#11661: Missing MonadFail instance for Q monad from TemplateHaskell -------------------------------------+------------------------------------- Reporter: PeterTrsko | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 8.0.1-rc2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by hvr): Yes... I have been wondering for some time already (which is the reason there is no `MonadFail` instance yet) how to properly define one given : {{{#!hs class Monad m => Quasi m where -- ... newtype Q a = Q { unQ :: forall m. Quasi m => m a } instance Monad Q where -- ... fail s = report True s >> Q (fail "Q monad failure") }}} Now obviously we can't {{{#!hs instance Fail.MonadFail Q where -- ... fail = Control.Monad.fail }}} as that would call `Q`'s inner `Monad(fail)`, whereas we'd need it to call `Q`'s inner `MonadFail(fail)` which would require changing `Quasi`'s superclass, and I'm not sure that's appropriate. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11661#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler