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It seems as though type class defaulting might be happening before the search.

This finds only undefined:

`

_traverse print "abc"

: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 :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 :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_:

`

_traverse print "abc" :: IO ()

: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 :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:

`

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

: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 :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.

Trac metadata

Trac field	Value
Version	8.4.1-alpha3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture