OptionsPattern does not match rule with compound left hand side

Question

OptionsPattern was introduced in Mathematica v6 so I have finally decided to give a try.

Look at this program:

Clear@foo;
foo[options : OptionsPattern[]] := {options}
foo[k2 -> 3]
foo[Indexed[k, 2] -> 3]

out:

{k2 -> 3}
foo[Indexed[k, {2}] -> 3]

The rule with compound left hand side has not matched OptionsPattern[].

You can replace Indexed by any function (Power, Subscript), it will be the same.

Now compare with old style option pattern:

Clear@foo;
foo[options___Rule] := {options}
foo[k2 -> 3]
foo[Indexed[k, 2] -> 3]

out:

{k2 -> 3}
{Indexed[k, {2}] -> 3}

Matching occurs in all cases.

Is there a bug in OptionsPattern?

Answer 1

The whole question is based on the wrong premise that old-style options were just simply arbitrary rules. This is not the case, in the past the special OptionQ predicate was used to distinguish options from general rules.

With proper option-testing, your example would look like:

Clear@foo;
foo[options___Rule?OptionQ] := {options}
foo[k2 -> 3]
foo[Indexed[k, 2] -> 3]

which gives the output identical to what OptionsPattern[] method does.

Answer 2

When OptionsPattern[] is used, OptionValue@k2 is supposed to work too, so behind the scene there is something like

k2/:OptionValue@k2=3

which fails if k2 is replaced by f1[k2]: you would get a "Tag too deep" error.

So the new system has simpler syntax but is less general.