Can a grammar be defined such that only a particular integer is allowed?

[Beakerboy]

In VBA, the following are valid:

GoTo 1
GoTo 2
On Error GoTo 1
On Error GoTo -1

But these are both invalid:

GoTo -1
On Error GoTo -2

In the official VBA syntax, an INTEGER is only a sequence of digits, and this is what my current grammar is expecting in these cases. In most mathematical cases, the MINUS is a unary operator defined in the parser.

I’m struggling with trying to figure out how to either define a MINUS_ONE token, or a Lexer rule that will only match “-1”. Should I instead just add `MINUS INTEGER” to the parser rule and let a Lint tool sort it out later if the minus exists and the value of the integer is anything other than a “1”?

[kaby76]

This is an example of when you need to move the syntax into a static semantics analysis. You should not be adding these checks into the grammar. Separate context-free syntax and semantic checks into completely different docs: the CFG is in the Antlr4 .g4 file; semantic checks need to be defined elsewhere. You can have a constraint system called post-parse on the entire tree, or during the parse on partial trees.

For example, with the grammars-v4/vba grammar, to handle On Error GoTo -2, I would specify the constraint using XPath.

06/07-08:45:50 ~/issues/g4-current/vba/Generated-CSharp
$ cat xxx
Public Sub Module()
    ON ERROR GOTO -2
End Sub
06/07-08:45:52 ~/issues/g4-current/vba/Generated-CSharp
$ trparse -l xxx | trxgrep ' //onErrorStmt[ON_ERROR]/valueStmt/literal/SHORTLITERAL[text() = "-2"]' | trcaret
CSharp 0 xxx success 0.1587228
L2:     ON ERROR GOTO -2
                      ^
06/07-08:45:57 ~/issues/g4-current/vba/Generated-CSharp
$

Note, the vba grammar defines a SHORTLITERAL with '-'. I would not define the integer literal with an operator.

[Beakerboy]

Thanks for this. I am working on a new grammar for VBA 7.1 based on the official Microsoft language specification. The new INTEGERLITERAL definition is a little different than what was put together for the existing VBA6 grammar.

I’m using this new grammar to build a Linting and static analysis tool, so I’ll make sure to place those checks there.