Hi all,
I tried to skip a rule in an if-then-else construct using gated symantic
predicates.
ifElse
scope {
boolean expResult;
} :
^(
IFTHEN b = expression { $ifElse::expResult = b; }
(
{$ifElse::expResult == true}?=> actionSequence
| . // if expResult == false, no action required but eat the token
)
)
|
^(
IFTHENELSE b = expression { $ifElse::expResult = b; }
(
{$ifElse::expResult == true}? actionSequence
. // if expResult == true, call the 'then' action and 'eat'
the else action
| . actionSequence // if expResult == false, 'eat' the 'then'
action and call the else action
)
);
leeds to
Eval.g: node from after line 1:21 no viable alternative at input 'DOWN
This works well until the subrule itself is a tree (i.e. nested
if-then-else). So it seems, that the '.' does not match the whole subtree?
Thanks for any hint, fridi
tree grammar Eval;
options {
tokenVocab = Simple;
language = Java;
ASTLabelType = CommonTree;
}
@header {
import java.util.Hashtable;
}
@members {
private Hashtable<String, Integer> vars = new Hashtable<String, Integer>();
public void setVars(Hashtable<String, Integer> vars) {
this.vars = vars;
}
public void setVar(String name, int value) {
vars.put(name, new Integer(value));
}
public int getVar(String name) {
if (vars.containsKey(name)) {
return vars.get(name).intValue();
}
return -1;
}
boolean expResult;
}
rule :
ifElse;
ifElse
scope {
boolean expResult;
} :
^(
IFTHEN b = expression { $ifElse::expResult = b; }
(
{$ifElse::expResult == true}?=> actionSequence
| . // if expResult == false, no action required but eat the token
)
)
|
^(
IFTHENELSE b = expression { $ifElse::expResult = b; }
(
{$ifElse::expResult == true}? actionSequence
. // if expResult == true, call the 'then' action and 'eat' the else
action
| . actionSequence // if expResult == false, 'eat' the 'then' action
and call the else action
)
);
actionSequence :
^(as = ASSIGN id = ID in = INT )
{
int value = 0;
try {
value = Integer.parseInt($in.text);
} catch (Exception e) {
}
setVar($id.text, value);
}
| ifElse;
expression returns [boolean b] :
^(op = OPERAND id = ID in = INT {
int var = getVar($id.text);
int value = 0;
try {
value = Integer.parseInt($in.text);
} catch (Exception e) {
}
$b = var > value;
} );
grammar Simple;
options {
language = Java;
output = AST;
}
tokens {
IFTHEN;
IFTHENELSE;
}
start
:
rule
;
rule
:
IF expression 'then' a1=actionSequence
(
'else' a2=actionSequence
->
^(IFTHENELSE expression $a1 $a2)
|
->
^(IFTHEN expression $a1)
)
'endif'
;
actionSequence
:
ID ASSIGN^ INT
| rule
;
expression
:
ID OPERAND^ INT
;
IF
:
'if'
;
ASSIGN
:
'='
;
OPERAND
:
'>' // reduced to > for simple example
;
ID
:
(
'a'..'z'
| 'A'..'Z'
| '_'
)
(
'a'..'z'
| 'A'..'Z'
| '0'..'9'
| '_'
)*
;
INT
:
'0'..'9'+
;
WS
:
(
' '
| '\t'
| '\r'
| '\n'
)
{$channel=HIDDEN;}
;
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