默认情况下,ANTLR 4 生成侦听器。但是如果你给出org.antlr.v4.Tool
命令行参数-visitor
,ANTLR 会为你生成访问者类。这些工作很像侦听器,但让您可以更好地控制遍历/访问哪些(子)树。如果您想排除某些(子)树(如 else/if 块,如您的情况),这将特别有用。虽然这可以使用侦听器来完成,但对访问者执行此操作要干净得多。使用侦听器,您需要引入全局变量来跟踪是否需要评估(子)树,而哪些不需要。
碰巧的是,我正在编写一个小型 ANTLR 4 教程。它还没有完成,但我将发布一个小的工作示例来演示这些访问者类和if
语句构造的使用。
1. 语法
if
这是一个支持基本表达式、 -、while
- 和-语句的简单语法log
:
穆格4
grammar Mu;
parse
: block EOF
;
block
: stat*
;
stat
: assignment
| if_stat
| while_stat
| log
| OTHER {System.err.println("unknown char: " + $OTHER.text);}
;
assignment
: ID ASSIGN expr SCOL
;
if_stat
: IF condition_block (ELSE IF condition_block)* (ELSE stat_block)?
;
condition_block
: expr stat_block
;
stat_block
: OBRACE block CBRACE
| stat
;
while_stat
: WHILE expr stat_block
;
log
: LOG expr SCOL
;
expr
: expr POW<assoc=right> expr #powExpr
| MINUS expr #unaryMinusExpr
| NOT expr #notExpr
| expr op=(MULT | DIV | MOD) expr #multiplicationExpr
| expr op=(PLUS | MINUS) expr #additiveExpr
| expr op=(LTEQ | GTEQ | LT | GT) expr #relationalExpr
| expr op=(EQ | NEQ) expr #equalityExpr
| expr AND expr #andExpr
| expr OR expr #orExpr
| atom #atomExpr
;
atom
: OPAR expr CPAR #parExpr
| (INT | FLOAT) #numberAtom
| (TRUE | FALSE) #booleanAtom
| ID #idAtom
| STRING #stringAtom
| NIL #nilAtom
;
OR : '||';
AND : '&&';
EQ : '==';
NEQ : '!=';
GT : '>';
LT : '<';
GTEQ : '>=';
LTEQ : '<=';
PLUS : '+';
MINUS : '-';
MULT : '*';
DIV : '/';
MOD : '%';
POW : '^';
NOT : '!';
SCOL : ';';
ASSIGN : '=';
OPAR : '(';
CPAR : ')';
OBRACE : '{';
CBRACE : '}';
TRUE : 'true';
FALSE : 'false';
NIL : 'nil';
IF : 'if';
ELSE : 'else';
WHILE : 'while';
LOG : 'log';
ID
: [a-zA-Z_] [a-zA-Z_0-9]*
;
INT
: [0-9]+
;
FLOAT
: [0-9]+ '.' [0-9]*
| '.' [0-9]+
;
STRING
: '"' (~["\r\n] | '""')* '"'
;
COMMENT
: '#' ~[\r\n]* -> skip
;
SPACE
: [ \t\r\n] -> skip
;
OTHER
: .
;
现在假设您想解析和评估输入,如下所示:
测试.mu
a = true;
b = false;
if a && b {
log "1 :: a=" + a +", b=" + b;
}
else if a || b {
log "2 :: a=" + a +", b=" + b;
}
else {
log "3 :: a=" + a +", b=" + b;
}
log "Done!";
2. 来访者 I
首先生成解析器和访问者类:
java -cp antlr-4.0-complete.jar org.antlr.v4.Tool Mu.g4 -visitor
上面的命令将生成文件MuBaseVisitor<T>
. 这是我们要扩展的没有自己的逻辑的类:
EvalVisitor.java
public class EvalVisitor extends MuBaseVisitor<Value> {
// ...
}
whereValue
只是我们语言的任何类型(String
, Boolean
, Double
)的包装器:
价值.java
public class Value {
public static Value VOID = new Value(new Object());
final Object value;
public Value(Object value) {
this.value = value;
}
public Boolean asBoolean() {
return (Boolean)value;
}
public Double asDouble() {
return (Double)value;
}
public String asString() {
return String.valueOf(value);
}
public boolean isDouble() {
return value instanceof Double;
}
@Override
public int hashCode() {
if(value == null) {
return 0;
}
return this.value.hashCode();
}
@Override
public boolean equals(Object o) {
if(value == o) {
return true;
}
if(value == null || o == null || o.getClass() != this.getClass()) {
return false;
}
Value that = (Value)o;
return this.value.equals(that.value);
}
@Override
public String toString() {
return String.valueOf(value);
}
}
3. 测试一
要测试这些类,请使用以下Main
类:
主.java
import org.antlr.v4.runtime.ANTLRFileStream;
import org.antlr.v4.runtime.CommonTokenStream;
import org.antlr.v4.runtime.tree.ParseTree;
public class Main {
public static void main(String[] args) throws Exception {
MuLexer lexer = new MuLexer(new ANTLRFileStream("test.mu"));
MuParser parser = new MuParser(new CommonTokenStream(lexer));
ParseTree tree = parser.parse();
EvalVisitor visitor = new EvalVisitor();
visitor.visit(tree);
}
}
并编译并运行源文件:
javac -cp antlr-4.0-complete.jar *.java
java -cp .:antlr-4.0-complete.jar Main
(在 Windows 上,最后一个命令是java -cp .;antlr-4.0-complete.jar Main
:)
运行后Main
,什么也没有发生(当然?)。这是因为我们没有在我们的EvalVisitor
类中实现任何规则。为了能够test.mu
正确评估文件,我们需要为以下规则提供正确的实现:
if_stat
andExpr
orExpr
plusExpr
assignment
idAtom
booleanAtom
stringAtom
log
#4。访客 II & 测试 II
以下是这些规则的实现:
import org.antlr.v4.runtime.misc.NotNull;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class EvalVisitor extends MuBaseVisitor<Value> {
// used to compare floating point numbers
public static final double SMALL_VALUE = 0.00000000001;
// store variables (there's only one global scope!)
private Map<String, Value> memory = new HashMap<String, Value>();
// assignment/id overrides
@Override
public Value visitAssignment(MuParser.AssignmentContext ctx) {
String id = ctx.ID().getText();
Value value = this.visit(ctx.expr());
return memory.put(id, value);
}
@Override
public Value visitIdAtom(MuParser.IdAtomContext ctx) {
String id = ctx.getText();
Value value = memory.get(id);
if(value == null) {
throw new RuntimeException("no such variable: " + id);
}
return value;
}
// atom overrides
@Override
public Value visitStringAtom(MuParser.StringAtomContext ctx) {
String str = ctx.getText();
// strip quotes
str = str.substring(1, str.length() - 1).replace("\"\"", "\"");
return new Value(str);
}
@Override
public Value visitNumberAtom(MuParser.NumberAtomContext ctx) {
return new Value(Double.valueOf(ctx.getText()));
}
@Override
public Value visitBooleanAtom(MuParser.BooleanAtomContext ctx) {
return new Value(Boolean.valueOf(ctx.getText()));
}
@Override
public Value visitNilAtom(MuParser.NilAtomContext ctx) {
return new Value(null);
}
// expr overrides
@Override
public Value visitParExpr(MuParser.ParExprContext ctx) {
return this.visit(ctx.expr());
}
@Override
public Value visitPowExpr(MuParser.PowExprContext ctx) {
Value left = this.visit(ctx.expr(0));
Value right = this.visit(ctx.expr(1));
return new Value(Math.pow(left.asDouble(), right.asDouble()));
}
@Override
public Value visitUnaryMinusExpr(MuParser.UnaryMinusExprContext ctx) {
Value value = this.visit(ctx.expr());
return new Value(-value.asDouble());
}
@Override
public Value visitNotExpr(MuParser.NotExprContext ctx) {
Value value = this.visit(ctx.expr());
return new Value(!value.asBoolean());
}
@Override
public Value visitMultiplicationExpr(@NotNull MuParser.MultiplicationExprContext ctx) {
Value left = this.visit(ctx.expr(0));
Value right = this.visit(ctx.expr(1));
switch (ctx.op.getType()) {
case MuParser.MULT:
return new Value(left.asDouble() * right.asDouble());
case MuParser.DIV:
return new Value(left.asDouble() / right.asDouble());
case MuParser.MOD:
return new Value(left.asDouble() % right.asDouble());
default:
throw new RuntimeException("unknown operator: " + MuParser.tokenNames[ctx.op.getType()]);
}
}
@Override
public Value visitAdditiveExpr(@NotNull MuParser.AdditiveExprContext ctx) {
Value left = this.visit(ctx.expr(0));
Value right = this.visit(ctx.expr(1));
switch (ctx.op.getType()) {
case MuParser.PLUS:
return left.isDouble() && right.isDouble() ?
new Value(left.asDouble() + right.asDouble()) :
new Value(left.asString() + right.asString());
case MuParser.MINUS:
return new Value(left.asDouble() - right.asDouble());
default:
throw new RuntimeException("unknown operator: " + MuParser.tokenNames[ctx.op.getType()]);
}
}
@Override
public Value visitRelationalExpr(@NotNull MuParser.RelationalExprContext ctx) {
Value left = this.visit(ctx.expr(0));
Value right = this.visit(ctx.expr(1));
switch (ctx.op.getType()) {
case MuParser.LT:
return new Value(left.asDouble() < right.asDouble());
case MuParser.LTEQ:
return new Value(left.asDouble() <= right.asDouble());
case MuParser.GT:
return new Value(left.asDouble() > right.asDouble());
case MuParser.GTEQ:
return new Value(left.asDouble() >= right.asDouble());
default:
throw new RuntimeException("unknown operator: " + MuParser.tokenNames[ctx.op.getType()]);
}
}
@Override
public Value visitEqualityExpr(@NotNull MuParser.EqualityExprContext ctx) {
Value left = this.visit(ctx.expr(0));
Value right = this.visit(ctx.expr(1));
switch (ctx.op.getType()) {
case MuParser.EQ:
return left.isDouble() && right.isDouble() ?
new Value(Math.abs(left.asDouble() - right.asDouble()) < SMALL_VALUE) :
new Value(left.equals(right));
case MuParser.NEQ:
return left.isDouble() && right.isDouble() ?
new Value(Math.abs(left.asDouble() - right.asDouble()) >= SMALL_VALUE) :
new Value(!left.equals(right));
default:
throw new RuntimeException("unknown operator: " + MuParser.tokenNames[ctx.op.getType()]);
}
}
@Override
public Value visitAndExpr(MuParser.AndExprContext ctx) {
Value left = this.visit(ctx.expr(0));
Value right = this.visit(ctx.expr(1));
return new Value(left.asBoolean() && right.asBoolean());
}
@Override
public Value visitOrExpr(MuParser.OrExprContext ctx) {
Value left = this.visit(ctx.expr(0));
Value right = this.visit(ctx.expr(1));
return new Value(left.asBoolean() || right.asBoolean());
}
// log override
@Override
public Value visitLog(MuParser.LogContext ctx) {
Value value = this.visit(ctx.expr());
System.out.println(value);
return value;
}
// if override
@Override
public Value visitIf_stat(MuParser.If_statContext ctx) {
List<MuParser.Condition_blockContext> conditions = ctx.condition_block();
boolean evaluatedBlock = false;
for(MuParser.Condition_blockContext condition : conditions) {
Value evaluated = this.visit(condition.expr());
if(evaluated.asBoolean()) {
evaluatedBlock = true;
// evaluate this block whose expr==true
this.visit(condition.stat_block());
break;
}
}
if(!evaluatedBlock && ctx.stat_block() != null) {
// evaluate the else-stat_block (if present == not null)
this.visit(ctx.stat_block());
}
return Value.VOID;
}
// while override
@Override
public Value visitWhile_stat(MuParser.While_statContext ctx) {
Value value = this.visit(ctx.expr());
while(value.asBoolean()) {
// evaluate the code block
this.visit(ctx.stat_block());
// evaluate the expression
value = this.visit(ctx.expr());
}
return Value.VOID;
}
}
当您重新编译并运行Main
时,以下内容将打印到您的控制台:
2 :: a=true, b=false
Done!
有关所有其他规则的实现,请参阅:https ://github.com/bkiers/Mu
编辑
来自@pwwpche,在评论中:
对于那些使用 jdk1.8 和遇到的人IndexOutOfBoundsException
来说,antlr 4.0 不知何故与 jdk1.8 不兼容。下载antlr-4.6-complete.jar,替换expr POW<assoc=right> expr
为<assoc=right>expr POW expr
将消除错误和警告。