main.swift

MainCommand.execute()

MainError.swift

import Guaka

enum MainError: Error, CustomStringConvertible {
    case example
    
    var description: String {
        switch self {
        case .example:
            return "This is an example error."
        }
    }
}

var MainCommand: Command {
    let command = Command(usage: "main")
    command.run = run(command: command, handler: execute)
    return command
}

fileprivate func execute(flags: Flags, args: [String]) throws {
    throw MainError.example
}

TryRun.swift

import Guaka

func run(command: Command, handler: @escaping (Flags, [String]) throws -> Void) -> Run {
    return { flags, args in
        do {
            try handler(flags, args)
        } catch let error as CustomStringConvertible {
            command.fail(statusCode: 1, errorMessage: error.description)
        } catch {
            command.fail(statusCode: 1, errorMessage: String(describing: error))
        }
    }
}