Question for Avdi

gistfile1.md

When checking for valid input, we could raise an exception to make it more helpful to the developer doing the implementation.

Current code:

def initialize(params)
  super
 
  raise ArgumentError, "Must provide results base dir." unless params.has_key?("dir")
  @dir = params["dir"]
end

According to Confident Ruby it seems this is a bad pattern. The right pattern, would be the following:

def initialize(params)
  super
 
  @dir = params.fetch("dir")
rescue KeyError
  raise ArgumentError, "Must provide results base dir."
end

Is this correct? If so, our issue with this is what if something in super raises KeyError?

I'd write this as:

def initialize(params)
  super

  @dir = params.fetch(:dir) do
    raise ArgumentError, "Must provide results base dir."
  end
end

I believe this strategy is covered in Confident Ruby.

@avdi

Okay. I think my example is too simplistic. If I want to use the @dir instance variable after fetching. How would you stripe it?

def initialize(params)
  super

  @dir = params.fetch(:dir) do
    raise ArgumentError, "Must provide results base dir."
  end

  ## Do something
end

I think the disconnect here is that I don't think of the #fetch--including the raise--as failure handling code. It is checking input (and possibly generating, but not handling, an exception). So the whole stanza falls under the heading of "handling input" to me.

I have to think on this a bit. Let me continue using the philosophy and post a more thorough example when I come across one. Thanks for being helpful.