Why Strings Are Evil

Given at Playgrounds 2017

1. Why Strings are Evil Samuel Giddins
2. Roadmap 1. Defining a String 2. Motivation 3. Strings in Our Programming Languages 4. Character Encodings 5. Unicode 6. Correctness, Performance, & Shipping
3. What is a String?
4. → Text → A bunch of letters and weird characters → Emoji → Control characters
5. Strings @segiddins
6. Strings import Foundation NSLog("Hello, world!")
7. Strings Ä__stubs__TEXTÄ0ÄÄ__stub_helper__TEXT∞`∞Ä__const__TEXTà__cstring_ TEXTòò__unwind_info__TEXT∞P∞Ë__DATA__nl_symbol_ptr__DATA__la_symbol_ptr__DATA@ H__LINKEDIT 0 †&"Ä0 pê ∏ ∞!x P h! /usr/lib/dyldvÛQùØ=ìRA¡SwE}$
8. Strings segiddins@segiddins.me
9. Strings → Unformatted data → Often has meaning beyond the bytes → The universal data type?
10. Motivation
11. Motivation → I think I'm pretty good at this → But I find strings really hard
12. String-Intensive Applications → Displaying rich text → Writing a JSON parser → Detecting data in forms → Literally anything that involves code → Communication
13. Motivation → I've spent more time writing string-related code than I want to → I want to work on projects that require understanding strings → I don't think I'm alone
14. Motivation → Strings are useful → Strings are hard → Strings aren't getting any less hard → I want to rationalize all the pain I've had working with Strings
15. Strings (in our languages)
16. In C char *string = "string"; char string[] = {'s', 't', 'r', 'i', 'n', 'g', 0};
17. In C → String is a sequence of characters → A character is a typedefd uint8_t → Nul-terminated → That's its
18. In Objective-C NSString *string = @"string\b!\0!";
19. In Objective-C An NSString object encodes a Unicode-compliant text string, represented as a sequence of UTF–16 code units. All lengths, character indexes, and ranges are expressed in terms of 16-bit platform- endian values, with index values starting at 0.
20. In Objective-C → Inspired by Pascal strings → NSString is a class cluster → Generally, a pointer to a buffer and a length → Canonically, UTF-16
21. In Swift let ! = """
22. In Swift A Unicode string value. A string is a series of characters. Strings in Swift are Unicode correct, locale insensitive, and designed to be efficient.
23. In Swift → String is a value-type struct → Composed of characters/extended grapheme clusters → 15 associated types → 75 initializers → 269 methods → 107 properties
24. Character Encodings
25. Character Encodings → Maps bytes to 'characters' → Map isn't necessarily invertible → Many choices → Lots of legacy cruft
26. Common Character Encodings → Morse → EBCDIC → ASCII → Windows-1252 → UTF-{8,16,32}
27. ASCII
28. ASCII → "plain text" → Great for English → Missing everything else
29. What's missing from ASCII? → Accents → Non-latin characters → Emoji → Math symbols → ...
30. Enter Unicode
31. Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language.
32. Why is Unicode important? → Puts "all" characters in a single encoding → Has idea of "canonical equivalence" → Several different underlying representations → Can choose between them depending upon the application
33. Swift Strings are Unicode-Aware "\u{006E}\u{0303}" == "\u{00F1}" // true "ñ" == "ñ" // true
34. Unicode Lessons → Byte equality string equality → String equality is (potentially) quadratic → Comparison, sorting, ... → Good luck! → Locales, casing, etc. are complicated
35. Unicode Lessons → Unicode is complicated → Because the languages we use are complicated
36. Correctness, Performance, & Shipping
37. Correctness, Performance, & Shipping Choose 2
38. Correctness → Being "unicode-aware" → Handling control characters → Detecting invalid byte sequences in a given encoding → Recognizing specific grammars in certain strings
39. Recognizing specific grammars in certain strings → Telephone numbers → Emails → JSON → Programming languages
40. Recognizing specific grammars in certain strings These all use strings in a certain format (defined by a grammar) to represent more than just their bytes
41. Performance → Indexing is → Length is → Looping can be either or → Checking equality → What does that even mean‽
42. Performance → Simple-looking code can be slow → Unicode adds performance wrinkles → String slicing can lead to memory leaks → Or string slicing can be slow
43. Performance → Changing encodings → Delineating lines/paragraphs/pages → Text layout → S ̵͘ ͝ ͖͎̯̱͕̣̝ u ̵̊̈́̓̈́̾̎̔̄̃ ̢͚͓̞̺ ͜ p ̶͓͔͕̞͆͆̇̂̄ 㸅 ͍̘̰̮̝̪̖ e ̴ ̚ ͑̿ ̚ ̙̬̜͕͓͈̝ r ̷̛͠ ͐̈́͌̎̒́͑ ̕ ͖ ̴̔̎̇ ́͆͐̔̚ ̢̳̲͔ ͎ l ̸̀̎̿̚ ͉̥ a ̴̾̅̿́ ̢̪̤͇͎̣ ͔̠ r ̷̓̈́̓͑ ̡̥̯͈ ̨̜̬̘ g ̵͛̅ ͚̤̰ e ̸̡͍͚̞̩͇̱̘̭͇̝̦͕̃́́̀̈́̌̄̆́ ̸̽̈́̃͘ ̚ ̐ ͒͝ ̕ ̓̽ ̨͍͍̬͖̭̠͓ s ̷̓̔͌̌ ̡ ̞͍͕̻̰̥ t ̵̆͂̍͐͝ ͋ ͕̭̯ r ̷̖͎̟̻̯̗̥̓̅ i̴ ̠̝̑̚ ͇̼͎̖̝̮ n ̸̛͉ g ̵̀͛̓̒͗̓͌͆̑͘̚ ͓ s ̷̏́͛͒ ̡̙̻̪
44. Shipping
45. Shipping All of these considerations mean designing fast & correct string handling is a massive undertaking.
46. Shipping Sometimes, sacrificing on one of them is OK. Sometimes, being incorrect or slow and shipping the rest of your app is a worthy tradeoff.
47. sɓuıɹʇS 'oS
48. So, Strings
49. So, Strings → Not evil, just complicated → An overloaded concept – pick the overload you need → Move from a string to structured data as soon as possible
50. So, Strings Not so simple in the end. Just as crucial.
51. @segiddins