Mundane Utility: A Functional Shell

Functional programming is a useful technique. We spend a lot of time discussing it in the context of hard, challenging or interesting problems, but no where nearly enough in the context of mundane problems.

The UNIX shell, or command line interpreter. A simple, but useful program too often mistaken as complex, or mysterious. Almost every programmer is exposed to shells from a users perspective, but far fewer have ever implemented one, or even know where to start.

This talk aims to be a fun look at using functional programming in and around a traditional, some would say mundane, system utility. We will work through the concepts involved with implementing your own shell, and live code our way to a basic functional shell implemented in Haskell.

From this talk, attendees will walk a way with a better understanding of a program they use every day, as well as ideas and inspiration around using functional programming to solve mundane programming problems.

NOTE: this is the lead in to a workshop https://github.com/markhibberd/xsh

Transcript

1. Mundane Utility by Mark Hibberd
2. “Awesomeness put to mundane use” TV Tropes on Mundane Utility
3. Unix Shell Userland Kernel Hardware
4. How did we get here…
5. How did we get here… firmware
6. How did we get here… firmware boot loader
7. How did we get here… firmware boot loader kernel
8. How did we get here… firmware boot loader kernel init
10. How did we get here… firmware boot loader kernel init
11. How did we get here… firmware boot loader kernel init getty note this is just one example sequence at this point onwards
12. How did we get here… firmware boot loader kernel init getty login
13. How did we get here… firmware boot loader kernel init getty login shell
14. How did we get here… firmware boot loader kernel init getty login shell ls
15. A shell…
16. why? A shell…
18. A shell… Line Editing Command Parsing Command Expansion Process Management Builtins
19. line editing… q w e r t y
20. command parsing…
21. command parsing… <program> ::= lists EOF | EOF <lists> ::= list | list <separator> list <list> ::= list | list <and> pipeline | list <or> pipeline <pipeline> ::= pipeline | pipeline <pipe> command <command> ::= word | command word <separator> ::= ; <and> ::= && <or> ::= || <pipe> ::= | <word> ::= …
22. command parsing… character input
23. command parsing… lexer
24. command parsing… token stream
25. command parsing… parser
26. command parsing… program
27. lexing words… word soft hard unquoted text variable text variable
28. process management…
29. ls | sort | uniq process management…
30. process management… fork() ls | sort | uniq uniq sort ls xsh xsh xsh xsh
31. fork() fork() ls | sort | uniq uniq sort ls xsh xsh xsh xsh process management…
32. fork() fork() exec() ls | sort | uniq uniq sort ls xsh xsh xsh xsh process management…
33. fork() fork() fork() exec() ls | sort | uniq uniq sort ls xsh xsh xsh xsh process management…
34. fork() fork() fork() exec() exec() ls | sort | uniq uniq sort ls xsh xsh xsh xsh process management…
35. fork() fork() fork() exec() exec() exec() ls | sort | uniq uniq sort ls xsh xsh xsh xsh process management…
36. ls | sort | uniq xsh xsh xsh xsh uniq sort ls fork() process management…
37. ls | sort | uniq xsh xsh xsh xsh uniq sort ls fork() fork() process management…
38. ls | sort | uniq xsh xsh xsh xsh uniq sort ls fork() fork() fork() process management…
39. ls | sort | uniq xsh xsh xsh xsh uniq sort ls fork() fork() fork() exec() process management…
40. ls | sort | uniq xsh xsh xsh xsh uniq sort ls fork() fork() fork() exec() exec() process management…
41. ls | sort | uniq xsh xsh xsh xsh uniq sort ls fork() fork() fork() exec() exec() exec() process management…
42. ls | sort | uniq fork() xsh xsh xsh xsh ls sort uniq process management…
43. ls | sort | uniq fork() exec() xsh xsh xsh xsh ls sort uniq process management…
44. ls | sort | uniq fork() fork() exec() xsh xsh xsh xsh ls sort uniq process management…
45. ls | sort | uniq fork() fork() exec() exec() xsh xsh xsh xsh ls sort uniq process management…
46. ls | sort | uniq fork() fork() fork() exec() exec() xsh xsh xsh xsh ls sort uniq process management…
47. ls | sort | uniq fork() fork() fork() exec() exec() exec() xsh xsh xsh xsh ls sort uniq process management…
48. pushing limits
49. workshop @ 1:15 https://github.com/markhibberd/xsh
50. Mundane Utility by Mark Hibberd