fail.swift

import Foundation
import Swift

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// Inspired by Maz Jaleel, https://github.com/Mazyod

/// Unconditionally prints a given message and stops execution,
/// allowing placement at any point where a contextual type
/// is required.
///
/// For example, use to fail on unwrap:
///
/// ```
/// // This URL is well formed
/// let normalURL = URL(string: "http://swift.org")
///                 ?? fail("URL is not well formed")
///
/// // This URL is not well formed and should raise a fatal error
/// let emojiURL =  URL(string: "😱")
///                 ?? fail("URL is not well formed")
/// ```
///
/// - Parameters:
///   - message: The string to print. The default is an empty string.
///   - file: The file name to print with `message`. The default is the file
///     where `fail(_:file:line:)` is called.
///   - line: The line number to print along with `message`. The default
///     is the line number where `fail(_:file:line:)` is called.
public func fail<T>(
  _ message: String = "",
  file: StaticString = #file,
  line: UInt = #line
) -> T {
  fatalError(message, file: file, line: line)
}

/// Unconditionally throw an error, allowing placement at any point where
/// a contextual type is required.
///
/// Note the use of `try` in the nil-coalescing calls in the following
/// example:
///
/// ```
/// public struct ErrorMessage: Error {
///   public let info: String
///   public init(_ info: String) {
///     self.info = info
///   }
/// }
///
/// let illformedURL = ErrorMessage("URL is not well formed")
///
/// do {
///   ///  This URL is well formed
///   let normalURL = try URL(string: "http:/// swift.org")
///                   ?? fail(illformedURL)
///
///   ///  This URL is not well formed and will throw
///   let emojiURL = try URL(string: "😱") ?? fail(illformedURL)
/// } catch {
///   print(error)
/// }
/// ```
/// - Parameters:
///   - error: The error to throw.
public func fail<T, E: Error>(_ error: E) throws -> T {
  throw error
}

public struct ErrorMessage: Error {
  public let info: String
  public init(_ info: String) {
    self.info = info
  }
}

let illformedURL = ErrorMessage("URL is not well formed")

do {
  ///  This URL is well formed
  let normalURL = try URL(string: "http:/// swift.org") ?? fail(illformedURL)
  
  ///  This URL is not well formed and will throw
  let emojiURL = try URL(string: "😱") ?? fail(illformedURL)
} catch {
  print(error)
}

// This URL is well formed
let normalURL = URL(string: "http://swift.org")
                ?? fail("URL is not well formed")

// This URL is not well formed and will fatal error
let emojiURL =  URL(string: "😱")
                ?? fail("URL is not well formed")