#12005: Constraint instances not shown in `:info` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 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 harpocrates): You may be interested in `:info!` which returns a more reliably complete listing of instances. {{{ Prelude Data.Kind> :i Defer class Defer (p :: Constraint) where defer :: (p => r) -> r {-# MINIMAL defer #-} -- Defined at <interactive>:2:1 instance [safe] Defer (() :: Constraint) -- Defined at <interactive>:3:10 }}} The reason `instance Defer (() :: Constraint)` is filtered out from the output of `:info` is due to `GHC.getInfo`. Unless instructed to not filter its output, this last function will check that //all// names in an instance head are in scope before reporting it. Due to some wrinkles around constraint tuples, `() :: Constraint` doesn't satisfy this check. I consider this a bug. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12005#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler