[4, 5, 6]
defInst.lean:8:26-8:32: error(lean.synthInstanceFailed): failed to synthesize instance of type class
  BEq Foo

Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
fun x y => sorry : (x y : Foo) → ?m x y
[4, 5, 6]
fun x y => x == y : Foo → Foo → Bool
[("hello", "hello")]
false
true
true
def instMonadM : Monad M :=
ReaderT.instMonad