purescript-hoist-error

Provides a typeclass and useful combinators for hoisting errors into a monad.

HoistError extends MonadThrow with hoistError, which enables lifting of partiality types such as Maybe and Either e into the monad.

For example, consider the following App monad that may throw BadPacket errors:

data AppError = BadPacket String

newtype App a = App (ExceptT AppError Aff) a

instance Functor App where ...
instance Apply App where ...
instance Applicative App where ...
instance Bind App where
instance Monad App
instance MonadThrow AppError where ...

We may have an existing function that parses a String into a Maybe Packet

parsePacket :: String -> Maybe Packet

which can be lifted into the App monad with hoistError

appParsePacket :: String -> App Packet
appParsePacket s = hoistError (parsePacket s) \_ -> BadPacket "no parse"

Similar instances exist for Either e and ExceptT e m.

Operator alias <%?> is provided for a syntactic convenience:

appParsePacket :: String -> App Packet
appParsePacket s = parsePacket s <%?> \_ -> BadPacket "no parse"

Inspired by: https://hackage.haskell.org/package/hoist-error-0.2.1.0/docs/Control-Monad-Error-Hoist.html