[SOLVED] what is the type signature of `Func` in gen

what is the type signature of `Func` in gen_server:start/4?

The OTP docs say the signature for gen_server:start/4 is this:

start_link(ServerName, Module, Args, Options) -> Result
ServerName = {local,Name} | {global,GlobalName}
  | {via,Module,ViaName}
 Name = atom()
 GlobalName = ViaName = term()
Module = atom()
Args = term()
Options = [Option]
 Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}
  Dbgs = [Dbg]
   Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}
  SOpts = [term()]
Result = {ok,Pid} | ignore | {error,Error}
 Pid = pid()
 Error = {already_started,Pid} | term()

https://erlang.org/doc/man/gen_server.html#start_link-4

What is Func? The docs just say:

This function is useful when a more complex initialization procedure is needed than the gen_server process behavior provides.

So now I know it is useful. But how many arguments does it take? Of what types?

Thanks for help.

Solution

The Func in that spec is an option for debug, so it has little to do with that doc.

The doc you have quoted refers to the gen_server:enter_loop function, not to Func.

Regarding your question, Func is handled with the sys module, like in sys:install/2, and has the following spec:

 dbg_fun() =
    fun((FuncState :: term(),
         Event :: system_event(),
         ProcState :: term()) ->
            done | (NewFuncState :: term()))