#10181: Lint check: arity invariant -------------------------------------+------------------------------------- Reporter: nomeata | Owner: nomeata Type: task | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by nomeata): * status: new => infoneeded Comment: GHC compiles with this lint check, but not the test suite: The invariant `idArity <= typeArity` fails to hold in the situation of #8743: {{{ T8743.hs-boot:3:10: Warning: [RHS of $fxToRowMaybe :: forall a_ani. ToRow (Maybe a_ani)] idArity 1 exceeds typeArity 0: $fxToRowMaybe }}} Here `ToRow` is a one-member class that is also imported with a self- source-import {{{ module T8743 where import {-# SOURCE #-} T8743 () class ToRow a where toRow :: a -> [()] instance ToRow (Maybe a) where toRow Nothing = [()] toRow (Just _) = [()] }}} So presumably due to the SOURCE import, `typeArity` is unable to look through the newtype `ToRow`. Maybe it is better done as a lint warning, and not a lint error? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10181#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler