Dimitry Golubovsky wrote:
=============
module Main where
import Data.Maybe
fun :: Show a => String -> Maybe a -> IO ()
fun s mb = do
putStrLn s
case mb of
Nothing -> return ()
Just a -> do putStrLn (show a)
return ()
main = fun "bla" Nothing
=============
compiles with Yhc and runs fine (even with only the first line of
main, so there is no mentioning that a String is wrapped in Maybe).
This program shouldn't compile, it doesn't compile in either ghc or hugs
and is definitely ambiguous. Looking at the core that Yhc generates:
Main.main =
let v234 = Prelude.Prelude.Show.Prelude.Integer
in Main.fun v234 Main._LAMBDA243 Prelude.Nothing
Main._LAMBDA243 = "bla"
It's arbitrarily chosen to give the Show dictionary for 'Integer' (which
is 'Prelude.Prelude.Show.Prelude.Integer'). Given that Yhc's type system
is unmodified since nhc98 it's likely that this is a 'carry over' bug
from nhc98.
Thanks
Tom
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