[Haskell-cafe] Parsing YAML with varying structure

[Johannes Strecha]

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"

The file test.yaml looks like

>v:
>  a: 4
>  b: 5

or, alternatively

>v:
>  c: "test"

The question is how I can decode test.yaml to get Var. Trying to code
readVar like readItem (having FromJSON Var like FromJSON Item) failed. For
convenience I attached the relevant source files.

Regards,
Johannes.
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