First-Class Commands, the 2017 Edition

NDC Oslo June 15, 2017

1. © 2017 Reginald Braithwaite. Some rights reserved. 1
2. First-Class Commands an unexpectedly fertile design pa!ern © 2017 Reginald Braithwaite. Some rights reserved. 2
3. why do we care about commands? © 2017 Reginald Braithwaite. Some rights reserved. 3
4. © 2017 Reginald Braithwaite. Some rights reserved. 4
5. the canonical example: mutable data © 2017 Reginald Braithwaite. Some rights reserved. 5
6. class Buffer { constructor (text = '') { this.text = text; } replaceWith (replacement, from = 0, to = this.text.length) { this.text = this.text.slice(0, from) + replacement + this.text.slice(to); return this; } toString () { return this.text; } } © 2017 Reginald Braithwaite. Some rights reserved. 6
7. let buffer = new Buffer(); buffer.replaceWith( "The quick brown fox jumped over the lazy dog" ); buffer.replaceWith("fast", 4, 9); buffer.replaceWith("canine", 40, 43); //=> The fast brown fox jumped over the lazy canine © 2017 Reginald Braithwaite. Some rights reserved. 7
8. buffer is an object © 2017 Reginald Braithwaite. Some rights reserved. 8
9. we treat objects as first-class entities © 2017 Reginald Braithwaite. Some rights reserved. 9
10. replaceWith is a method © 2017 Reginald Braithwaite. Some rights reserved. 10
11. we can treat methods as first- class entities © 2017 Reginald Braithwaite. Some rights reserved. 11
12. buffer.replaceWith("fast", 4, 9) is an invocation © 2017 Reginald Braithwaite. Some rights reserved. 12
13. what does it mean to treat an invocation as a first-class entity? © 2017 Reginald Braithwaite. Some rights reserved. 13
14. storing invocations © 2017 Reginald Braithwaite. Some rights reserved. 14
15. class Edit { constructor (buffer, {replacement, from, to}) { this.buffer = buffer; Object.assign(this, {replacement, from, to}); } doIt () { this.buffer.text = this.buffer.text.slice(0, this.from) + this.replacement + this.buffer.text.slice(this.to); return this.buffer; } } © 2017 Reginald Braithwaite. Some rights reserved. 15
16. class Buffer { constructor (text = '') { this.text = text; } replaceWith (replacement, from = 0, to = this.text.length) { return new Edit(this, {replacement, from, to}); } toString () { return this.text; } } © 2017 Reginald Braithwaite. Some rights reserved. 16
17. let buffer = new Buffer(), jobQueue = []; jobQueue.push( buffer.replaceWith( "The quick brown fox jumped over the lazy dog" ) ); jobQueue.push( buffer.replaceWith("fast", 4, 9) ); jobQueue.push( buffer.replaceWith("canine", 40, 43) ); while (jobQueue.length > 0) { jobQueue.shift().doIt(); } //=> The fast brown fox jumped over the lazy canine © 2017 Reginald Braithwaite. Some rights reserved. 17
18. job queues © 2017 Reginald Braithwaite. Some rights reserved. 18
19. © 2017 Reginald Braithwaite. Some rights reserved. 19
20. querying invocations © 2017 Reginald Braithwaite. Some rights reserved. 20
21. class Edit { netChange () { return this.from - this.to + this.replacement.length; } } © 2017 Reginald Braithwaite. Some rights reserved. 21
22. let buffer = new Buffer(); buffer.replaceWith( "The quick brown fox jumped over the lazy dog" ).netChange(); //=> 44 buffer.replaceWith("fast", 4, 9).netChange(); //=> -1 © 2017 Reginald Braithwaite. Some rights reserved. 22
23. command state © 2017 Reginald Braithwaite. Some rights reserved. 23
24. © 2017 Reginald Braithwaite. Some rights reserved. 24
25. command history © 2017 Reginald Braithwaite. Some rights reserved. 25
26. © 2017 Reginald Braithwaite. Some rights reserved. 26
27. transforming invocations © 2017 Reginald Braithwaite. Some rights reserved. 27
28. class Edit { reversed () { let replacement = this.buffer.text.slice(this.from, this.to), from = this.from, to = from + this.replacement.length; return new Edit(buffer, {replacement, from, to}); } } © 2017 Reginald Braithwaite. Some rights reserved. 28
29. let buffer = new Buffer( "The quick brown fox jumped over the lazy dog" ); let doer = buffer.replaceWith("fast", 4, 9), undoer = doer.reversed(); doer.doIt(); //=> The fast brown fox jumped over the lazy dog undoer.doIt(); //=> The quick brown fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 29
30. all together now © 2017 Reginald Braithwaite. Some rights reserved. 30
31. class Buffer { constructor (text = '') { this.text = text; this.history = []; this.future = []; } } © 2017 Reginald Braithwaite. Some rights reserved. 31
32. class Buffer { replaceWith (replacement, from = 0, to = this.length()) { let doer = new Edit(this, {replacement, from, to}), undoer = doer.reversed(); this.history.push(undoer); this.future = []; return doer.doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 32
33. undo © 2017 Reginald Braithwaite. Some rights reserved. 33
34. class Buffer { undo () { let undoer = this.history.pop(), redoer = undoer.reversed(); this.future.unshift(redoer); return undoer.doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 34
35. let buffer = new Buffer( "The quick brown fox jumped over the lazy dog" ); buffer.replaceWith("fast", 4, 9) //=> The fast brown fox jumped over the lazy dog buffer.replaceWith("canine", 40, 43) //=> The fast brown fox jumped over the lazy canine © 2017 Reginald Braithwaite. Some rights reserved. 35
36. buffer.undo() //=> The fast brown fox jumped over the lazy dog buffer.undo() //=> The quick brown fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 36
37. redo © 2017 Reginald Braithwaite. Some rights reserved. 37
38. class Buffer { redo () { let redoer = this.future.shift(), undoer = redoer.reversed(); this.history.push(undoer); return redoer.doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 38
39. buffer.redo() //=> The fast brown fox jumped over the lazy dog buffer.redo() //=> The fast brown fox jumped over the lazy canine © 2017 Reginald Braithwaite. Some rights reserved. 39
40. that's the basic command pa!ern © 2017 Reginald Braithwaite. Some rights reserved. 40
41. invocations as first-class entities: we stored them; we queried them; we transformed them. © 2017 Reginald Braithwaite. Some rights reserved. 41
42. question! © 2017 Reginald Braithwaite. Some rights reserved. 42
43. class Buffer { replaceWith (replacement, from = 0, to = this.length()) { let doer = new Edit(this, {replacement, from, to}), undoer = doer.reversed(); this.history.push(undoer); this.future = []; return doer.doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 43
44. why do we have to throw the future away? © 2017 Reginald Braithwaite. Some rights reserved. 44
45. class Buffer { replaceWith (replacement, from = 0, to = this.length()) { let doer = new Edit(this, {replacement, from, to}), undoer = doer.reversed(); this.history.push(undoer); // this.future = []; return doer.doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 45
46. let buffer = new Buffer( "The quick brown fox jumped over the lazy dog" ); buffer.replaceWith("fast", 4, 9); //=> The fast brown fox jumped over the lazy dog buffer.undo(); //=> The quick brown fox jumped over the lazy dog buffer.replaceWith("My", 0, 3); //=> My quick brown fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 46
47. what happens when we evaluate buffer.redo()? © 2017 Reginald Braithwaite. Some rights reserved. 47
48. "My qfastbrown fox jumped over the lazy dog" © 2017 Reginald Braithwaite. Some rights reserved. 48
49. © 2017 Reginald Braithwaite. Some rights reserved. 49
50. let's consider commands as a history © 2017 Reginald Braithwaite. Some rights reserved. 50
51. let buffer = new Buffer("The quick brown fox jumped over the lazy dog"); "The quick brown fox jumped over the lazy dog" // PAST // FUTURE © 2017 Reginald Braithwaite. Some rights reserved. 51
52. buffer.replaceWith("fast", 4, 9) "The fast brown fox jumped over the lazy dog" // PAST replaceWith("fast", 4, 9) // FUTURE © 2017 Reginald Braithwaite. Some rights reserved. 52
53. buffer.undo() "The quick brown fox jumped over the lazy dog" // PAST // FUTURE replaceWith("fast", 4, 9) © 2017 Reginald Braithwaite. Some rights reserved. 53
54. buffer.replaceWith("My", 0, 3) "My quick brown fox jumped over the lazy dog" // PAST replaceWith("My", 0, 3) // FUTURE replaceWith("fast", 4, 9) © 2017 Reginald Braithwaite. Some rights reserved. 54
55. buffer.redo() "My qfastbrown fox jumped over the lazy dog" // PAST replaceWith("My", 0, 3) replaceWith("fast", 4, 9) // FUTURE © 2017 Reginald Braithwaite. Some rights reserved. 55
56. every command depends on the history of commands preceding it © 2017 Reginald Braithwaite. Some rights reserved. 56
57. prepending a command into its history alters the command © 2017 Reginald Braithwaite. Some rights reserved. 57
58. let buffer = new Buffer( "The quick brown fox jumped over the lazy dog" ); let fast = new Edit( buffer, { replacement: "fast", from: 4, to: 9 } ); let my = new Edit( buffer, { replacement: "My", from: 0, to: 3 } ); © 2017 Reginald Braithwaite. Some rights reserved. 58
59. let buffer = new Buffer( "The quick brown fox jumped over the lazy dog" ); let fast = new Edit( buffer, { replacement: "fast", from: 4, to: 9 } ); let my = new Edit( buffer, { replacement: "My", from: 0, to: 3 } ); © 2017 Reginald Braithwaite. Some rights reserved. 59
60. class Edit { isBefore (other) { return other.from >= this.to; } } fast.isBefore(my); //=> false my.isBefore(fast); //=> true © 2017 Reginald Braithwaite. Some rights reserved. 60
61. class Edit { prependedWith (other) { if (this.isBefore(other)) { return this; } else if (other.isBefore(this)) { let change = other.netChange(), {replacement, from, to} = this; from = from + change; to = to + change; return new Edit(this.buffer, {replacement, from, to}) } } } © 2017 Reginald Braithwaite. Some rights reserved. 61
62. my.prependedWith(fast) //=> buffer.replaceWith("My", 0, 3) fast.prependedWith(my) //=> buffer.replaceWith("fast", 3, 8) © 2017 Reginald Braithwaite. Some rights reserved. 62
63. my.prependedWith(fast) //=> buffer.replaceWith("My", 0, 3) fast.prependedWith(my) //=> buffer.replaceWith("fast", 3, 8) © 2017 Reginald Braithwaite. Some rights reserved. 63
64. class Buffer { replaceWith (replacement, from = 0, to = this.length()) { let doer = new Edit(this, {replacement, from, to}), undoer = doer.reversed(); this.history.push(undoer); this.future = this.future.map( (edit) => edit.prependedWith(doer) ); return doer.doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 64
65. let's start over © 2017 Reginald Braithwaite. Some rights reserved. 65
66. let buffer = new Buffer( "The quick brown fox jumped over the lazy dog" ); buffer.replaceWith("fast", 4, 9); //=> The fast brown fox jumped over the lazy dog buffer.undo(); //=> The quick brown fox jumped over the lazy dog buffer.replaceWith("My", 0, 3); //=> My quick brown fox jumped over the lazy dog buffer.redo(); © 2017 Reginald Braithwaite. Some rights reserved. 66
67. "My fast brown fox jumped over the lazy dog" © 2017 Reginald Braithwaite. Some rights reserved. 67
68. what did fixing redo teach us about invocations as first- class entities? © 2017 Reginald Braithwaite. Some rights reserved. 68
69. "People assume that time is a strict progression of cause to effect, but actually from a non- linear, non-subjective viewpoint— it's more like a big ball of wibbly wobbly… time-y wimey… stuff." © 2017 Reginald Braithwaite. Some rights reserved. 69
70. reversed() and prependedWith() show us we can change both the direction and order of time. © 2017 Reginald Braithwaite. Some rights reserved. 70
71. © 2017 Reginald Braithwaite. Some rights reserved. 71
72. let alice = new Buffer( "The quick brown fox jumped over the lazy dog" ); let bob = new Buffer( "The quick brown fox jumped over the lazy dog" ); © 2017 Reginald Braithwaite. Some rights reserved. 72
73. for simplicity, we'll omit undo , redo and reversed class Buffer { constructor (text = '') { this.text = text; this.history = []; } } © 2017 Reginald Braithwaite. Some rights reserved. 73
74. class Buffer { replaceWith (replacement, from = 0, to = this.length()) { let edit = new Edit(this, {replacement, from, to} ); this.history.push(edit); return edit.doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 74
75. alice.replaceWith("My", 0, 3); //=> My quick brown fox jumped over the lazy dog bob.replaceWith("fast", 4, 9); //=> The fast brown fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 75
76. © 2017 Reginald Braithwaite. Some rights reserved. 76
77. class Buffer { append (theirEdit) { this.history.forEach( (myEdit) => { theirEdit = theirEdit.prependedWith(myEdit); }); return new Edit(this, theirEdit).doIt(); } } © 2017 Reginald Braithwaite. Some rights reserved. 77
78. class Buffer { appendAll(otherBuffer) { otherBuffer.history.forEach( (theirEdit) => this.append(theirEdit) ); return this; } } © 2017 Reginald Braithwaite. Some rights reserved. 78
79. alice.appendAll(bob); //=> My fast brown fox jumped over the lazy dog bob.appendAll(alice); //=> My fast brown fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 79
80. ! © 2017 Reginald Braithwaite. Some rights reserved. 80
81. © 2017 Reginald Braithwaite. Some rights reserved. 81
82. let GUID = () => ??? class Buffer { constructor (text = '', history = []) { let befores = new Set(history.map(e => e.guid)); history = history.slice(0); Object.assign(this, {text, history, befores}); } share () { return new Buffer(this.text, this.history); } } © 2017 Reginald Braithwaite. Some rights reserved. 82
83. class Edit { constructor (buffer, { guid = GUID(), befores = new Set(), replacement, from, to }) { this.buffer = buffer; befores = new Set(befores); Object.assign(this, {guid, replacement, from, to, befores}); } } © 2017 Reginald Braithwaite. Some rights reserved. 83
84. class Buffer { has (edit) { return this.befores.has(edit.guid); } perform (edit) { if (!this.has(edit)) { this.history.push(edit); this.befores.add(edit.guid); return edit.doIt(); } } } © 2017 Reginald Braithwaite. Some rights reserved. 84
85. class Buffer { replaceWith (replacement, from = 0, to = this.length()) { let befores = this.befores, let edit = new Edit(this, {replacement, from, to, befores} ); return this.perform(edit); } } © 2017 Reginald Braithwaite. Some rights reserved. 85
86. class Buffer { append (theirEdit) { this.history.forEach( (myEdit) => { theirEdit = theirEdit.prependedWith(myEdit); }); return this.perform(new Edit(this, theirEdit)); } } © 2017 Reginald Braithwaite. Some rights reserved. 86
87. class Buffer { appendAll(otherBuffer) { otherBuffer.history.forEach( (theirEdit) => this.has(theirEdit) || this.append(theirEdit) ); return this; } } © 2017 Reginald Braithwaite. Some rights reserved. 87
88. class Edit { prependedWith (other) { if (this.isBefore(other) || this.befores.has(other.guid) || this.guid === other.guid) return this; let change = other.netChange(), {guid, replacement, from, to, befores} = this; from = from + change; to = to + change; befores = new Set(befores); befores.add(other.guid); return new Edit(this.buffer, {guid, replacement, from, to, befores}); } } © 2017 Reginald Braithwaite. Some rights reserved. 88
89. © 2017 Reginald Braithwaite. Some rights reserved. 89
90. let alice = new Buffer( "The quick brown fox jumped over the lazy dog" ); let bob = alice.share(); //=> The quick brown fox jumped over the lazy dog alice.replaceWith("My", 0, 3); //=> My quick brown fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 90
91. let carol = alice.share(); //=> My quick brown fox jumped over the lazy dog bob.replaceWith("fast", 4, 9); //=> The fast brown fox jumped over the lazy dog alice.appendAll(bob); //=> My fast brown fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 91
92. bob.appendAll(alice); //=> My fast brown fox jumped over the lazy dog alice.replaceWith("spotted", 8, 13); //=> My fast spotted fox jumped over the lazy dog bob.appendAll(alice); //=> My fast spotted fox jumped over the lazy dog carol.appendAll(bob); //=> My fast spotted fox jumped over the lazy dog © 2017 Reginald Braithwaite. Some rights reserved. 92
93. "Unfortunately, implementing OT sucks. There's a million algorithms with different tradeoffs, mostly trapped in academic papers. The algorithms are really hard and time consuming to implement correctly." © 2017 Reginald Braithwaite. Some rights reserved. 93
94. perhaps we should borrow a trick from react, and periodically scan a "shadow buffer" for diffs that we exchange with collaborators… © 2017 Reginald Braithwaite. Some rights reserved. 94
95. differential synchronization © 2017 Reginald Braithwaite. Some rights reserved. 95
96. how does differential synchronization differ from the command pa!ern? © 2017 Reginald Braithwaite. Some rights reserved. 96
97. this is a very big problem space © 2017 Reginald Braithwaite. Some rights reserved. 97
98. © 2017 Reginald Braithwaite. Some rights reserved. 98
99. class Buffer { replaceWith () { ... } share () { ... } append () { ... } appendAll () { ... } } © 2017 Reginald Braithwaite. Some rights reserved. 99
100. class Branch { commit () { ... } fork () { ... } cherryPick () { ... } merge () { ... } } © 2017 Reginald Braithwaite. Some rights reserved. 100
101. distributed version control © 2017 Reginald Braithwaite. Some rights reserved. 101
102. with invocations as first-class entities, we can build distributed algorithms and protocols; we can master time and change © 2017 Reginald Braithwaite. Some rights reserved. 102
103. © 2017 Reginald Braithwaite. Some rights reserved. 103
104. © 2017 Reginald Braithwaite. Some rights reserved. 104
105. down, tiger! © 2017 Reginald Braithwaite. Some rights reserved. 105
106. separation of concerns © 2017 Reginald Braithwaite. Some rights reserved. 106
107. Reg Braithwaite PagerDuty, Inc. h!ps://speakerdeck.com/raganwald/ first-class-commands-an- unexpectedly-fertile-design-pa!ern raganwald.com @raganwald © 2017 Reginald Braithwaite. Some rights reserved. 107