#15008: Type synonyms with hidden, determined type variables -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Research | needed Component: Compiler | Version: 8.5 Resolution: worksforme | Keywords: | QuantifiedConstraints, wipT2893 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 AntC): Replying to [comment:3 Iceland_jack]:
{{{#!hs -- GHC transforms it into type MyMonad m = (forall xx. (MonadReader xx m, Show xx)) }}}
The power of `QuantifiedConstraints` seems to get closer to 'magic', the more I see of them! If we didn't have a FunDep on `MonadReader`, could we simulate it with an implication constraint thusly? {{{#!hs -- user writes or GHC transforms it into type MyMonad m = (forall xx. (MonadReader xx m, MonadReader xx m => Show xx)) }}} Would we then need a superclass constraint `Show xx` on `MonadReader xx m`? Or would it be sufficient if the instances were for types in `Show`? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15008#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler