[Haskell-cafe] Surprising behavior with wildcard pattern and unlifted type

[Tom Ellis]

On Tue, Jan 03, 2023 at 03:09:58AM -0500, David Feuer wrote:
> Ouch ... that looks ... not great. I don't know if that was intentional,
> but it's weird enough to deserve a bug report.

Bug report filed:

https://gitlab.haskell.org/ghc/ghc/-/issues/22719





> On Mon, Jan 2, 2023, 2:01 PM Tom Ellis <
> tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> 
> > Is there supposed to be this surprising difference between ex1 and ex2?
> >
> >     {-# LANGUAGE UnliftedDatatypes #-}
> >
> >     {-# OPTIONS_GHC -Wall #-}
> >
> >     module Main where
> >
> >     import GHC.Exts (UnliftedType)
> >
> >     type T :: UnliftedType
> >     data T = T
> >
> >     ex1 :: ()
> >     ex1 = let _ = undefined :: T in ()
> >
> >     ex2 :: ()
> >     ex2 = let _a = undefined :: T in ()
> >
> >
> >     ghci> ex1
> >     ()
> >     ghci> ex2
> >     *** Exception: Prelude.undefined
> >     CallStack (from HasCallStack):
> >       undefined, called at test17.hs:53:16 in fake-package-0-inplace:Main