Your problem is the type system and that there is different [procedural
types](https://nim-lang.org/docs/manual.html#types-procedural-type).
A normal proc in nim always is of type `nimcall`. A normal **unnamed** proc
(like you have when you use it as a type to store it in `TestOptions`) is
always of type `closure` so you can have an environment.
You have 2 options, either:
1. you explicitly tell your Callback type that it stores a proc of type
`nimcall` (which you should do if you **don 't** access global variables in
that proc) or
2. you define your `show` proc as an unnamed proc that you assign to a show
variable`closure` (which you should do if you _do_ plan on accessing global
variables).
For 1 simply change your callback definition: ` Callback = Option[proc(name:
string, length: int) {.nimcall.}]`
For 2 simply change your proc definition:
const show =proc(name: string, length: int) {.closure.} =
echo "Callback executed: ", name, " is ", length, " inches long"
Run
