def f1 (x : Nat) : IO Unit :=
  unless x > 10 do
    IO.println s!"x: {x}"

#eval f1 0
#eval f1 100

def f2 (x : Nat) (ref : IO.Ref Nat) : IO Nat :=
  return x + (←ref.get)

#eval id (α := IO Nat) do f2 5 (← IO.mkRef 10)

def f3 (x : Nat) : IO Nat :=
  try
    IO.println x
    throw $ IO.userError "failed"
  catch
    | IO.Error.userError msg => IO.println s!"at catch: {msg}"; pure 0
    | ex => throw ex

#eval f3 5

def f4 (xs : List Nat) : IO Unit :=
  for x in xs do
    IO.println x

#eval f4 [1,2,3]