How to resolve "The following alternatives can never be matched" - antlr

Question

I have been struggling to resolve a "multiple alternatives" error in my parser for a couple of days now but with no success. I have been converting Bart Kiers excellent Tiny Language(TL) tutorial code to C# using Sam Harwell's port of ANTLR3 and VS2010. Kudos to both these guys for their excellent work. I believe I have followed Bart's tutorial accurately but as I am a newbie with ANTLR I can't be sure.
I did have the TL code working nicely on a pure math basis i.e. no "functions" or "if then else" or "while" (see screenshot of a little app)
but when I added the code for the missing pieces to complete the tutorial I get a parsing error in "functionCall" and in "list" (see the code below)
grammar Paralex2;
options {
language=CSharp3;
TokenLabelType=CommonToken;
output=AST;
ASTLabelType=CommonTree;
}
tokens {
BLOCK;
RETURN;
STATEMENTS;
ASSIGNMENT;
FUNC_CALL;
EXP;
EXP_LIST;
ID_LIST;
IF;
TERNARY;
U_SUB;
NEGATE;
FUNCTION;
INDEXES;
LIST;
LOOKUP;
}
#lexer::namespace{Paralex2}
#parser::namespace{Paralex2}
/*
* Parser Rules
*/
#parser::header {using System; using System.Collections.Generic;}
#parser::members{
public SortedList<string, Function> functions = new SortedList<string, Function>();
private void defineFunction(string id, Object idList, Object block) {
// `idList` is possibly null! Create an empty tree in that case.
CommonTree idListTree = idList == null ? new CommonTree() : (CommonTree)idList;
// `block` is never null.
CommonTree blockTree = (CommonTree)block;
// The function name with the number of parameters after it the unique key
string key = id + idListTree.Children.Count();
functions.Add(key, new Function(id, idListTree, blockTree));
}
}
public parse
: block EOF -> block
;
block
: (statement | functionDecl)* (Return exp ';')? -> ^(BLOCK ^(STATEMENTS statement*) ^(RETURN exp?))
;
statement
: assignment ';' -> assignment
| functionCall ';' -> functionCall
| ifStatement
| forStatement
| whileStatement
;
assignment
: Identifier indexes? '=' exp
-> ^(ASSIGNMENT Identifier indexes? exp)
;
functionCall
: Identifier '(' expList? ')' -> ^(FUNC_CALL Identifier expList?)
| Assert '(' exp ')' -> ^(FUNC_CALL Assert exp)
| Size '(' exp ')' -> ^(FUNC_CALL Size exp)
;
ifStatement
: ifStat elseIfStat* elseStat? End -> ^(IF ifStat elseIfStat* elseStat?)
;
ifStat
: If exp Do block -> ^(EXP exp block)
;
elseIfStat
: Else If exp Do block -> ^(EXP exp block)
;
elseStat
: Else Do block -> ^(EXP block)
;
functionDecl
: Def Identifier '(' idList? ')' block End
{defineFunction($Identifier.text, $idList.tree, $block.tree);}
;
forStatement
: For Identifier '=' exp To exp Do block End
-> ^(For Identifier exp exp block)
;
whileStatement
: While exp Do block End -> ^(While exp block)
;
idList
: Identifier (',' Identifier)* -> ^(ID_LIST Identifier+)
;
expList
: exp (',' exp)* -> ^(EXP_LIST exp+)
;
exp
: condExp
;
condExp
: (orExp -> orExp)
| ( '?' a=exp ':' b=exp -> ^(TERNARY orExp $a $b)
| In exp -> ^(In orExp exp)
)?
;
orExp
: andExp ('||'^ andExp)*
;
andExp
: equExp ('&&'^ equExp)*
;
equExp
: relExp (('==' | '!=')^ relExp)*
;
relExp
: addExp (('>=' | '<=' | '>' | '<')^ addExp)*
;
addExp
: mulExp ((Add | Sub)^ mulExp)*
;
mulExp
: powExp ((Mul | Div)^ powExp)*
;
powExp
: unaryExp ('^'^ unaryExp)*
;
unaryExp
: Sub atom -> ^(U_SUB atom)
| '!' atom -> ^(NEGATE atom)
| atom
;
atom
: Nmber
| Bool
| Null
| lookup
;
list
: '[' expList? ']' -> ^(LIST expList?)
;
lookup
: list indexes? -> ^(LOOKUP list indexes?)
| functionCall indexes? -> ^(LOOKUP functionCall indexes?)
| Identifier indexes? -> ^(LOOKUP Identifier indexes?)
| String indexes? -> ^(LOOKUP String indexes?)
| '(' exp ')' indexes? -> ^(LOOKUP exp indexes?)
;
indexes
: ('[' exp ']')+ -> ^(INDEXES exp+)
;
/*
* Lexer Rules
*/
Assert : 'assert';
Size : 'size';
Def : 'def';
If : 'if';
Else : 'else';
Return : 'return';
For : 'for';
While : 'while';
To : 'to';
Do : 'do';
End : 'end';
In : 'in';
Null : 'null';
Or : '||';
And : '&&';
Equals : '==';
NEquals : '!=';
GTEquals : '>=';
LTEquals : '<=';
Pow : '^';
GT : '>';
LT : '<';
Add : '+';
Sub : '-';
Mul : '*';
Div : '/';
Modulus : '%';
OBrace : '{';
CBrace : '}';
OBracket : '[';
CBracket : ']';
OParen : '(';
CParen : ')';
SColon : ';';
Assign : '=';
Comma : ',';
QMark : '?';
Colon : ':';
Bool
: 'true'
| 'false'
;
Nmber
: Int ('.' Digit*)?
;
Identifier
: ('a'..'z' | 'A'..'Z' | '_') ('a'..'z' | 'A'..'Z' | '_' | Digit)*
;
String
#after {
setText(getText().substring(1, getText().length()-1).replaceAll("\\\\(.)", "$1"));
}
: '"' (~('"' | '\\') | '\\' ('\\' | '"'))* '"'
| '\'' (~('\'' | '\\') | '\\' ('\\' | '\''))* '\''
;
Comment
: '//' ~('\r' | '\n')* {Skip();}
| '/*' .* '*/' {Skip();}
;
Space
: (' ' | '\t' | '\r' | '\n' | '\u000C') {Skip();}
;
fragment Int
: '1'..'9' Digit*
| '0'
;
fragment Digit
: '0'..'9'
;
The error messages I get are
Decision can match input such as "CParen" using multiple alternatives: 1, 2 : Line 79:20
and
Decision can match input such as "CBracket" using multiple alternatives: 1, 2 : Line 176:10
The errors relate to the functionCall and list rules. I have examined the parser file in ANTLRWorks 1.5 and confirmed the same errors there. The syntax diagrams for the two rules look like this;
and this;
I have tried several changes to try to solve the problem but I don't seem to be able to get the syntax right. I would appreciate any help you guys could provide and can email the images if that would help.
Thanks in advance
Ian Carson

Answer 1

You have an OR-operator too many in the condExp rule making the grammar ambiguous.
You have:
condExp
: ( orExp -> orExp)
| ( '?' a=exp ':' b=exp -> ^(TERNARY orExp $a $b)
| In exp -> ^(In orExp exp)
)?
;
corresponding to:
But it should be:
condExp
: ( orExp -> orExp)
( '?' a=exp ':' b=exp -> ^(TERNARY orExp $a $b)
| In exp -> ^(In orExp exp)
)?
;
corresponding to: