Little Language [Grand]

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1 Little Language [Grand] Intent Given the grammar of a simple language, provide a parser. Motivation Many problems can be expressed using small grammars. Applications then must provide a parser that, for example, allows to build an abstract syntax tree (AST) which in turn can be interpreted by an interpreter (see also the Interpreter pattern). Such problems often fall into the following parts: the definition of the lexicals of the language; the definition of the syntax of a language; the definition of its semantics. The task of the scanner (also referred to as lexical analyzer) is to extract the lexicals of the language. The details of a scanner, especially the specification of lexicals by using regular expressions, are not discussed here. The Java class java.io.streamtokenizer provides a simple and configurable scanner. Design Patterns Little Language [Grand] 1

2 The grammar can be defined by a set of production P i : P i --> P i1 P i2... P in To determine the top-level production, one of the production P i is uniquely specified (not shown, here, the first one). Example of a grammar: expression --> orexpression orexpression --> andexpression orexpression --> andexpression "+" orexpression andexpression --> simpleexpression andexpression --> simpleexpression "*" andexpression simpleexpression --> "(" orexpression ")" simpleexpression --> notexpression simpleexpression --> variableexpression simpleexpression --> constantexpression notexpression --> "~" variableexpression notexpression --> "~" constantexpression variableexpression --> "name" constantexpression --> "true" constantexpression --> "false" Design Patterns Little Language [Grand] 2

3 Note that the above grammar does not produce the "natural" grouping of composed expressions. For example, expression a * b * c would be grouped as a * (b * c). During the process of recognizing productions, the parser needs to invoke semantic actions on the productions recognized so far. If, for example, an AST must be built, then the corresponding node of a tree must be created, possibly with zero or more sub-trees corresponding to the non-terminal symbols of the RHS of the production. The production orexpression --> andexpression "+" orexpression can result in a corresponding method of a class Parser which might look like: Applicability private BooleanExp orexpression() { BooleanExp exp = andexpression(); while (token == LexicalAnalyzer.OR) { nexttoken(); exp = new OrExp(exp, orexpression()); Use Little Language if the grammar is simple and small. Use parser generators if the grammar is complex. Design Patterns Little Language [Grand] 3

4 Structure Client <<returns>> Parser Abstract Expression parse(reader): AbstractExpression interpret(context) * LexicalAnalyzer nexttoken(): Token Reader <<creates>> Terminal Expression interpret(context) <<creates>> Nonterminal Expression interpret(context) Design Patterns Little Language [Grand] 4

5 Participants AbstractExpression: declares an abstract interprete method that is common to all node in the abstract syntax tree TerminalExpression: implements the interprete method associated with terminal symbols of the grammar a concrete class is required for each terminal symbol of the grammar NonterminalExpression: one of such class is required for each rule R ::= R 1 R 2... R n maintains Collaborations Client creates instances of Reader and Parser. Parser creates an instance of LexicalAnalyzer. Parser uses a LexicalAnalyzer to get the next Token. Parser creates an AbstractExpression. Client interpretes an instance of AbstractExpression. Design Patterns Little Language [Grand] 5

6 Consequences It is easy to write a parser for a small language. Little Command creates an AST with heterogeneous nodes. ASTs with homogenous nodes could easily be created, though. The AST is suitable for being interpreted, see Interpreter. Implementation Parser logic is captured in one class only. This is not quite object-oriented, but makes perfect sense in this case (high cohesion of the Parser class). Sample Code The excerpt of a lexical analyzer is: import java.io.*; public class LexicalAnalyzer { private StreamTokenizer input; private int lasttoken; Design Patterns Little Language [Grand] 6

7 // Tokens. static final int INVALID_CHAR = -1; static final int NO_TOKEN = 0; static final int AND = 1; // etc. for remaining tokens, then: static final int EOF = 99; public LexicalAnalyzer(Reader r) { input = new StreamTokenizer(r); // configure StreamTokenizer: input.resetsyntax(); input.eolissignificant(false); input.wordchars( a, z ); input.ordinarychar( + ); // or input.ordinarychar( * ); // and input.ordinarychar( ~ ); // not input.ordinarychar( ( ); input.ordinarychar( ) ); input.whitespacechars( \u0000, ); Design Patterns Little Language [Grand] 7

8 public int nexttoken() { int token; try { switch (input.nexttoken ()) { case StreamTokenizer.TT_EOF: token = EOF; break; case StreamTokenizer.TT_WORD: token = NAME; break; case * : token = AND; break; // etc. default: token = INVALID_CHAR; break; catch (IOException ex) { token = EOF; return token; Design Patterns Little Language [Grand] 8

9 See exercises for a complete lexical analyzer class. Note also that it is very helpful for debugging purposes to add a main method to LexicalAnalyzer. The excerpt of class Parser looks like: public class Parser { private LexicalAnalyzer lexer; private int token; public BooleanExp parse(reader input) { lexer = new LexicalAnalyzer(input); nexttoken(); BooleanExp exp = orexpression(); expect(lexicalanalyzer.eof); private BooleanExp orexpression() { BooleanExp exp = andexpression(); while (token == LexicalAnalyzer.OR) { nexttoken(); exp = new OrExp(exp, orexpression()); Design Patterns Little Language [Grand] 9

10 private BooleanExp andexpression() { BooleanExp exp = null; // Rest of body: left as an exercise. private BooleanExp simpleexpression() { BooleanExp exp = null; if (token == LexicalAnalyzer.LPAR) { nexttoken(); exp = orexpression(); expect(lexicalanalyzer.rpar); nexttoken(); else if (token == LexicalAnalyzer.NOT) { // Treatement of remaining cases: left as an exercise. Design Patterns Little Language [Grand] 10

11 private BooleanExp notexpression() { BooleanExp exp = null; // Rest of body: left as an exercise. private BooleanExp variableexpression() { BooleanExp exp = null; // Rest of body: left as an exercise. private BooleanExp constantexpression() { BooleanExp exp = null; // Rest of body: left as an exercise. private void expect(int t) { if (token!= t) { // Print error message... Design Patterns Little Language [Grand] 11

12 private void nexttoken() { token = lexer.nexttoken(); // Recommended: main() method, left as an exercise. Related Patterns Interpreter pattern to interpret the generated AST. Visitor allows to move the interpretation logic away from the node classes of the AST into a separate class. Design Patterns Little Language [Grand] 12

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