Sadly.
The current Error based instance in transformers is somewhat lacking in
#9588: Add `MonadPlus (Either e)` and `Alternative (Either e)` instances -------------------------------------+------------------------------------- Reporter: hvr | Owner: hvr Type: task | Status: new Priority: normal | Milestone: ⊥ Component: Core Libraries | Version: Resolution: | Keywords: report-impact Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10755 #12160 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Replying to [comment:12 ekmett]: this and other regards. I mindlessly applied that and it broke a parser in our code :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9588#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler