Hello, I have this class with two data type familly class DataSource a where data DataSourcePath a :: Type data DataSourceAcq a :: Type withDataSourceP :: (Location l, MonadSafe m) => l -> DataSourcePath a -> (DataSourceAcq a -> m r) -> m r instance DataSource Double where data DataSourcePath Double = DataSourcePath'Double (Hdf5Path Z Double) | DataSourcePath'Double'Const Double deriving (Generic, Show, FromJSON, ToJSON) data DataSourceAcq Double = DataSourceAcq'Double Dataset | DataSourceAcq'Double'Const Double withDataSourceP f (DataSourcePath'Double p) g = withHdf5PathP f p $ \ds -> g (DataSourceAcq'Double ds) withDataSourceP _ (DataSourcePath'Double'Const a) g = g (DataSourceAcq'Double'Const a) the purpose of this is to let our users gives multiple way to extract a value from different sources. In the previous case, from an hdf5 file or from a constante value. The withHdf5PathP method can fail and raise an Exception. So I would like a way to provide a fallback like (pseudo code) withDataSourceP f (datapath `or` fallback) g = try datapath and if it fails fallback My question is howto write this `or` method, it is not clear to me and it seems to me that this method should be usable by all the instance. I am also not use if this is the right way to design all this. Thanks for your help. Frederic