#9588: Add `MonadPlus {IO,Either e}` and `Alternative (Either e)` instances -------------------------------------+------------------------------------- Reporter: hvr | Owner: hvr Type: task | Status: new Priority: normal | Milestone: 7.10.1 Component: | Version: libraries/base | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by hvr): I'm hitting a problem while trying to implement this. While the `IO` instances are rather straightforward (and btw, there's also an `Alternative IO` instance defined in `transformers`): {{{#!hs instance MonadPlus IO where mzero = ioError (userError "mzero") m `mplus` n = m `catchIOError` \ _ -> n instance Alternative IO where empty = mzero (<|>) = mplus }}} The instances for `Either`, however, depend on an `Error` class in `transformers` to produce an `empty`/`mzero` value: {{{#!hs instance (Error e) => Alternative (Either e) where empty = Left noMsg Left _ <|> n = n m <|> _ = m instance (Error e) => MonadPlus (Either e) where mzero = Left noMsg Left _ `mplus` n = n m `mplus` _ = m }}} how shall this issue be resolved? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9588#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler