Adding a new key to the context based on a metadata key

field.hs

-- | Creates a new field based on the item's metadata. If the metadata field is not present then no
-- field will actually be created. Otherwise, the value will be passed to the given function and the
-- result of that function will be used as the field's value.
transformedMetadataField :: String -> String -> (String -> String) -> Context a
transformedMetadataField key itemName f = field key $ \item -> do
    fieldValue <- getMetadataField (itemIdentifier item) itemName
    return $ maybe (fail $ "Value of " ++ itemName ++ " is missing") f fieldValue