#12160: MonadFail instance for (Either String)? -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: libraries | Version: 8.0.1 (other) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #9588 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ekmett): The danger here is that the `(Either String)` specific instance proposed here rules out reasoning uniformly about any other instance someone might want to put on `Either` that works for more types, which could get to be be common in things like test suites. This lets one 'pointwise' instance override anybody's ability to grab a more general one. e.g. the old `Error` class from `MonadError` approach. I'm not advocating for that class per se, as there is no need for `noMsg` for `MonadFail`, making that class "too big". `IsString` on the other hand, does have the remarkable property that it has precisely the right size. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12160#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler