponse r) asyncEvent
return ()
я получаю ошибку
Xhr00.hs:24:36:
No instance for (aeson-0.9.0.1:Data.Aeson.Types.Class.FromJSON b0)
arising from a use of ‘decodeXhrResponse’
The type variable ‘b0’ is ambiguous
Relevant bindings include
rspApod :: Event Spider b0 (bound at Xhr00.hs:24:9)
Note: there is a potential instance available:
instance (aeson-0.9.0.1:Data.Aeson.Types.Class.FromJSON a,
aeson-0.9.0.1:Data.Aeson.Types.Class.FromJSON b) =>
aeson-0.9.0.1:Data.Aeson.Types.Class.FromJSON
(Data.These.These a b)
-- Defined in ‘Data.These’
In the expression: decodeXhrResponse r
In the first argument of ‘fmapMaybe’, namely
‘(\ r -> decodeXhrResponse r)’
In the expression:
fmapMaybe (\ r -> decodeXhrResponse r) asyncEvent
Failed, modules loaded: none.
Я встраиваю библиотечную функцию reflex-dom decodeXhrResponse
(а также decodeText
). Я меняю сигнатуру типа FromJSON a => XhrResponse -> Maybe a
на сигнатуру без переменной типа XhrResponse -> Maybe Apod
. Затем программа успешно компилируется.
{-# LANGUAGE RecursiveDo, ScopedTypeVariables, DeriveGeneric, OverloadedStrings #-}
import Reflex
import Reflex.Dom hiding (decodeXhrResponse, decodeText)
-- import Reflex.Dom
import Data.Aeson
import GHC.Generics
import qualified Data.Text as T
import Control.Monad
import qualified Data.ByteString.Lazy as BL
import Data.Text.Encoding
data Apod = Apod { copyright :: T.Text
, date :: T.Text
, explanation :: T.Text
, hdurl :: T.Text
, media_type :: T.Text
, service_version :: T.Text
, title :: T.Text
, url :: T.Text
} deriving (Generic, Show)
instance FromJSON Apod
main :: IO ()
main = do
mainWidget $ el "div" $ do
buttonEvent <- button "GET"
let nasa = "https://api.nasa.gov/planetary/apod?api_key=DEMO_KEY"
let defaultReq = xhrRequest "GET" nasa def
asyncEvent <- performRequestAsync (tag (constant defaultReq) buttonEvent)
let rspApod :: Event Spider Apod = fmapMaybe (\r -> decodeXhrResponse r) asyncEvent
return ()
-- Inlined and changed library function:
-- decodeXhrResponse :: FromJSON a => XhrResponse -> Maybe a
decodeXhrResponse :: XhrResponse -> Maybe Apod
decodeXhrResponse = join . fmap decodeText . _xhrResponse_responseText
-- Inlined and changed library function:
-- decodeText :: FromJSON a => T.Text -> Maybe a
decodeText :: T.Text -> Maybe Apod
decodeText = decode . BL.fromStrict . encodeUtf8
Я попытался добавить переменную типа с областью действия для rspApod, например rspApod :: Event t Apod
или rspApod :: Event Spider Apod
, но это не помогло.
Вопросы:
Как мне изменить первую программу для успешной компиляции? (встраивание и изменение библиотечной функции — очень плохой хак!)
Почему компилятор не находит и не использует экземпляр FromJSON
для типа данных Apod
?
decodeXhrResponse r :: Maybe Apod
. - person Reid Barton   schedule 28.12.2016let rspApod = fmapMaybe ( \r -> decodeXhrResponse r :: Maybe Apod) asyncEvent
не решает проблему: выдает ту же ошибкуNo instance for
. К сожалению ... - person Jogger   schedule 28.12.2016