Build fails using GHC 9.0.1

[mmhat]

I tried to build this library with the latest Stackage Nightly snapshot (nightly-2021-07-01) and got the following errors:

src/Control/Ev/Util.hs:160:24: error:
    • Couldn't match type: forall a1. Op () a1 e' ans
                     with: Op () a e' ans
      Expected: Choose e' ans -> Op () a e' ans
        Actual: Choose e' ans -> forall a. Op () a e' ans
    • In the first argument of ‘perform’, namely ‘none’
      In the expression: perform none ()
      In an equation for ‘empty’: empty = perform none ()
    • Relevant bindings include
        empty :: Eff e a (bound at src/Control/Ev/Util.hs:160:3)
    |
160 |   empty      = perform none ()
    |                        ^^^^

src/Control/Ev/Util.hs:165:25: error:
    • Couldn't match type: forall a1. Op () a1 e' ans
                     with: Op () a e' ans
      Expected: Choose e' ans -> Op () a e' ans
        Actual: Choose e' ans -> forall a. Op () a e' ans
    • In the first argument of ‘perform’, namely ‘none’
      In the expression: perform none ()
      In an equation for ‘mzero’: mzero = perform none ()
    • Relevant bindings include
        mzero :: Eff e a (bound at src/Control/Ev/Util.hs:165:3)
    |
165 |   mzero       = perform none ()
    |

[noughtmare]

This is due to the simplified subsumption, a simple (but ugly) fix is to eta expand none:

diff --git a/src/Control/Ev/Util.hs b/src/Control/Ev/Util.hs
index 035a7cf..020e75b 100644
--- a/src/Control/Ev/Util.hs
+++ b/src/Control/Ev/Util.hs
@@ -157,11 +157,11 @@ chooseAll
           }
 
 instance (Choose :? e) => Alternative (Eff e) where
-  empty      = perform none ()
+  empty      = perform (\h -> none h) ()
   m1 <|> m2  = do x <- perform choose 2
                   if (x==1) then m1 else m2
 
 instance (Choose :? e) => MonadPlus (Eff e) where
-  mzero       = perform none ()
+  mzero       = perform (\h -> none h) ()
   mplus m1 m2 = do x <- perform choose 2
                    if (x==1) then m1 else m2

[xnning]

Thanks for the issue report! I will take a look soon.

[xnning]

This is indeed due to simplified subsumption. It's a bit annoying but there seems to be no solution other than manual eta-expansion. I added an eta-expanded version of none, called none', so now when performing operations we can do perform none' (), or manually perform (\h -> none h) ().