SDF vs Context-Free Grammars for Compiler Construction
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Presentation slides for lecture 5 of course IN4303 on Compiler Construction at TU Delft.
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SDF vs Context-Free Grammars for Compiler Construction
- 1. Declarative Syntax Definition pretty printing Guido Wachsmuth Delft Course IN4303 2012/13 University of Technology Compiler Construction Challenge the future
- 2. Assessment last lecture Pretty Printing 2
- 3. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Pretty Printing 2
- 4. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Discuss additional description means provided by SDF and why they are needed in compiler construction. Pretty Printing 2
- 5. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Discuss additional description means provided by SDF and why they are needed in compiler construction. • regular expressions Pretty Printing 2
- 6. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Discuss additional description means provided by SDF and why they are needed in compiler construction. • regular expressions • layout Pretty Printing 2
- 7. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Discuss additional description means provided by SDF and why they are needed in compiler construction. • regular expressions • layout • AST constructors Pretty Printing 2
- 8. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Discuss additional description means provided by SDF and why they are needed in compiler construction. • regular expressions • layout • AST constructors • follow restrictions Pretty Printing 2
- 9. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Discuss additional description means provided by SDF and why they are needed in compiler construction. • regular expressions • layout • AST constructors • follow restrictions • annotations for associativity Pretty Printing 2
- 10. Assessment last lecture Compare SDF with the plain formalism of context-free grammars. Discuss additional description means provided by SDF and why they are needed in compiler construction. • regular expressions • layout • AST constructors • follow restrictions • annotations for associativity • priorities Pretty Printing 2
- 11. Overview today’s lecture Lexical Analysis 3
- 12. Overview today’s lecture plagues of traditional parsing algorithms • paradise lost • paradise regained Lexical Analysis 3
- 13. Overview today’s lecture plagues of traditional parsing algorithms • paradise lost • paradise regained pretty-printing • from trees to text • box layout in pretty-print tables Lexical Analysis 3
- 14. Overview today’s lecture plagues of traditional parsing algorithms • paradise lost • paradise regained pretty-printing • from trees to text • box layout in pretty-print tables template language • alternative to SDF • generation of pretty-printing strategies • generation of completion templates Lexical Analysis 3
- 15. I paradise lost & regained Pretty Printing 4
- 16. Pure and Declarative Syntax Definition: Paradise Lost and Regained Lennart Kats Eelco Visser Guido Wachsmuth Delft University of Technology
- 18. PARADISE
- 19. PARADISE LOST
- 21. PARADISE DENIED
- 22. PARADISE
- 23. WORDS
- 24. TREES
- 25. GRAMMARS
- 27. LANGUAGES
- 28. GRAMMARS
- 29. NATURAL
- 30. PURE
- 31. BEAUTIFUL
- 34. NOT NATURAL
- 35. NOT PURE
- 36. NOT BEAUTIFUL
- 38. NATURAL
- 39. PURE
- 40. BEAUTIFUL
- 41. THE FALL
- 43. PARADISE LOST
- 44. PAIN
- 45. SWEAT
- 46. NOT NATURAL
- 47. NOT PURE
- 48. NOT BEAUTIFUL
- 49. THE PLAGUES
- 50. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0) GRAMMAR CLASSES
- 51. DISAMBIGUATION
- 52. LEXICAL SYNTAX
- 54. EVOLUTION
- 55. COMPOSITION
- 56. RESTRICTION TO PARSERS
- 58. PARADISE
- 59. WORDS WERE MADE THROUGH GRAMMARS
- 60. GRAMMARS
- 61. Num ➝ Digit Num Num ➝ Digit Digit ➝ “0” Digit ➝ “1”
- 62. Num ➝ Digit Num Num ➝ Digit Digit ➝ “0” Digit ➝ “1” production rules
- 63. Num ➝ Digit Num Num ➝ Digit Digit ➝ “0” Digit ➝ “1” terminal symbols production rules
- 64. Num ➝ Digit Num Num ➝ Digit Digit ➝ “0” Digit ➝ “1” nonterminal symbols terminal symbols production rules
- 65. Num ➝ Digit Num Num ➝ Digit Digit ➝ “0” Digit ➝ “1” nonterminal symbols terminal symbols production rules start symbol
- 66. WORDS
- 67. Num
- 68. Num Num ➝ Digit Num Digit Num
- 69. Num Num ➝ Digit Num Digit Num Num ➝ Digit Digit Digit
- 70. Num Num ➝ Digit Num Digit Num Num ➝ Digit Digit Digit Digit ➝ “0” Digit 0
- 71. Num Num ➝ Digit Num Digit Num Num ➝ Digit Digit Digit Digit ➝ “0” Digit 0 Digit ➝ “1” 10
- 72. SENTENCES
- 73. Exp ➝ Exp “+” Exp Exp ➝ Exp “*” Exp Exp ➝ Num production rules
- 74. Exp ➝ Exp “+” Exp Exp ➝ Exp “*” Exp Exp ➝ Num terminal symbols production rules
- 75. Exp ➝ Exp “+” Exp Exp ➝ Exp “*” Exp Exp ➝ Num nonterminal symbols terminal symbols production rules
- 76. Exp ➝ Exp “+” Exp Exp ➝ Exp “*” Exp Exp ➝ Num nonterminal symbols terminal symbols production rules start symbol
- 77. Exp
- 78. Exp Exp ➝ Exp “+” Exp Exp + Exp
- 79. Exp Exp ➝ Exp “+” Exp Exp + Exp Exp ➝ Exp “*” Exp Exp * Exp + Exp
- 80. Exp Exp ➝ Exp “+” Exp Exp + Exp Exp ➝ Exp “*” Exp Exp * Exp + Exp Exp ➝ Num 3 * Exp + Exp
- 81. Exp Exp ➝ Exp “+” Exp Exp + Exp Exp ➝ Exp “*” Exp Exp * Exp + Exp Exp ➝ Num 3 * Exp + Exp Exp ➝ Num 3 * 7 + Exp
- 82. Exp Exp ➝ Exp “+” Exp Exp + Exp Exp ➝ Exp “*” Exp Exp * Exp + Exp Exp ➝ Num 3 * Exp + Exp Exp ➝ Num 3 * 7 + Exp Exp ➝ Num 3 * 7 + 21
- 83. THEY MADE LANGUAGES BY MAKING GRAMMARS
- 84. GRAMMAR
- 85. LANGUAGE
- 86. TRUTH
- 87. Exp ➝ Exp “+” Exp Exp “+” Exp ➝ Exp Exp ➝ Exp “*” Exp Exp “*” Exp ➝ Exp Exp ➝ Num Num ➝ Exp productive reductive
- 88. 3 * 7 + 21
- 89. 3 * 7 + 21 Num ➝ Exp 3 * 7 + Exp
- 90. 3 * 7 + 21 Num ➝ Exp 3 * 7 + Exp Num ➝ Exp 3 * Exp + Exp
- 91. 3 * 7 + 21 Num ➝ Exp 3 * 7 + Exp Num ➝ Exp 3 * Exp + Exp Num ➝ Exp Exp * Exp + Exp
- 92. 3 * 7 + 21 Num ➝ Exp 3 * 7 + Exp Num ➝ Exp 3 * Exp + Exp Num ➝ Exp Exp * Exp + Exp Exp “*” Exp ➝ Exp Exp + Exp
- 93. 3 * 7 + 21 Num ➝ Exp 3 * 7 + Exp Num ➝ Exp 3 * Exp + Exp Num ➝ Exp Exp * Exp + Exp Exp “*” Exp ➝ Exp Exp + Exp Exp “+” Exp ➝ Exp Exp
- 94. THEY TURNED WORDS INTO TREES
- 95. SENTENCES
- 96. STRUCTURE
- 97. Exp ➝ Exp “+” Exp Exp “+” Exp ➝ Exp Exp ➝ Exp “*” Exp Exp “*” Exp ➝ Exp Exp ➝ Num Num ➝ Exp productive reductive
- 98. Exp Exp Exp Num Exp + Exp Exp * Exp tree construction
- 99. 3 * 7 + 21
- 100. Exp Num Exp Exp Exp 3 * 7 + 21
- 101. Exp Exp Exp + Exp * Exp Exp Exp 3 * 7 + 21
- 102. Exp Exp Exp Exp + Exp Exp Exp Exp 3 * 7 + 21
- 103. ONE FORMALISM
- 104. THREE READINGS
- 105. PURE
- 106. DECLARATIVE
- 107. BEAUTIFUL
- 108. PARADISE LOST
- 109. EFFICIENCY
- 110. THE FIRST PLAGUE WERE GRAMMAR CLASSES
- 112. context-free grammars LL(0)
- 113. context-free grammars LL(1) LL(0)
- 114. context-free grammars LL(k) LL(1) LL(0)
- 115. context-free grammars LL(k) LL(1) LL(0) LR(0)
- 116. context-free grammars LL(k) LL(1) LR(1) LL(0) LR(0)
- 117. context-free grammars LL(k) LR(k) LL(1) LR(1) LL(0) LR(0)
- 118. context-free grammars LL(k) LR(k) LL(1) LR(1) SLR LL(0) LR(0)
- 119. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 120. Exp “+” Exp ➝ Exp Term (“+” Term)* ➝ Exp Exp “*” Exp ➝ Exp Fact (“*” Fact)* ➝ Term Num ➝ Exp Num ➝ Fact paradise grammar classes
- 121. Exp * Exp * Exp Term Term Term Exp Exp Exp Fact Fact Fact 3 + 7 + 21 3 + 7 + 21 paradise grammar classes
- 122. THE SECOND PLAGUE WAS DISAMBIGUATION
- 123. Exp Exp Exp Exp Exp Exp Exp Exp Exp Exp 3 * 7 + 21 3 * 7 + 21
- 124. precedence operators associativity 1 ( ), [ ] non-associative 2 new non-associative 3 . left-associative 4 ++, -- non-associative 5 -, +, !, ~, ++, --, (type) right-associative 6 *, /, % left-associative 7 +, - left-associative … … … text books
- 125. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 126. context-free grammars unambigous LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 127. Exp “+” Term ➝ Exp Term ➝ Exp Term “*” Fact ➝ Term Fact ➝ Term Num ➝ Fact grammar classes
- 128. FALSE PROPHETS
- 129. L = {ab, a} L = {ab, a} “a” “b” ➝ A “a” “b” / “a” ➝ A “a” ➝A paradise PEGs
- 130. L = {ab, a} L = {a} “a” ➝ A “a” / “a” “b” ➝ A “a” “b” ➝ A paradise PEGs
- 131. if c1 then if c2 then s1 else s2 dangling else
- 132. “if ” E “then” S “else” S / “if ” E “then” S / “if ” E “then” S ➝S “if ” E “then” S “else” S ➝ S PEGs
- 133. THE THIRD PLAGUE WAS LEXICAL SYNTAX
- 134. morphology & syntax
- 135. LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0) limited look-ahead
- 136. scanners
- 137. 3 * 7 + 21
- 138. 3 * 7 + 21
- 139. parsers
- 140. 3 * 7 + 21
- 141. Exp Exp Exp Exp Exp 3 * 7 + 21
- 142. x = 1. * .10 y : array [ 1 .. 10 ] of integer
- 143. x = 1. * .10 y : array [ 1 .. 10 ] of integer
- 144. x = 1. * .10 y : array [ 1. .10 ] of integer
- 145. THE FOURTH PLAGUE WAS TREE CONSTRUCTION
- 146. Exp Add Exp Add Exp Exp Exp Const Const Const 3 + 7 + 21 3 7 21 paradise
- 147. Exp * Add * Add Term Term Term Const Const Const Fact Fact Fact 3 7 21 3 + 7 + 21 grammar classes
- 148. expr: INTEGER { $$ = con($1); } | expr '+' expr { $$ = opr('+', 2, $1, $3);} | expr '*' expr { $$ = opr('*', 2, $1, $3);} ; semantic actions
- 149. THE FIFTH PLAGUE WAS EVOLUTION
- 150. Exp “+” Term ➝ Exp Exp “+” Exp ➝ Exp Term ➝ Exp Exp “*” Exp ➝ Exp Term “*” Fact ➝ Term Num ➝ Exp Fact ➝ Term Num ➝ Fact paradise grammar classes
- 151. CExp “+” Term ➝ CExp Term ➝ CExp Exp “+” Exp ➝ Exp Term “*” Fact ➝ Term Exp “*” Exp ➝ Exp Fact ➝ Term Num ➝ Exp Num ➝ Fact Exp “=” Exp ➝ Exp Exp “=” CExp ➝ Exp Exp “<” Exp ➝ Exp Exp “<” CExp ➝ Exp Exp “>” Exp ➝ Exp Exp “>” CExp ➝ Exp CExp ➝ Exp paradise grammar classes
- 152. THE SIXTH PLAGUE WAS COMPOSITION
- 153. parsers
- 154. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 155. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 156. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 157. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 158. scanners
- 159. public boolean authenticate(String user, String pw) { SQL stm = <| SELECT id FROM Users WHERE name = ${user} AND password = ${pw} |>; return executeQuery(stm).size() != 0; }
- 160. THE SEVENTH PLAGUE WAS RESTRICTION TO PARSERS
- 161. PRETTY PRINTERS
- 162. SENTENCE GENERATORS
- 163. AST ACCESS
- 164. IDE SUPPORT
- 165. PARADISE REGAINED
- 166. GENERALISED PARSING
- 167. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 169. context-free syntax Exp “+” Exp ➝ Exp Exp “*” Exp ➝ Exp Exp "+" Exp -> Exp Num ➝ Exp Exp "*" Exp -> Exp NUM -> Exp paradise SDF
- 171. context-free grammars unambigous LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 172. context-free grammars unambigous
- 174. precedence operators associativity 1 ( ), [ ] non-associative 2 new non-associative 3 . left-associative 4 ++, -- non-associative 5 -, +, !, ~, ++, --, (type) right-associative 6 *, /, % left-associative 7 +, - left-associative … … … text books
- 175. context-free priorities Exp "*" Exp -> Exp {left} > Exp "+" Exp -> Exp {left}
- 176. context-free syntax "if" E "then" S -> S {prefer} "if" E "then" S "else" S -> S
- 177. SCANNERLESS PARSING
- 178. morphology & syntax
- 179. lexical syntax [0-9]+ -> NUM [ tn] -> LAYOUT "//" ~[n]* [n] -> LAYOUT
- 180. parser
- 181. parser
- 183. Exp Add Exp Add Exp Exp Exp Const Const Const 3 + 7 + 21 3 7 21 paradise
- 184. context-free syntax Exp "+" Exp -> Exp {cons("Add")} Exp "*" Exp -> Exp {cons("Mul")} NUM -> Exp {cons("Const")} SDF
- 185. Add Add( Add( Add Const("3"), Const("7") Const Const Const ), Const("21") 3 7 21 ) paradise SDF
- 186. SEAMLESS EVOLUTION
- 187. context-free syntax Exp "+" Exp -> Exp {cons("Add")} Exp "*" Exp -> Exp {cons("Mul")} NUM -> Exp {cons("Const")} Exp "=" Exp -> Exp {cons("Eq")} Exp ">" Exp -> Exp {cons("Gt")} Exp "<" Exp -> Exp {cons("Lt")}
- 188. MODULAR COMPOSITION
- 189. context-free grammars LL(k) LR(k) LL(1) LR(1) LALR(1) SLR LL(0) LR(0)
- 191. public boolean authenticate(String user, String pw) { SQL stm = <| SELECT id FROM Users WHERE name = ${user} AND password = ${pw} |>; return executeQuery(stm).size() != 0; }
- 192. module Java-SQL imports Java SQL exports context-free syntax "<|" Query "|>" -> Exp {cons("ToSQL")} "${" Exp "}" -> SqlExp {cons("FromSQL")}
- 193. BEYOND PARSERS
- 194. PRETTY PRINTERS
- 195. SENTENCE GENERATORS
- 196. AST ACCESS
- 197. IDE SUPPORT
- 198. PARADISE DENIED
- 199. still around
- 200. still around
- 201. still have to use it
- 202. still have to learn LL, LR, SLR, LALR
- 203. still think using parser generators is hard
- 206. PARADISE OPEN
- 208. coffee break Pretty Printing 181
- 209. II pretty-printing Pretty Printing 182
- 210. Recap: Parsing & AST construction let function fact(n : int): int = if n < 1 then 1 else n * fact(n - 1) in printint(fact(10)) end Let( [ FunDec( "fact" , [FArg("n", Tid("int"))] , Tid("int") , IfThenElse( Lt(Var("n"), Int("1")) , Int("1") , Times(Var("n"), Call("fact", [Minus(Var("n"), Int("1"))])) ) ) ] , [Call("printint", [Call("fact", [Int("10")])])] ) Pretty Printing 183
- 211. Pretty Printing from ASTs to text • keywords • layout: spaces, line breaks, indentation specification • partially defined in grammar • missing layout Pretty Printing 184
- 212. Box Layout basic boxes “foo” KW [ “foo” ] _1 Pretty Printing 185
- 213. Box Layout horizontal boxes H hs=x [ B B B ] B B B Pretty Printing 186
- 214. Box Layout vertical boxes V vs=y is=i [ B hs=x B B ] B B B Pretty Printing 187
- 215. Pretty Print Tables generation "if" Exp "then" Exp -> Exp {"IfThen"} "let" Dec* "in" {Exp ";"}* "end" -> Exp {"Let"} Exp.IfThen -- KW["if"] _1 KW["then"] _2, Exp.Let -- KW["let"] _1 KW["in"] _2 KW["end"], Exp.Let.1:iter-star -- _1, Exp.Let.2:iter-star-sep -- _1 KW[";"] Pretty Printing 188
- 216. Pretty Print Tables manual override "if" Exp "then" Exp -> Exp {"IfThen"} "let" Dec* "in" {Exp ";"}* "end" -> Exp {"Let"} Exp.Let -- V vs=1 is=0 [ V vs=1 is=2 [KW["let"] _1] V vs=1 is=2 [KW["in"] _2] KW["end"] ] Pretty Printing 189
- 217. III syntax templates Pretty Printing 190
- 218. Recap: SDF sorts Exp context-free syntax LValue ":=" Exp -> Exp "let" Dec* "in" {Exp ";"}* "end" -> Exp "if" Exp "then" Exp -> Exp "if" Exp "then" Exp "else" Exp -> Exp "while" Exp "do" Exp -> Exp "for" Id ":=" Exp "to" Exp "do" Exp -> Exp "break" -> Exp "(" ")" -> Exp "(" Exp ")" -> Exp "(" {Exp ";"}+ ")" -> Exp Pretty Printing 191
- 219. Pretty Print Tables manual overrides Exp.Let -- V vs=1 is=0 [ V vs=1 is=2 [KW["let"] _1] V vs=1 is=2 [KW["in"] _2] KW["end"] ] Pretty Printing 192
- 220. Recap: Code Completion module Tiger-Completions imports Tiger-Completions.generated completions completion keyword : "function" completion template Exp: "if " <e> " thennt" <s> (blank) completion proposer : editor-complete Pretty Printing 193
- 221. Templates explicit layout in syntax definitions context-free syntax Exp "+" Exp -> Exp {left, "Plus"} Exp "-" Exp -> Exp {left, "Minus"} context-free syntax Exp.Plus = <<Exp> + <Exp>> {left} Exp.Minus = <<Exp> - <Exp>> {left} Pretty Printing 194
- 222. Templates explicit layout in syntax definitions context-free syntax "let" Dec* "in" {Exp ";"}* "end" " Exp {"Let"} context-free syntax Exp.Let = < let <Dec*; separator="n"> in <Exp*; separator=";n"> end> Pretty Printing 195
- 223. IV summary Pretty Printing 196
- 224. Summary lessons learned Pretty Printing 197
- 225. Summary lessons learned Why are efficient parsing algorithms problematic? • not longer pure, declarative, beautiful • paradise lost: seven plagues • paradise regained: scannerless generalised parsing Pretty Printing 197
- 226. Summary lessons learned Why are efficient parsing algorithms problematic? • not longer pure, declarative, beautiful • paradise lost: seven plagues • paradise regained: scannerless generalised parsing How can we specify pretty-printers? • generated from SDF • manual overrides • one stop shop: TemplateLanguage Pretty Printing 197
- 227. Summary lessons learned Why are efficient parsing algorithms problematic? • not longer pure, declarative, beautiful • paradise lost: seven plagues • paradise regained: scannerless generalised parsing How can we specify pretty-printers? • generated from SDF • manual overrides • one stop shop: TemplateLanguage Pretty Printing 197
- 228. Literature learn more Pretty Printing 198
- 229. Literature learn more declarative syntax definition Lennart C. L. Kats, Eelco Visser, Guido Wachsmuth: Pure and Declarative Syntax Definition - Paradise Lost and Regained. SPLASH 2010 Pretty Printing 198
- 230. Literature learn more declarative syntax definition Lennart C. L. Kats, Eelco Visser, Guido Wachsmuth: Pure and Declarative Syntax Definition - Paradise Lost and Regained. SPLASH 2010 pretty-printing Marc G.J. van den Brand, Eelco Visser: Generation of Formatters for Context- Free Languages. ACM TOSEM 5(1):1-41, 1996. Merijn de Jonge. Pretty-Printing for Software Reengineering. ICSM 2002 Pretty Printing 198
- 231. Literature learn more declarative syntax definition Lennart C. L. Kats, Eelco Visser, Guido Wachsmuth: Pure and Declarative Syntax Definition - Paradise Lost and Regained. SPLASH 2010 pretty-printing Marc G.J. van den Brand, Eelco Visser: Generation of Formatters for Context- Free Languages. ACM TOSEM 5(1):1-41, 1996. Merijn de Jonge. Pretty-Printing for Software Reengineering. ICSM 2002 template language Tobi Vollebregt, Lennart C. L. Kats, Eelco Visser. Declarative Specification of Template-Based Textual Editors. LDTA 2012 Pretty Printing 198
- 232. Outlook beyond IN4303 Model-driven Software Development • domain-specific languages • build your own language with Spoofax Seminar Metaprogramming • science behind the scenes Master theses • theory & practice • WebDSL & mobl • Spoofax Pretty Printing 199
- 233. Outlook coming next declarative semantics definition • Lecture 4: Term Rewriting • Lecture 5: Static Analysis and Error Checking • Lecture 6: Code Generation Labs • Sep 27 syntax definition • Oct 4 code completion & pretty printing Pretty Printing 200
- 234. copyrights Pretty Printing 201
- 235. Pretty Printing 202
- 236. Pictures attribution & copyrights Slide 1: Latin Bible by Gerard Brils (photo by Adrian Pingstone), public domain Slide 188: West Cornwall Pasty Co. by Dominica Williamson, some rights reserved Slide 199-200: Ostsee by Mario Thiel, some rights reserved Slides 5-180: Pure and Declarative Syntax Definition: Paradise Lost and Regained, some rights reserved Pretty Printing 203
Notes de l'éditeur
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- My name is Guido Wachsmuth, \n
- and I will tell you a sad story.\n
- A story about the paradise of pure and declarative syntax definition,\n
- about how this paradise once was lost,\n
- and about how it was regained later,\n
- but also about how this paradise is denied until these days. \n
- In the beginning\n
- were the words, and the words\n
- were trees, and the trees were words. \n
- All words were made through grammars, and without grammars was not any word made that was made. \n
- Those were the days of the garden of Eden. \nAnd there where language engineers strolling through the garden. \n
- They made languages which were sets of words by making grammars full of beauty. \n
- And with these grammars, they turned words into trees and trees into words. \n
- And the trees were natural, \n
- and pure, \n
- and beautiful, as were the grammars.\n
- Among them were software engineers who made software as the language engineers made languages. And they dwelt with them and they were one people. The language engineers were software engineers and the software engineers were language engineers. \n
- And the language engineers made language software. They made recognisers to know words, and generators to make words, and parsers to turn words into trees, and formatters to turn trees into words.\n
- But the software they made was not as natural, \n
- and pure, \n
- and beautiful as the grammars they made. \n
- So they made software to make language software and began to make language software by making syntax definitions. And the syntax definitions were grammars and grammars were syntax definitions. With their software, they turned syntax definitions into language software. And the syntax definitions were language software and language software were syntax definitions.\n
- And the syntax definitions were natural, \n
- and pure, \n
- and beautiful, as were the grammars.\n
- Now the serpent was more crafty than any other beast of the field. He said to the language engineers\n\nDid you actually decide not to build any parsers?\n\nAnd the language engineers said to the serpent,\n\nWe build parsers, but we decided not to build others than general parsers, nor shall we try it, lest we lose our syntax definitions to be natural, and pure, and beautiful.\n\nBut the serpent said to the language engineers,\n\nYou will not surely lose your syntax definitions to be natural, and pure, and beautiful. For you know that when you build particular parsers your benchmarks will be improved, and your parsers will be the best, running fast and efficient.\n
- So when the language engineers saw that restricted parsers were good for efficiency, and that they were a delight to the benchmarks, they made software to make efficient parsers and began to make efficient parsers by making parser definitions.\n
- Those days, the language engineers went out from the garden of Eden. \n
- In pain they made parser definitions all the days of their life. But the parser definitions were not grammars and grammars were not parser definitions.\n
- And by the sweat of their faces they turned parser definitions into efficient parsers. \n
- But the parser definitions were not natural, \n
- nor pure, \n
- nor beautiful, as the grammars had been before.\n
- Their software was full of plagues. \n
- The first plague were grammar classes. Only few grammars could be turned directly into parser definitions. And language engineers massaged their grammars all the days of their life to make them fit into a grammar class. And the parser definitions became unnatural, and impure, and ugly. And there was weeping and mourning.\n
- The second plague was disambiguation. Their new parsers were deterministic. So the language engineers encoded precedence in parser definitions. And the parser definitions became unnatural, and impure, and ugly.\n\n
- The third plague was lexical syntax. The new software could not handle lexical syntax definitions. So the language engineers made another software to turn lexical syntax definitions into scanners. But lexical syntax definitions were less expressive than the grammars they used before. And they were separated from parser definitions, as were scan- ners from parsers. And there was weeping and wailing.\n\n
- The fourth plague was tree construction. The language engineers wanted the efficient parsers to turn words into trees, as their old parsers did. So they added code to their parser definitions. And the parser definitions became unnatural, and impure, and ugly. And those who were oblivious to the working of the efficient parsers made parsers that turn the right words into the wrong trees.\n\n
- The fifth and sixth plague were evolution\n
- and composition. Once the language engineers added a new rule to their parser definitions, those tended to break. And they massaged them by the sweat of their faces to make them fit again into the grammar class. And they were not able to compose two parser definitions to a single parser definition because of grammar classes and separate scanners. And there was weeping and groaning.\n
- The seventh plague was the restriction to parsers. The language engineers turned parser definitions into recognizers and into parsers. But they could not turn them into generators or formatters. That was because parser definitions were not grammars.\n
- Many have undertaken to compile a narrative of the things that have been accomplished among us. It seemed good to us also, having followed all things closely for some time past, to tell an orderly account for you that you may have certainty concerning the things you have been taught.\n
- It all starts with the beauty of grammars and trees\n
- and it all starts with Noam Chomsky.\n\nLike other linguists, he was thinking about the infinite productivity of language.\nWe can produce sentences that no-one produced before.\n\n
- But how can we put this productivity into finite models?\nChomsky&#x2019;s came up with grammars as generative devices.\nFinite models, declaring how words are made from letters and sentences are made from words.\n\n
- This is a grammar for generating binary numbers.\n
- It consists of 4 production rules\n
- At the RHS of these rules we see terminal symbols.\nThese are the symbols binary numbers are made of.\n
- At the LHS of each rule we see a nonterminal symbol.\nNonterminal symbols are variables in the generation.\nThey can also occur on the right-hand side of a production rule.\nWe say: The LHS symbol produces the symbols on the RHS.\n
- The start symbol is a particular nonterminal symbol.\n
- When we generate words through grammars,\n
- we start with this start symbol.\n
- and apply production rules by replacing nonterminal symbols.\nWe replace a nonterminal symbol from the LHS of a rule with the symbols of the RHS.\n
- We do this as long as there are nonterminal symbols for which we can find production rules.\nWe are free to choose any nonterminal symbol and apply any rule.\nSo let&#x2019;s take another rule for NUM this time.\n
- Now we can replace one of the both symbols for digits.\nLet&#x2019;s apply the first rule.\n
- There is still one nonterminal symbol left.\nLet&#x2019;s take the other rule for digits this time.\nWe are done. \nWe generated the word 10.\n
- But with grammars we can not only generate words but also sentences.\n
- Here is a grammar for generating expressions.\nAgain we have production rules\n
- with terminal symbols occuring on the RHS of these rules.\nHere, we have two different kinds of terminal symbols:\n+ and * each representing a particular morphem,\nand NUM, representing a whole class of morphems, let&#x2019;s assume in this case decimal numbers.\n
- We have only a single nonterminal symbol\n
- which is also the start symbol.\n
- We can now generate an expression.\n
- We replace the start symbol by applying the first rule for EXP.\n
- Now let&#x2019;s replace the first EXP and let&#x2019;s use the second rule this time.\n
- Let&#x2019;s take again the first EXP and replace it by a morphem from the morphem class NUM.\n
- When we do this two more times\n
- we have generated an expression.\n
- We can use grammars to describe languages by making a grammar that generates all the words of a given language.\n
- For example, we can come up with a grammar for Latin.\n
- Which should generate all these Latin sentences.\nAnd all other correct Latin sentences.\n
- But a grammar can also define a language. \nIt defines the language of sentences that it generates.\nAnd we can make new languages my making grammars.\nAnd we can ask the grammar if a sentence is in the language or not.\nThe grammar is the only truth.\n
- For this, we read the production rules of a grammar as reduction rules.\nThe symbols of the RHS can be reduced to the nonterminal symbol of the LHS.\nTo emphasise this reading, we can write the rules in a reductive way, switching LHS and RHS.\n
- Now let&#x2019;s see if this is a valid expression according to our grammar we&#x2019;ve seen before.\n
- We apply reduction rules where ever we can, finding the LHS of a rule and replacing it with the nonterminal symbol from the RHS.\nHere we can reduce 21 to EXP.\n
- The same for 7\n
- and 3.\n
- Now we can replace EXP*EXP by EXP\n
- and finally EXP+EXP by EXP.\nIt remains the start symbol of our grammar, telling us the truth:\n3*7+21 is a valid expression.\n
- We can use grammars also to turn words into trees.\n
- We do so because we are not only interested if a sentence is an element of a language,\n
- but also its structure. \nAs we know from primary school, \nthis structure can typically be represented as trees.\n
- we saw already two readings of grammars\n
- now, here we have a third one: rules as tree construction rules. \nnonterminal symbol as the root, symbols constituting the root as leaves\n
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- non-intuitive classification\nnon-intuitive error messages\ncomplex parsing algorithms\n
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- declarative disambiguation\n
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- interleaved with evaluation of the parser\n
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- overlapping morphem classes\nglobally reserved words\n
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- what did before the scanner, now does the parser\n
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- Fear not! Stand firm! See the naturalness, and the pureness, and the beauty of declarative syntax definitions, which will work for you. \nFor the parser definitions that you see today, you shall never see again.\n
- Go out to the promised land! \nMake new software to make parsers and begin to make parsers by making syntax definitions again. \nLet the syntax definitions be grammars again and grammars be syntax definitions again. \nAnd the syntax definitions will be natural, and pure, and beautiful again, as will the grammars.\n
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- WebDSL & mobl: Flash to HTML5, query optimisation, data synchronisation, partitioning, e-learning platform\nSpoofax: debugging, integration, testing, visualisation, incremental compilation, DSLs for robotics\n
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- round-up on every lecture\n\nwhat to take with you\n\ncheck yourself, pre- and post-paration\n
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