Changing options inside a function

Question

I would like to make a function that can perform checks on its option values, and resets them to legal values if necessary. The option should still be accessible using OptionValue. I am looking for a way to do this without touching the function's default option values. A minimal example might look something like this:

ClearAll[f];
Options[f] = {"Opt1" -> 1};

f[opts:OptionsPattern[]] := 
  If[
   OptionValue["Opt1"] < 2, Print["Option value is fine."]; OptionValue["Opt1"],
   Print["Option value is illegal."]; resetOption["Opt1", -1]; OptionValue["Opt1"]
   ];

f["Opt1" -> 4]

This should return -1, but leave the default value at 1. Is there a good way to do this?

EDIT: My real-world function is using OptionValue in several places, and I am looking for a clean way to modify options without having to introduce additional variables. Can this be done?

Answer 1

This will work on your minimal example,

ClearAll[f];
Options[f] = {"Opt1" -> 1};

f[opts : OptionsPattern[]] := If[OptionValue["Opt1"] < 2,
   Print["Option value is fine."];
   OptionValue["Opt1"],
   Print["Option value is illegal."];
   OptionValue["Opt1"] /. {OptionValue["Opt1"] -> -1}
   ];

f["Opt1" -> 4]
Options[f]

"Option value is illegal."
(* -1 *)
(* {"Opt1" -> 1} *)

Sadly, it will not work if you are trying to do anything beyond this minimal example, as for some reason ReplaceAll won't go inside any other Head. That is, if you replace OptionValue["Opt1"] /. {OptionValue["Opt1"] -> -1} with (OptionValue["Opt1"] + 1)/. {OptionValue["Opt1"] -> -1} then the result is 5 and not 0 as you would hope.

Answer 2

If, for some reason, you really need to access modified option value using OptionValue["Opt1"], then you could localize OptionValue symbol using Internal`InheritedBlock and assign something to local OptionValue like OptionValue["Opt1"] = something.

An environment that performs necessary localization:

ClearAll[withLocalOptionValue]
SetAttributes[withLocalOptionValue, HoldFirst]
withLocalOptionValue[body_] :=
    Internal`InheritedBlock[{OptionValue},
        Unprotect[OptionValue];
        body
    ]

Now you can wrap whole body of your function with withLocalOptionValue and inside it simply assign values to OptionValue:

ClearAll[f];
Options[f] = {"Opt1" -> 1};
f[OptionsPattern[]] :=
    withLocalOptionValue@If[OptionValue["Opt1"] < 2,
        Print["Option value is fine."];
        OptionValue["Opt1"]
    (* else *),
        Print["Option value is illegal."];
        OptionValue["Opt1"] = -1; 
        OptionValue["Opt1"]
   ]

f["Opt1" -> 4]
(* Option value is illegal. *)
(* -1 *)

f["Opt1" -> 0]
(* Option value is fine. *)
(* 0 *)

But this is a strange requirement, why not simply use Module or With and assign correct option value to local symbol, as suggested in comment by Jason B?

ClearAll[f];
Options[f] = {"Opt1" -> 1};
f[OptionsPattern[]] :=
    Module[{opt1 = OptionValue["Opt1"]},
        If[opt1 < 2,
            Print["Option value is fine."]
        (* else *),
            Print["Option value is illegal."];
            opt1 = -1
        ];
        opt1
    ]

f["Opt1" -> 4]
(* Option value is illegal. *)
(* -1 *)

f["Opt1" -> 0]
(* Option value is fine. *)
(* 0 *)

or

ClearAll[f];
Options[f] = {"Opt1" -> 1};
f[OptionsPattern[]] :=
    With[{opt1 = Replace[OptionValue["Opt1"], val_ /; val >= 2 -> -1]},
        opt1
    ]

f["Opt1" -> 4]
(* -1 *)

f["Opt1" -> 0]
(* 0 *)

Answer 3

Ok, found an acceptable solution using the OptionValue[f, opts, name] construct. Thanks!

ClearAll[f];

Options[f] = {"Opt1" -> 1};

f[opts : OptionsPattern[]] := Module[{options},

   options = List[opts];

   If[
    OptionValue[f, options, "Opt1"] < 2,
    Print["Option value is fine."]; OptionValue[f, options, "Opt1"],
    Print["Option value is illegal."]; 
    PrependTo[options, "Opt1" -> -1]; OptionValue[f, options, "Opt1"]
    ]
   ];

f[]
(* Option value is fine. *)
(* 1 *)

f["Opt1" -> 4]
(* Option value is illegal. *)
(* -1 *)