"Illegal schematic variable(s)" in code generated by proof (cases rule: ...)`

Question

I defined a cases rule for case_option in the hope of making some proofs more readable. However, when applying it with proof (cases rule: ...) and using the code snippet suggested by the proof statement, the Isar case syntax tells me Illegal schematic variable(s) in case ..., even though the rule works in other cases.

lemma case_option_cases[case_names None Some]: (* removing the "case_names" tag does not solve the issue *)
  assumes "x = None ==> P a"
    and "!!y. x = Some y ==> P (b y)"
  shows "P (case x of None => a | Some y => b y)"
  using assms unfolding option.split_sel by blast

notepad
begin

  fix P :: "'y => bool" and x :: "'x option" and a :: "'y" and b :: "'x => 'y"

  (* sanity check *)
  assume "x = None ==> P a" and "!!y. x = Some y ==> P (b y)"
  then have "P (case x of None => a | Some y => b y)"
    by (cases rule: case_option_cases) (* also works just "by (rule ...)" *)

  have "P (case x of None => a | Some y => b y)"
  proof (cases rule: case_option_cases) (* this line generates and suggests the following structure *)
    case None (* Illegal schematic variable(s) in case "None" *)
    then show ?thesis sorry
  next
    case (Some y) (* same here *)
    then show ?thesis sorry
  qed

end

Is there a way to fix this?

Answer 1

This can be caused/fixed by a few possible things:

• If you make modifications to your PATH, you'll have to reopen your command prompt for the changes to take effect.

• If you have multiple versions of Python installed, you'll have multiple Scripts/ folders, so make sure the Python version you want is not only in the PATH, but also appears in the PATH before any other Python version installed.

Path modification steps for Windows:

• From the desktop Start Menu, search for "environment variables" (will be in the Control Panel). In the "System Properties" window, click on the "Advanced" tab. In the "Advanced" tab, click the "Environment Variables..." button. Search for the "Path" variable under the "Systems variables" section and click the "Edit" button. Make sure the "...Scripts/" folder of the Python version you want appears at the top before any other "Scripts/" folder. Click "OK", then close/reopen any command prompts.