Clarify Rule: Name unused function parameters as underscores

[Jager-yoo]

Summary

• This PR proposes clarifying the rule name unused function parameters as underscores to be more specific and understandable way.
• Plus, adds the missing return type.

Reasoning

• In Add rule to name unused function paremeters as underscores #136, calda said it was a 'real world' example, and I totally understand that, but the Context type is a bit ambiguous compared to other easy types used in this style guide. (like Universe, Galaxy, Planet, Star, etc.)
• so, I clarified this example with a more easy type, Weather.
• Open to suggestions on this, thanks! 😄

Reviewers

cc. @calda

Please react with 👍/👎 if you agree or disagree with this proposal.

[Jager-yoo]

Note that I added the missing return type here! -> Weather

the full code of this Weather example is:

import SwiftUI

struct WeatherView: View {

  @State private var currentWeather = Weather(condition: .sunny)

  var body: some View {
    VStack {
      weatherIcon(type: currentWeather.condition)
        .font(.largeTitle)

      Button("change weather") {
        currentWeather = currentWeather.updateWeather(.rainy)
      }
    }
  }

  private func weatherIcon(type: Weather.WeatherCondition) -> some View {
    switch type {
    case .sunny:
      return Image(systemName: "sun.max.fill")
    case .rainy:
      return Image(systemName: "cloud.rain.fill")
    }
  }
}

struct Weather {
  var condition: WeatherCondition

  enum WeatherCondition {
    case sunny
    case rainy
  }

  func updateWeather(_ newCondition: WeatherCondition) -> Weather {
    var updatedWeather = self
    updatedWeather.condition = condition // logical error that is hard to notice!
    return updatedWeather
  }
}

struct WeatherView_Previews: PreviewProvider {
  static var previews: some View {
    WeatherView()
  }
}

[calda]

This is a nice improvement, thank you!