Let’s write a parser! [RUG::B edition]

1. Let’s write a parser! DENIS DEFREYNE / RUG˸˸B / MAY 12TH, 2016
2. 1. Language 2
3. I am Denis. 3
4. But how do you know that I am Denis? 4
5. But how do you know that I am Denis? I told you. I wrote it down. Tobi introduced me. You might have seen me before. Etc. 5
6. But how do you know that I am Denis? You understand English. 6
7. Computers are stupid. 7
8. 8 $ git commit --message="Fix bugs"
9. 9 def greet(name) puts "Hello, #{name}" end
10. Text forms a language, but computers don’t know that. 10
11. 2. Parsing 11
12. Basic idea: 12 Parser objects that are small, composable, and purely functional.
13. 13 def read(input, pos)
14. 14 def read(input, pos) Success.new(pos + 1) end
15. 15 def read(input, pos) Failure.new(pos) end
16. 16 char("H") Succeeds if the next character is the given one.
17. 17 char("H").apply("Hello")
18. 17 H e l l o char("H").apply("Hello")
19. 17 H e l l o 0 1 2 3 4 char("H").apply("Hello")
20. 17 H e l l o 0 1 2 3 4 char("H").apply("Hello")
21. 17 H e l l o 0 1 2 3 4 char("H").apply("Hello")
22. 17 H e l l o 0 1 2 3 4 char("H").apply("Hello") Success(position = 1)
23. 18 char("H").apply("Adiós")
24. 18 A d i ó s 0 1 2 3 4 char("H").apply("Adiós")
25. 18 A d i ó s 0 1 2 3 4 char("H").apply("Adiós")
26. 18 A d i ó s 0 1 2 3 4 char("H").apply("Adiós")
27. Failure(position = 0) 18 A d i ó s 0 1 2 3 4 char("H").apply("Adiós")
28. 19 if input[pos] == @char Success.new(pos + 1) else Failure.new(pos) end
29. 20 seq(a, b) Succeeds if both given parsers succeed in sequence.
30. 21 seq(char("H"), char("e")).apply("Hello")
31. H e l l o 21 0 1 2 3 4 seq(char("H"), char("e")).apply("Hello")
32. H e l l o 21 0 1 2 3 4 seq(char("H"), char("e")).apply("Hello")
33. H e l l o 21 0 1 2 3 4 seq(char("H"), char("e")).apply("Hello")
34. H e l l o 21 0 1 2 3 4 seq(char("H"), char("e")).apply("Hello")
35. H e l l o 21 0 1 2 3 4 seq(char("H"), char("e")).apply("Hello") Success(position = 2)
36. 22 seq( char("H"), char("e"), char("l"), char("l"), char("o"), )
37. 23 string(s) Succeeds if all characters in the given string can be read in sequence.
38. H e l l o 24 0 1 2 3 4 string("Hello").apply("Hello")
39. H e l l o 24 0 1 2 3 4 string("Hello").apply("Hello")
40. H e l l o 24 0 1 2 3 4 string("Hello").apply("Hello")
41. H e l l o 24 0 1 2 3 4 string("Hello").apply("Hello") Success(position = 5)
42. 25 eof() Succeeds at the end of input; fails otherwise.
43. H e l l o 26 0 1 2 3 4 seq(string("Hello"), eof).apply("Hello")
44. H e l l o 26 0 1 2 3 4 seq(string("Hello"), eof).apply("Hello")
45. H e l l o 26 0 1 2 3 4 seq(string("Hello"), eof).apply("Hello")
46. H e l l o 26 0 1 2 3 4 seq(string("Hello"), eof).apply("Hello")
47. H e l l o 26 0 1 2 3 4 seq(string("Hello"), eof).apply("Hello") Success(position = 5)
48. 27 0 1 2 3 4 5 H e l l o ! seq(string("Hello"), eof).apply("Hello!")
49. 27 0 1 2 3 4 5 H e l l o ! seq(string("Hello"), eof).apply("Hello!")
50. 27 0 1 2 3 4 5 H e l l o ! seq(string("Hello"), eof).apply("Hello!")
51. 27 0 1 2 3 4 5 H e l l o ! seq(string("Hello"), eof).apply("Hello!")
52. 27 0 1 2 3 4 5 Failure(position = 5) H e l l o ! seq(string("Hello"), eof).apply("Hello!")
53. 28 alt(a, b) Succeeds if either of the given parsers succeed.
54. A d i ó s 29 0 1 2 3 4 alt(char("H"), char("A")).apply("Adiós")
55. A d i ó s 29 0 1 2 3 4 alt(char("H"), char("A")).apply("Adiós")
56. A d i ó s 29 0 1 2 3 4 alt(char("H"), char("A")).apply("Adiós")
57. A d i ó s 29 0 1 2 3 4 alt(char("H"), char("A")).apply("Adiós") Success(position = 1)
58. 30 whitespace_char = alt(char(" "), char("\t"))
59. 31 optional(p) Succeeds always, but only advances if p succeeds.
60. 32 repeat(p) Succeeds always, and attempts to apply p as often as possible.
61. 33 repeat(whitespace_char)
62. 34 intersperse(a, b) Alternates between a and b., always ending with a.
63. 35 intersperse(char("a"), char(",")).apply("a,a,b")
64. a , a , b 35 0 1 2 3 4 intersperse(char("a"), char(",")).apply("a,a,b")
65. a , a , b 35 0 1 2 3 4 intersperse(char("a"), char(",")).apply("a,a,b")
66. a , a , b 35 0 1 2 3 4 intersperse(char("a"), char(",")).apply("a,a,b")
67. a , a , b 35 0 1 2 3 4 intersperse(char("a"), char(",")).apply("a,a,b")
68. a , a , b 35 0 1 2 3 4 intersperse(char("a"), char(",")).apply("a,a,b") Success(position = 3)
69. 36 etc.
70. 37 720 6 29530
71. 38 
 digit = alt( *('0'..'9') .map { |c| char(c) } )
72. 39 
 digit = char_in('0'..'9')
 
 

73. 40 nat_number = seq(digit, repeat(digit))
74. 41 nat_number = repeat1(digit)
75. 42 nat_number = repeat1(digit) .capture
76. 42 nat_number = repeat1(digit) .capture Success(position = 3, data = "123")
77. 43 nat_number = repeat1(digit) .capture .map(&:to_i)
78. 43 nat_number = repeat1(digit) .capture .map(&:to_i) Success(position = 3, data = 123)
79. 44 def read(input, pos)
80. 45 def read(input, pos) Success.new(pos + 1) end
81. 46 def read(input, pos) Success.new(pos + 1, "blahblah") end
82. 47 first,last,age Denis,Defreyne,29
84. 48 field =
 repeat(char_not(',', "\n")).capture

85. 48 field =
 repeat(char_not(',', "\n")).capture
 line =
 field.intersperse(char(','))

86. 48 field =
 repeat(char_not(',', "\n")).capture
 line =
 field.intersperse(char(','))
 file =
 seq(
 line.intersperse(char("\n")),
 end_of_input,
 )
87. 49 [ ["first_name", "last_name", "age"], ["Denis", "Defreyne", "29"], ]
88. 50 add(1, mul(2, 3)) mul(2, 3)
89. 51 lparen = char('(') rparen = char(')') comma = char(',')
90. 52 expr = alt(lazy { funcall }, nat_number)
91. 53 funcall = seq( identifier, lparen, arglist, rparen, )
92. 54 letter =
 char_in('a'..'z')
 identifier =
 repeat1(letter).capture
94. 55 arglist =
 seq(expr, arglist_tail)

95. 55 arglist =
 seq(expr, arglist_tail)
 arglist_tail =
 repeat(seq(comma, whitespace, expr))
97. 56 expr_list =
 expr.intersperse(char("\n"))

98. 56 expr_list =
 expr.intersperse(char("\n"))
 program =
 seq(expr_list, eof)
99. 57 [ ["add", 1, ["mul", 2, 3]], ["mul", 2, 3], ]
100. And that is how you can write a parser. 58
101. github.com/ddfreyne/d-parse 59
102. github.com/ddfreyne/d-parse 59
103. github.com/ddfreyne/d-parse 59
104. 60 My name is Denis Defreyne. Ready to parse your questions. Find me at denis@stoneship.org, or @ddfreyne on Twitter.