Translate String of Variable Name into Variable Value

[spam@...]

I'm trying to expand some existing LUA code in a project. Below is a
simplified version of an existing function to explain what I'm after:

 const foo = 4

 function compare_value(a,b)
  return a == b
 end

I need to write an equivalent function that does the same as
"compare_value" when:

The value being passed to me for "a" the string name of one of several
pre-declared constants, like "foo" above.
The value being passed for "b" is an integer value out of the set of
possible constants above, in this case "4".

What I want to do is translate variable "a" from the string "foo" into its
equivalent named variable value of "4", so that:

 compare_label("foo",4)

is true.

TIA for help. I'm missing something obvious, aren't I?