Prototype implementation of a .strings file parser written in Swift, using Parser Combinators, as explained by PointFree.co. See https://www.pointfree.co/collections/parsing/parser-combinators for all videos on this topic.

StringsFileParser.swift

public protocol FileParser {
  func parse(string: String) -> [Entry]
}

public struct FileParserFactory {
  public static func unordered() -> FileParser {
    UnorderedFileParser()
  }
  public static func ordered() -> FileParser {
    LineOrderedFileParser(unorderedParser: unordered())
  }
}

struct UnorderedFileParser: FileParser {
  func parse(string: String) -> [Entry] {
    stringsFile.run(string).match ?? []
  }
}

struct LineOrderedFileParser: FileParser {
  let unorderedParser: FileParser

  func parse(string: String) -> [Entry] {
    unorderedParser
      .parse(string: string)
      .enumerated()
      .map { index, entry in
        entry.map(withOrder: index)
      }
  }
}

// MARK: Parser helpers

private let quote = literal("\"")
private let newLine = Character("\n")

private let stringsFile: Parser<[Entry]> = zeroOrMore(
  oneOf([
    entry,
    singleLineSlashComment.map { Entry(comment: $0) },
    singleLineSlashAsteriskComment.map { Entry(comment: $0) }
  ]),
  separatedBy: zeroOrMore(char: newLine)
)

private let entry = zip(
  operand,
  assignOperator,
  operand,
  semicolon
).map { key, _, value, _ in
    Entry(
      key: String(key),
      value: String(value),
      order: 0
    )
}

private let operand = zip(quote, anyString(until: quote)).map { $1 }

private let assignOperator = zip(
  zeroOrMoreSpaces,
  literal("="),
  zeroOrMoreSpaces
).map { _ in () }

private let semicolon = literal(";")

private let singleLineSlashComment = zip(
  zeroOrMoreSpaces,
  literal("//"),
  zeroOrMoreSpaces,
  prefix(while: { $0 != newLine })
).map { _, _, _, commentText in
  String(commentText)
}

private let singleLineSlashAsteriskComment = zip(
  zeroOrMoreSpaces,
  literal("/*"),
  anyString(until: zip(
    literal("*/"),
    zeroOrMore(char: newLine)
  ))
).map { _, _, commentText in
  commentText.trimmingCharacters(in: .whitespaces)
}

StringsFileParserTests.swift

import XCTest
import Data

class StringsFileParserTests: XCTestCase {
  
  func testSeparatedByNewlines() throws {
    let entries = FileParserFactory.ordered().parse(string:
      """
      "editor.button.save" = "Save";
      "editor.button.open" = "Open";
      """
    )
    
    XCTAssertEqual(entries.count, 2)

    XCTAssertEqual(entries[0].content, .key("editor.button.save", value: "Save"))
    XCTAssertEqual(entries[0].order, 0)
    
    XCTAssertEqual(entries[1].content, .key("editor.button.open", value: "Open"))
    XCTAssertEqual(entries[1].order, 1)
  }
  
  func testSeparatedBySemicolonsOnly() throws {
    let entries = FileParserFactory.ordered().parse(string:
      """
      "editor.button.save" = "Save";"editor.button.open" = "Open";
      """
    )
    
    XCTAssertEqual(entries.count, 2)
    
    XCTAssertEqual(entries[0].content, .key("editor.button.save", value: "Save"))
    XCTAssertEqual(entries[0].order, 0)
    
    XCTAssertEqual(entries[1].content, .key("editor.button.open", value: "Open"))
    XCTAssertEqual(entries[1].order, 1)
  }
  
  func testLineOrder() throws {
    let entries = FileParserFactory.ordered().parse(string:
      """
      "line.0" = "Line 0";
      "line.1" = "Line 1";
      "line.2" = "Line 2";
      "line.3" = "Line 3";
      "line.4" = "Line 4";
      """
    )
    
    XCTAssertEqual(entries.count, 5)
    XCTAssertEqual(entries, [
      Entry(key: "line.0", value: "Line 0", order: 0),
      Entry(key: "line.1", value: "Line 1", order: 1),
      Entry(key: "line.2", value: "Line 2", order: 2),
      Entry(key: "line.3", value: "Line 3", order: 3),
      Entry(key: "line.4", value: "Line 4", order: 4)
    ])
  }

  func testSingleLineSlashComment() throws {
    var entries: [Entry]

    // In the middle
    entries = FileParserFactory.unordered().parse(string:
      """
      "line.0" = "Line 0";
      // A single line comment
      "line.1" = "Line 1";
      """
    )
    XCTAssertEqual(entries.count, 3)
    XCTAssertEqual(entries, [
      Entry(key: "line.0", value: "Line 0"),
      Entry(comment: "A single line comment"),
      Entry(key: "line.1", value: "Line 1")
    ])

    // As first line
    entries = FileParserFactory.unordered().parse(string:
      """
      // A single line comment
      "line.0" = "Line 0";
      "line.1" = "Line 1";
      """
    )
    XCTAssertEqual(entries.count, 3)
    XCTAssertEqual(entries, [
      Entry(comment: "A single line comment"),
      Entry(key: "line.0", value: "Line 0"),
      Entry(key: "line.1", value: "Line 1")
    ])

    // As last line
    entries = FileParserFactory.unordered().parse(string:
      """
      "line.0" = "Line 0";
      "line.1" = "Line 1";
      // A single line comment
      """
    )
    XCTAssertEqual(entries.count, 3)
    XCTAssertEqual(entries, [
      Entry(key: "line.0", value: "Line 0"),
      Entry(key: "line.1", value: "Line 1"),
      Entry(comment: "A single line comment")
    ])

    // Leading and trailing whitespace handling
    entries = FileParserFactory.unordered().parse(string:
      """
      "line.0" = "Line 0";
             // A single line comment
      "line.1" = "Line 1";
      """
    )
    XCTAssertEqual(entries.count, 3)
    XCTAssertEqual(entries, [
      Entry(key: "line.0", value: "Line 0"),
      Entry(comment: "A single line comment"),
      Entry(key: "line.1", value: "Line 1")
    ])

    // Only comment
    entries = FileParserFactory.unordered().parse(string:
      """
      // A single line comment
      """
    )
    XCTAssertEqual(entries.count, 1)
    XCTAssertEqual(entries, [
      Entry(comment: "A single line comment")
    ])
  }

  func testSingleLineSlashAsteriskComment() throws {
    var entries: [Entry]

    // In the middle
    entries = FileParserFactory.unordered().parse(string:
      """
      "line.0" = "Line 0";
      /* A single line comment */
      "line.1" = "Line 1";
      """
    )
    XCTAssertEqual(entries.count, 3)
    XCTAssertEqual(entries, [
      Entry(key: "line.0", value: "Line 0"),
      Entry(comment: "A single line comment"),
      Entry(key: "line.1", value: "Line 1")
    ])
  }
}