问题
I have a language which basically is meant to map columns to a new structure in an array. The language is meant for product managers to define mappings without having to know a lot of programming details. I'm sure there is a lot more to improve here but this is what I have.
The language works, mostly. The problem I have is with conditional statements.
My parser has the following rule:
conditionalexpr : IF^ LPAREN! (statement) RPAREN! THEN! LCURLY! statement RCURLY! (ELSE! LCURLY! statement RCURLY!)?;
Which works to generate a tree with three children.
My problem is to avoid evaluating the statements if the condition doesn't allow it.
Very naively I did:
conditionalexpr returns[Object o]:
^(IF a=statement b=statement (c=statement)?)
{
$o = (Boolean)$a.o ? $b.o : $c.o != null ? $c.o : "";
}
;
Obviously this will not work.
I have been playing around with syntactic predicates but I can't make those work properly.
statement returns an object currently. Mostly the language deals in Strings but I need to support booleans and numbers (integer and decimal) as well.
If I add anything like {$a.o}?=> I get a $a in the generated code.
I have looked on the antlr-interest list but this question is not really answered well there, most likely because it seems obvious to them.
I am willing to post the complete grammar but have left it out to keep this short.
回答1:
If you don't want certain sub-trees to be evaluated, you'll need to let the tree rules return nodes instead of actual values. You can either extend CommonTree and provide a custom TreeAdaptor to help build your own nodes, but personally, I find it easiest to create a custom node class (or classes) and use them instead. A demo to clarify:
T.g
grammar T;
options {
output=AST;
}
tokens {
ASSIGNMENT;
}
parse
: statement+ EOF -> statement+
;
statement
: ifStatement
| assignment
;
ifStatement
: IF a=expression THEN b=expression (ELSE c=expression)? -> ^(IF $a $b $c?)
;
assignment
: ID '=' expression -> ^(ASSIGNMENT ID expression)
;
expression
: orExpression
;
orExpression
: andExpression (OR^ andExpression)*
;
andExpression
: equalityExpression (AND^ equalityExpression)*
;
equalityExpression
: relationalExpression (('==' | '!=')^ relationalExpression)*
;
relationalExpression
: atom (('<=' | '<' | '>=' | '>')^ atom)*
;
atom
: BOOLEAN
| NUMBER
| ID
| '(' expression ')' -> expression
;
IF : 'if';
THEN : 'then';
ELSE : 'else';
OR : 'or';
AND : 'and';
BOOLEAN : 'true' | 'false';
ID : ('a'..'z' | 'A'..'Z')+;
NUMBER : '0'..'9'+ ('.' '0'..'9'+)?;
SPACE : (' ' | '\t' | '\r' | '\n') {skip();};
Main.java
I've created a Node interface that has a eval(): Object method, and also created an abstract class BinaryNode which implements Node and will have always 2 children. As you can see in the tree grammar that follows after these Java classes, all rule now return a Node.
import org.antlr.runtime.*;
import org.antlr.runtime.tree.*;
public class Main {
public static void main(String[] args) throws Exception {
String source = "a = 3 b = 4 if b > a then b==b else c==c";
TLexer lexer = new TLexer(new ANTLRStringStream(source));
TParser parser = new TParser(new CommonTokenStream(lexer));
TWalker walker = new TWalker(new CommonTreeNodeStream(parser.parse().getTree()));
Node root = walker.walk();
System.out.println(root.eval());
}
}
interface Node {
Object eval();
}
abstract class BinaryNode implements Node {
protected Node left;
protected Node right;
public BinaryNode(Node l, Node r) {
left = l;
right = r;
}
}
class AtomNode implements Node {
private Object value;
public AtomNode(Object v) {
value = v;
}
@Override
public Object eval() {
return value;
}
}
class OrNode extends BinaryNode {
public OrNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return (Boolean)super.left.eval() || (Boolean)super.right.eval();
}
}
class AndNode extends BinaryNode {
public AndNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return (Boolean)super.left.eval() && (Boolean)super.right.eval();
}
}
class LTNode extends BinaryNode {
public LTNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return (Double)super.left.eval() < (Double)super.right.eval();
}
}
class LTEqNode extends BinaryNode {
public LTEqNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return (Double)super.left.eval() <= (Double)super.right.eval();
}
}
class GTNode extends BinaryNode {
public GTNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return (Double)super.left.eval() > (Double)super.right.eval();
}
}
class GTEqNode extends BinaryNode {
public GTEqNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return (Double)super.left.eval() >= (Double)super.right.eval();
}
}
class EqNode extends BinaryNode {
public EqNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return super.left.eval().equals(super.right.eval());
}
}
class NEqNode extends BinaryNode {
public NEqNode(Node left, Node right) { super(left, right); }
@Override
public Object eval() {
return !super.left.eval().equals(super.right.eval());
}
}
class VarNode implements Node {
private java.util.Map<String, Object> memory;
private String var;
VarNode(java.util.Map<String, Object> m, String v) {
memory = m;
var = v;
}
@Override
public Object eval() {
Object value = memory.get(var);
if(value == null) {
throw new RuntimeException("Unknown variable: " + var);
}
return value;
}
}
class IfNode implements Node {
private Node test;
private Node ifTrue;
private Node ifFalse;
public IfNode(Node a, Node b, Node c) {
test = a;
ifTrue = b;
ifFalse = c;
}
@Override
public Object eval() {
return (Boolean)test.eval() ? ifTrue.eval() : ifFalse.eval();
}
}
TWalker.g
tree grammar TWalker;
options {
tokenVocab=T;
ASTLabelType=CommonTree;
}
@members {
private java.util.Map<String, Object> memory = new java.util.HashMap<String, Object>();
}
walk returns [Node n]
: (statement {$n = $statement.n;})+
;
statement returns [Node n]
: ifStatement {$n = $ifStatement.n;}
| assignment {$n = null;}
;
assignment
: ^(ASSIGNMENT ID expression) {memory.put($ID.text, $expression.n.eval());}
;
ifStatement returns [Node n]
: ^(IF a=expression b=expression c=expression?) {$n = new IfNode($a.n, $b.n, $c.n);}
;
expression returns [Node n]
: ^(OR a=expression b=expression) {$n = new OrNode($a.n, $b.n);}
| ^(AND a=expression b=expression) {$n = new AndNode($a.n, $b.n);}
| ^('==' a=expression b=expression) {$n = new EqNode($a.n, $b.n);}
| ^('!=' a=expression b=expression) {$n = new NEqNode($a.n, $b.n);}
| ^('<=' a=expression b=expression) {$n = new LTEqNode($a.n, $b.n);}
| ^('<' a=expression b=expression) {$n = new LTNode($a.n, $b.n);}
| ^('>=' a=expression b=expression) {$n = new GTEqNode($a.n, $b.n);}
| ^('>' a=expression b=expression) {$n = new GTNode($a.n, $b.n);}
| BOOLEAN {$n = new AtomNode(Boolean.valueOf($BOOLEAN.text));}
| NUMBER {$n = new AtomNode(Double.valueOf($NUMBER.text));}
| ID {$n = new VarNode(memory, $ID.text);}
;
If you now run the main class, and evaluate:
a = 3
b = 4
if b > a then
b==b
else
c==c
true is being printed to the console:
bart@hades:~/Programming/ANTLR/Demos/T$ java -cp antlr-3.3.jar org.antlr.Tool T.g
bart@hades:~/Programming/ANTLR/Demos/T$ java -cp antlr-3.3.jar org.antlr.Tool TWalker.g
bart@hades:~/Programming/ANTLR/Demos/T$ javac -cp antlr-3.3.jar *.java
bart@hades:~/Programming/ANTLR/Demos/T$ java -cp .:antlr-3.3.jar Main
true
But if you check if b < a, causing the else to be executed, you'll see the following:
Exception in thread "main" java.lang.RuntimeException: Unknown variable: c
at VarNode.eval(Main.java:140)
at EqNode.eval(Main.java:112)
at IfNode.eval(Main.java:160)
at Main.main(Main.java:11)
For implementations of more complicated language constructs (scopes, functions, etc.), see my blog.
Good luck!
来源：https://stackoverflow.com/questions/6598975/if-then-else-conditional-evaluation