>> f: func [x /a][either a [x + 2] [x + 1]]
== func [x /a][either a [x + 2] [x + 1]]
>> b: /a
== /a
>> f/b 1
*** Script Error: f has no refinement called b
*** Where: f
*** Stack: f
>> f/:b 1
*** Script Error: f has no refinement called :b
*** Where: f
*** Stack: f
You can see that the function f has a refinement a, and I bind /a to b. When calling f with its refinement /a by b, it fails.
What is the correct way to pass a refinement which needs to be evaluated before to its function? Or, is there a way to convert a path! to function!?
Refinements are limited, in a way that they can be passed only by being listed literally in a function call. On the other hand, you can construct such function call (a path! value followed by all the arguments) whichever way you want:
>> b: 'a
== a
>> do probe reduce [to path! reduce ['f b] 1]
[f/a 1]
== 3
Note that elements of the path in this case are word!s, not refinement!s. In general, such use-case is covered by apply. However, only Rebol has it natively, and with an awkward calling convention:
>> f: func [x /a][either a [x + 2] [x + 1]]
>> b: yes
== true
>> apply :f [1 b]
== 3
>> apply :f [1 no]
== 2
You can easily write your own high-level version of it if that's what you need.