Handling Grammar Ambiguity in ANTLR4 - parsing

Question

I have a grammar that should parse the following snippet (as an example):
vmthread programm_start
{
CALL main
}
subcall main
{
// Declarations
DATAF i
CALL i
// Statements
MOVEF_F 3 i
}
The problem is the ambiguity between the CALL statement. This op code is valid in the vmthread section (and only the CALL!) but also in those subcall sections. If I define a OP_CODES token with all op codes and an additional OC_CALL token, the lexer can't handle the situation (obviously).
The following listings are snippets of my grammar (first lexer, second parser):
VMTHREAD
: 'vmthread'
;
SUBCALL
: 'subcall'
;
CURLY_OPEN
: '{'
;
CURLY_CLOSE
: '}'
;
OP_CODES
: 'DATA8'
| 'DATAF'
| 'MOVE8_8'
| 'MOVEF_F'
| 'CALL'
;
OC_CALL
: 'CALL'
;
lms
: vmthread subcalls+
;
vmthread
: VMTHREAD name = ID CURLY_OPEN vmthreadCall CURLY_CLOSE
;
vmthreadCall
: oc = OC_CALL name = ID
;
subcalls
: SUBCALL name = ID CURLY_OPEN ins = instruction* CURLY_CLOSE
;
//instruction+
instruction
: oc = OP_CODES args = argumentList
;
argumentList
: arguments+
;
arguments
: INTEGER
| NUMBER
| TEXT
| ID
;
To continue my work I've switched the OC_CALL token in the vmthreadCall parser rule with the OP_CODES token. That solves the problem for now, because the code is auto generated. But there's the possibility that a user can type this code so this could go wrong.
Is there a solution for this or should I move the validation into the parser. There I can easily determine if the statement in the vmthread section contains just the call statement.
For clarification: In the vmthread there's only the CALL allowed. In the subcall (could be more than one) every op code is allowed (CALL + every other op code defined). And I do not want to distinguish between those different CALL statements. I know that's not possible in a context free grammar. I will handle this in the parser. I just want to restrict the vmthread to the one CALL statement and allow all statements (all op codes) in the subcalls. Hopefully that's more clear.

Answer 1

Change your lexer rules like this:
OP_CODES
: 'DATA8'
| 'DATAF'
| 'MOVE8_8'
| 'MOVEF_F'
| OP_CALL
;
OC_CALL
: 'CALL'
;
or alternatively so:
OP_CODES
: 'DATA8'
| 'DATAF'
| 'MOVE8_8'
| 'MOVEF_F'
| CALL
;
OC_CALL
: CALL
;
fragment CALL: 'CALL';
Btw, I recommend that you create explicit lexer rules for your literals (like that CALL fragment), which will make later processing easier. ANTLR assigns generic names to implicitly created literals, which makes it hard to find out which token belongs to which literal.