C callback example on docs causes compile errors

[tomjw64]

System:
OS: WSL running Ubuntu 16.04
Crystal Version Info:

Crystal 0.24.1 (2017-12-22)

LLVM: 4.0.0
Default target: x86_64-unknown-linux-gnu

Issue:
Thought I'd try my hand at passing a closure to a C function using the example on the crystal-lang docs as a learning exercise. I ran into a couple issues trying to make the example work.

An exact copy (aside from an @Link I added) of the example code yields the following error:

Error in ticker.cr:7: class variable '@@box' of Ticker is not nilable (it's Box(Proc(Int32, Nil))) so it must have an initializer

  @@box : Box(Int32 ->)

I thought I'd just initialize it as an empty Array to just get past that line, given that there's probably some obvious solution that I missed. I changed lines in the example from:
@@box : Box(Int32 ->) and @@box = boxed_data
to
@@box : Array(Box(Int32 ->)) = [] of Box(Int32 ->) and @@box << boxed_data, respectively. I then get the following error:

Error in ticker.cr:29: instantiating 'Ticker:Module#on_tick()'

Ticker.on_tick do |tick|
       ^~~~~~~

in ticker.cr:16: no overload matches 'Array(Box(Proc(Int32, Nil)))#<<' with type Pointer(Void)
Overloads are:
 - Array(T)#<<(value : T)

    @@box << boxed_data

which sort of makes sense because Box is supposed to give us a Pointer(Void), but why do the types not match?

So it seems that either the syntax or the the types used in the module-level variable are causing problems. Are the docs out of date or have I done something wrong myself? I can't really find any alternative examples of this process.

[straight-shoota]

It seems like the documentation is just wrong here.

This should be a somewhat working example, only on carc.in it doesn't link: https://carc.in/#/r/3c7j

[tomjw64]

Even the linking is not the problem, however.
@[Link(ldflags: "#{__DIR__}/ticker.o")] will be sufficient if you have some compiled C, ticker.o, with the correct function declaration. I think it was implied that this exists in the docs, although, it wouldn't hurt to have a full example.
The original issue assumes a correct link statement. I've updated some wording to indicate this.

[tomjw64]

Ah! The example differs on the official language reference and the "Passing a Proc to a C Function" section in the API Docs. The API Docs for Proc contain a correct example using @@box : Pointer(Void)? thus the language reference should be updated.

[asterite]

Closed in favor of crystal-lang/crystal-book#186