I am trying to write a FromJSON implementation which would parse a list of objects while at the same time skipping some of them - those which contain a certain json property.

I have the code like this, but without proper handling of mzero it returns an error once it encounters a value with "exclude: true".

newtype Response = Response [Foo]
newtype Foo = Foo Text

instance FromJSON Response where
  parseJSON = withArray "Foos" $ \arr -> do
    -- can I filter out here the ones which return `mzero`?
    foos <- mapM parseJSON arr
    pure $ Response (toList foos)

instance FromJSON Foo where
  parseJSON = withObject "Foo" $ \foo -> do
    isExcluded <- foo .: "exclude"
    if isExcluded
      then mzero
      else do
        pure $ Foo "bar"

I've found a few questions which hint at using parseMaybe, but I can't figure out how I can use it from within the FromJSON definition, it seems to be more suited to running the parser from "outside". Is it possible to do skipping "inside"? Or am I going the wrong route here?

CodePudding user response：

What you're looking for is the optional function. It can be a bit tricky to find because it's very general-purpose and not simply a helper function in aeson. For your purposes, it will have the type Parser a -> Parser (Maybe a) and used in conjunction with catMaybes should do what you want.