Swift 3 errors with additional data

Question

In Swift 3, what is the recommended way to put (potentially lots of) additional information in an error/exception that the catcher can use to solve/handle the problem? In all th

Answer 1

I don't know if there is a "recommended" way, perhaps someone else can answer that or provide a better solution.

But one possible approach would be to use a struct (with properties) as the error type and use optional properties for values which need not be provided. Example:

struct ParserError: Error {
    enum Reason {
        case invalidCharacter
        case unexpectedEOF
    }
    let reason: Reason
    let line: Int?
    let column: Int?

    init(reason: Reason, line: Int? = nil, column: Int? = nil) {
        self.reason = reason
        self.line = line
        self.column = column
    }
}

One might also want to adopt the LocalizedError protocol to provide sensible error descriptions even if the concrete error type is not known by the catcher (compare How to provide a localized description with an Error type in Swift?):

extension ParserError: LocalizedError {
    public var errorDescription: String? {
        var description: String
        switch reason {
        case .invalidCharacter:
            description = "Invalid Character in input file"
        case .unexpectedEOF:
            description = "Unexpected end of file"
        }
        if let line = line {
            description += ", line \(line)"
        }
        if let column = column {
            description += ", column \(column)"
        }
        return description
    }
}

Usage example:

func parse() throws {
    // Throw error with line number, but without column:
    throw ParserError(reason: .invalidCharacter, line: 13)
}

do {
    try parse()
} catch let error {
    print(error.localizedDescription)
}

Output:

Invalid Character in input file, line 13