[英]if then else conditional evaluation
I have a language which basically is meant to map columns to a new structure in an array.我有一种语言,基本上是为了将 map 列转换为数组中的新结构。 The language is meant for product managers to define mappings without having to know a lot of programming details.
该语言旨在让产品经理无需了解大量编程细节即可定义映射。 I'm sure there is a lot more to improve here but this is what I have.
我确信这里还有很多需要改进的地方,但这就是我所拥有的。
The language works, mostly.大多数情况下,该语言都有效。 The problem I have is with conditional statements.
我遇到的问题是条件语句。
My parser has the following rule:我的解析器有以下规则:
conditionalexpr : IF^ LPAREN! (statement) RPAREN! THEN! LCURLY! statement RCURLY! (ELSE! LCURLY! statement RCURLY!)?;
Which works to generate a tree with three children.这可以生成具有三个孩子的树。
My problem is to avoid evaluating the statements if the condition doesn't allow it.我的问题是避免在条件不允许的情况下评估语句。
Very naively I did:我很天真地做了:
conditionalexpr returns[Object o]:
^(IF a=statement b=statement (c=statement)?)
{
$o = (Boolean)$a.o ? $b.o : $c.o != null ? $c.o : "";
}
;
Obviously this will not work.显然这是行不通的。
I have been playing around with syntactic predicates but I can't make those work properly.我一直在玩语法谓词,但我无法使它们正常工作。
statement returns an object currently.语句当前返回 object。 Mostly the language deals in Strings but I need to support booleans and numbers (integer and decimal) as well.
大多数语言处理字符串,但我也需要支持布尔值和数字(整数和十进制)。
If I add anything like {$ao}?=> I get a $a in the generated code.如果我添加类似 {$ao}?=> 我在生成的代码中得到一个 $a 。
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.我查看了 antlr-interest 列表,但这个问题在那里并没有得到很好的回答,很可能是因为这对他们来说似乎很明显。
I am willing to post the complete grammar but have left it out to keep this short.我愿意发布完整的语法,但为了保持简短而将其省略。
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.如果您不希望评估某些子树,则需要让树规则返回节点而不是实际值。 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.您可以扩展
CommonTree
并提供自定义TreeAdaptor
来帮助构建自己的节点,但就个人而言,我发现创建自定义节点 class(或类)并使用它们来代替是最简单的。 A demo to clarify:一个演示来澄清:
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();};
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.我创建了一个具有
eval(): Object
方法的Node
接口,还创建了一个抽象的 class BinaryNode
实现Node
并且总是有 2 个孩子。 As you can see in the tree grammar that follows after these Java classes, all rule now return a Node
.正如您在这些 Java 类之后的树语法中所见,所有规则现在都返回一个
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();
}
}
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, $ID.text);}
;
If you now run the main class, and evaluate:如果您现在运行主 class,并评估:
a = 3
b = 4
if b > a then
b==b
else
c==c
true
is being printed to the console: true
正在打印到控制台:
bart@hades:~/Programming/ANTLR/Demos/T$ 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:但是,如果您检查 if
b < a
,导致else
被执行,您将看到以下内容:
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 .有关更复杂的语言结构(范围、函数等)的实现,请参阅我的博客。
Good luck!祝你好运!
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