#14858: Typed hole subtitution search fails in the REPL -------------------------------------+------------------------------------- Reporter: paf31 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.4.1-alpha3 (Type checker) | Keywords: typed holes | Operating System: Unknown/Multiple substitutions | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- It seems as though type class defaulting might be happening before the search. This finds only undefined: {{{#!text

_traverse print "abc"

<interactive>:20:1: error: • Found hole: _traverse :: (() -> IO ()) -> [Char] -> t Where: ‘t’ is a rigid type variable bound by the inferred type of it :: t at <interactive>:20:1-21 Or perhaps ‘_traverse’ is mis-spelled, or not in scope • In the expression: _traverse In the expression: _traverse print "abc" In an equation for ‘it’: it = _traverse print "abc" • Relevant bindings include it :: t (bound at <interactive>:20:1) Valid substitutions include undefined :: forall (a :: TYPE r). GHC.Stack.Types.HasCallStack => a (imported from ‘Prelude’ (and originally defined in ‘GHC.Err’)) }}} Annotating the return type helps, but we still don't find traverse_: {{{#!text

_traverse print "abc" :: IO ()

<interactive>:22:1: error: • Found hole: _traverse :: (() -> IO ()) -> [Char] -> IO () Or perhaps ‘_traverse’ is mis-spelled, or not in scope • In the expression: _traverse In the expression: _traverse print "abc" :: IO () In an equation for ‘it’: it = _traverse print "abc" :: IO () • Relevant bindings include it :: IO () (bound at <interactive>:22:1) Valid substitutions include mempty :: forall a. Monoid a => a (imported from ‘Prelude’ (and originally defined in ‘GHC.Base’)) undefined :: forall (a :: TYPE r). GHC.Stack.Types.HasCallStack => a (imported from ‘Prelude’ (and originally defined in ‘GHC.Err’)) }}} (note how print seems to have been defaulted to ()) Annotating the type of print helps: {{{#!text

_traverse (print :: Char -> IO ()) "abc" :: IO ()

<interactive>:23:1: error: • Found hole: _traverse :: (Char -> IO ()) -> [Char] -> IO () Or perhaps ‘_traverse’ is mis-spelled, or not in scope • In the expression: _traverse In the expression: _traverse (print :: Char -> IO ()) "abc" :: IO () In an equation for ‘it’: it = _traverse (print :: Char -> IO ()) "abc" :: IO () • Relevant bindings include it :: IO () (bound at <interactive>:23:1) Valid substitutions include mempty :: forall a. Monoid a => a (imported from ‘Prelude’ (and originally defined in ‘GHC.Base’)) undefined :: forall (a :: TYPE r). GHC.Stack.Types.HasCallStack => a (imported from ‘Prelude’ (and originally defined in ‘GHC.Err’)) foldMap :: forall (t :: * -> *). Foldable t => forall m a. Monoid m => (a -> m) -> t a -> m (imported from ‘Prelude’ (and originally defined in ‘Data.Foldable’)) mapM_ :: forall (t :: * -> *) (m :: * -> *) a b. (Foldable t, Monad m) => (a -> m b) -> t a -> m () (imported from ‘Prelude’ (and originally defined in ‘Data.Foldable’)) traverse_ :: forall (t :: * -> *) (f :: * -> *) a b. (Foldable t, Applicative f) => (a -> f b) -> t a -> f () (imported from ‘Data.Foldable’) }}} This was found with 8.4.1-rc.1. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14858 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler