Expose stderr and stdout for commands

[aaronmallen]

Introduce @stderr and @stdout instance variables to make error and output streams accessible within commands. This enables more flexible error handling and message output customization.

[aaronmallen]

The idea here is to not have to change the signature of Command#initialize causing a breaking change

[alassek]

How about initializing a pair of StringIO in the command, and then

def perform_command(arguments)
  command, args = parse(kommand, arguments, [])
  command.call(**args)
  IO.copy_stream(command.out, out)
  IO.copy_stream(command.err, err)
end

This would make testing the stdout and stderr of commands much simpler.

[aaronmallen]

The intention is to utilize the out/err passed to the CLI in the first place. dry-cil uses them to print help and errors and "inline" commands. We should also allow the commands to use the user specified out/err. Testing is already trivial as the values are DI

[timriley]

@aaronmallen I like the idea of this. We already do similar in Hanami CLI, which I think represents good and typical usage of dry-cli:

# in hanami/cli/command.rb

      def initialize(out:, err:, fs:, inflector:)
        super()
        @out = out
        @err = err
        @fs = fs
        @inflector = inflector
      end

I'd love to pull this down into the core library so it's there for everyone who uses it. Since we already make out and err injectable via Dry::CLI#call (good for testing), I think it only makes sense to thread those through to the commands it invoked, and begin setting the expectation that the commands themselves use the provided IO streams, and not manage their own (unless they have good reason to, and in that case, they know they're deviating from the defaults).

In your change here you're conscious to avoid API breakage, but I wonder, perhaps this is worth breaking API for? This feels like a valuable enough thing to move to 2.0 for. And if we did that, is there anything else you'd like to see change with the command interface?

[aaronmallen]

And if we did that, is there anything else you'd like to see change with the command interface?

@timriley The only other thing I modify about Dry::CLI locally would actually be a small breaking change. I typically change the example interface to behave like this:

class MyCommand < Dry::CLI::Command
  example "--with-option", "Do a thing with an option"
  example "--without-option", "Do a thing without an option"
end

via

def example(example, description)
  @examples += [example, description].freeze
end

I then implement:

def examples
  indent = @examples.map { |example, _| example.length }.max + 5
  @examples.map { |example, description| "#{example.ljust(indent)}# #{description}" }
end

The idea being I always have a aligned example/description print out:

Examples:

foo --with-option        # Do a thing with an option
foo --without-option     # Do a thing without an option

[aaronmallen]

I would even go so far as to say, it would be cool if Dry::CLI::Banner took the width of everything into account and aligned everything in a similar fashion.