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Implementing a Scanner

sc4312. Lecture 2b. 2. Scanner as a DFA. A scanner is a big DFA.. . 0. " ". . 1. . letter. letter. digit. . 2. . digit. digit. . . 3. . (. . 4. . >. . 5. . =. .... ident. number. lpar. gtr. geq. sc4312. Lecture 2b. 3. Scanning. Scanners tend to be built in two waysad-hoc or handwritten using goto or nested case statementsfor more details see Figure 2.11table-driven DFA.

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Implementing a Scanner

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    1. sc4312 Lecture 2b 1 Implementing a Scanner SC4312 Compiler Construction Originally prepared by Dr.Songsak Channarukul Modified by Dr.Kwankamol Nongpong

    2. sc4312 Lecture 2b 2 Scanner as a DFA A scanner is a big DFA.

    3. sc4312 Lecture 2b 3 Scanning Scanners tend to be built in two ways ad-hoc or handwritten using goto or nested case statements for more details see Figure 2.11 table-driven DFA

    4. sc4312 Lecture 2b 4 Scanning Ad-hoc generally yields the fastest, most compact code by doing lots of special-purpose things, though good automatically-generated scanners come very close Table-driven DFA is what scanner generators like lex and scangen produce lex (flex) in the form of C code scangen in the form of numeric tables and a separate driver (for details see Figure 2.12)

    5. sc4312 Lecture 2b 5 The Rule (almost universal) The scanner always accepts the longest-possible token from the input. One legitimate token could be a prefix of another token. However, we cant tell whether a longer token is possible without peeking at more than one character ahead (lookahead).

    6. sc4312 Lecture 2b 6 Lookahead Take Pascal for example, when you have a 3 and you a see a dot, you need to peek at the character beyond the dot. 3.14 (a single token designating a real number) 3..5 (three tokens designating a range)

    7. sc4312 Lecture 2b 7 DFA Implementation (Hand-Coded) A DFA can be implemented by hand-coding the states in the source code.

    8. sc4312 Lecture 2b 8 DFA Implementation (Table-Driven) A DFA can be implemented as a matrix of d.

    9. sc4312 Lecture 2b 9 Scanner Generator A scanner generator is a program that generates a source code for a scanner from a specification file in a target language. Compiler generators usually include a scanner generator (e.g., lex, flex) and a parser generator (e.g., yacc, bison). In this class, we will use a compiler generator, Coco/R, which generates both scanner and parser in C++, C#, Java and etc.

    10. sc4312 Lecture 2b 10 Overview of Coco/R Coco/R takes an attributed grammar of a source language as an input. Its output includes Scanner, Parser, and related classes. Other semantic classes (e.g., symbol table, code generator) will have to be hand-coded. The main program will create a Parser object and start the compilation process from there.

    11. sc4312 Lecture 2b 11 Generated Scanner The scanner generated by Coco/R is implemented as a DFA. Therefore, the lexical rules must be specified by an EBNF grammar. Tokens must be made up of characters from the extended ASCII set (256 values). The scanner can be made case-sensitive or case-insensitive.

    12. sc4312 Lecture 2b 12 EBNF in Coco/R = separates the sides of a production. . terminates a production. | separates alternatives. () groups alternatives. [] specifies an option. {} specifies an iteration (zero or more).

    13. sc4312 Lecture 2b 13 Structure of Coco/R Specification The Coco/R specification has the following structure: Cocol = [Imports] COMPILER ident [GlobalFieldsAndMethods] ScannerSpecification ParserSpecification END ident .

    14. sc4312 Lecture 2b 14 Scanner Specification The scanner specification consists of five optional parts: ScannerSpecification = [IGNORECASE] [CHARACTERS {SetDecl}] [TOKENS {TokenDecl}] [PRAGMAS {PragmaDecl}] {CommentDecl} {WhiteSpaceDecl}.

    15. sc4312 Lecture 2b 15 Character Sets Character sets are defined to be used in later sections. Examples: digit = 0123456789. hexDigit = digit + ABCDEF. letter = A .. Z. eol = \r. noDigit = ANY digit.

    16. sc4312 Lecture 2b 16 Tokens Tokens may be divided into two groups. Literals have a fixed representation in the source language (e.g., while, >=). Token classes have a certain structure that must be explicitly declared by a regular expression in EBNF.

    17. sc4312 Lecture 2b 17 Tokens Specification A token specification is as follow: TokenDecl = Symbol [= TokenExpr .]. TokenExpr = TokenTerm {| TokenTerm}. TokenTerm = TokenFactor {TokenFactor} [CONTEXT ( TokenExpr )]. TokenFactor = Symbol | ( TokenExpr ) | { TokenExpr } | [ TokenExpr ]. Symbol = ident | string | char.

    18. sc4312 Lecture 2b 18 Examples ident = letter {letter | digit | _}. number = digit {digit} | 0x hexDigit hexDigit hexDigit hexDigit. float = digit {digit} . {digit} [E [+|-] digit {digit}]. while = while. public = public.

    19. sc4312 Lecture 2b 19 Comments Comments in programming languages are usually hard to specify with regular expressions. Nested comments are even harder. Coco/R allows us to specify comments easily as follows: COMMENTS FROM // TO eol COMMENTS FROM (* TO *) NESTED

    20. sc4312 Lecture 2b 20 White Spaces White spaces are not relevant to a source program therefore they must be discarded by a scanner. The specification of white spaces in Coco/R is as follows: IGNORE \t + \r + \n

    21. sc4312 Lecture 2b 21 The Main Class of a Compiler public class Compiler { public static void Main(string[] arg) { Scanner scanner = new Scanner(arg[0]); Parser parser = new Parser(scanner); parser.Parse(); Console.WriteLine(parser.errors.count + errors detected.); } }

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