defInst.lean:6:0-6:12: error: expression
  test 4
has type
  Foo
but instance
  Lean.Eval Foo
failed to be synthesized, this instance instructs Lean on how to display the resulting value, recall that any type implementing the `Repr` class also implements the `Lean.Eval` class
defInst.lean:8:26-8:32: error: failed to synthesize instance
  BEq Foo
fun x y => sorryAx (?m x y) : (x y : Foo) → ?m x y
[4, 5, 6]
fun x y => x == y : Foo → Foo → Bool
[("hello", "hello")]
false
true
true
def instMMonad : Monad M :=
ReaderT.instMonadReaderT