Dear Cafe,
I need help parsing YAML files with varying structure. The minimal example is: a yaml file that has one key, which can hold either one key-value pair or two key value pairs. I have data types for the one key, and the two possible sub-keys:
>data Var = Var {
> v' :: Either MyList Item
> } deriving Show
>
>data MyList = MyList {
> a' :: Int,
> b' :: Int
> } deriving Show
>
>data Item = Item {
> content' :: String
> } deriving Show
To read MyList and Item from the file I wrote FromJSON instances
>instance FromJSON MyList where
> parseJSON (Object m) = MyList <$> m .: (pack "a") <*> m .: (pack "b")
> parseJSON x = fail ("not an object: " ++ show x)
>
>instance FromJSON Item where
> parseJSON (Object m) = Item <$> m .: (pack "c")
> parseJSON x = fail ("not an object: " ++ show x)
I also have read functions for MyList and Item:
>readItem :: IO Item
>readItem = either (error.show) id <$> decodeFileEither "test.yaml"
>
>readMyList :: IO MyList
>readMyList = either (error.show) id <$> decodeFileEither "test.yaml"