Crystal: Can't use Log.for in an instance variable.

[didactic-drunk]

Code:

class C
        getter log : Log

        def initialize(log = nil)
                # @log = log || Log.for(self)
                @log = Log.for(self)
        end
end

C.new

Output:

 > 4 |             @log : Log
                   ^---
Error: instance variable '@log' of C was not initialized directly in all of the 'initialize' methods, rendering it nilable. Indirect initialization is not supported.

Crystal 0.35.0-dev [74b7aa188] (2020-04-12)

[asterite]

Because self is passed to a method, there's a chance @log is read before it's used. The compiler prevents this.

One way to solve it is to make @log nilable. You can use getter! log : Log for this.

[straight-shoota]

Using a nilable ivar won't help in this case. Log.for(self) only works in class scope, not inside a method. Log.for receives either a string or a class (which is then transformed into a string).

I'm not sure if you actually need the log as an instance variable. Usually you would just do Log = ::Log.for(self) at the class scope.

[RX14]

Either way, the equivalent invocation would be @log = Log.for(C), though @straight-shoota is correct that that's rather unneccesary vs a constant.

[Blacksmoke16]

Main benefit would be able to inject a test logger. Otherwise currently there isn't a way, that I can think of ATM, that would allow you to mock it out for unit tests.

I suppose you would have to reopen the type in the spec and change the Log = xx const. Although wouldn't it complain that that constant is already defined?

[z64]

MemoryBackend is provided by std, imagined for testing purposes.

[Blacksmoke16]

Right yea, but how would you change the logger instance to use that backend if it is a constant? As you would run into Error: already initialized constant Foo::Log.

[z64]

That's not how Log works. You would route the log source to the memory backend using the top-level Log as shown in the docs:

https://crystal-lang.org/api/0.34.0/Log.html#configure-logging-explicitly-in-the-code

[didactic-drunk]

Thank you all. The solution i went with:

class C
  Log = ::Log.for(self)

  def initialize(@log = Log)
  end
end

Why? I have a class with 2 distinct uses where I need debugging on one or the other but rarely both (Mostly just one). Turning them both on adds too much noise.