Should an empty begin/end block be classified as `ntEmptyStatement` ?

[NickRing]

Hi,

Came across a piece of code that is like this:

if <expression> then
begin
end
else
begin
  // statements
end;

Yes, there is nothing in the initial begin/end group. The XML output is below. Since there is a type of ntEmptyStatement (used within Case statement - ntCaseSelector), I am just asking if ntEmptyStatement should be used in this kind of empty statement (not saying it is a bug, not saying it is a feature request... )

Cheers,
Nick

      <IF line="5010" col="3">
        <EXPRESSION line="5010" col="6">
          <... snipped ... >
        </EXPRESSION>
        <THEN line="5010" col="65">
          <STATEMENTS begin_line="5011" begin_col="3" end_line="5013" end_col="3"/>
        </THEN>
        <ELSE line="5013" col="3">
          <STATEMENTS begin_line="5014" begin_col="3" end_line="5018" end_col="6">

[bogdanpolak]

@NickRing can you elaborate what kid of problem do you have with existing code, because personally for me this is hard to guess what do you expect. Do you want to have updated XML generator?

[JackTrapper]

I would say that this behavior is fine and correct. Because ntStatements means there is a begin/end block:

if expression then                           // ntIf ntExpression ntThen
begin                                        // ntStatements
end                                          // 
else                                         // ntElse
begin                                        // ntStatements
   // statements
end;

Which parses into a tree:

ntIf                                         ;if 
├─ ntExpression                              
│  ╰─ ntIdentifier @anName="expression"      ;expression	
├─ ntThen                                    ;then			
│  ╰─ ntStatements                           ;begin...end
╰─ ntElse                                    ;else
   ╰─ ntStatements                           ;begin...end

The ntStatements means you had a begin/end.

Omitting begin/end to omit ntStatements

If you omitted the begin/end from the ntThen section:

if expression then                           // ntIf ntExpression ntThen
   // No-op in this case
else                                         // ntElse
begin                                        // ntStatements
   // statements
end;

then you will get an AST that omits ntStatements—because there is no begin/end:

ntIf                                         ;if      
├─ ntExpression                              
│  ╰─ ntIdentifier @anName="expression"      ;expression
├─ ntThen                                    ;then
│                                            ; Notice: No begin/end means no ntStatements
╰─ ntElse                                    ;else
   ╰─ ntStatements 		                     ;begin...end

It's important to remember that ntStatements is a collection of ntStatement. When we have the statements, it means we have the begin/end.

begin                                ; /-start of ntStatements
   ...                               ; |   series of ntStatement
end                                  ; \-end of ntStatements

The ntEmptyStatement is reserved for where a statement can be empty.

• it's empty when the statement is empty
• not when a collection of statements (ntStatements) is empty

It means there is supposed to be a statement, but instead all they got was a semicolon (;).

For example you are allowed to have an empty statement in a case statement. The following is allowed:

case Foo of
3: Print('Fizz');
5: Print('Buzz');
7: {nop}; //notice the empty statement, it even ends with a semicolon
end

The reason it returns an ntEmptyStatement is because there is a statement there; the developer forced it into existance with the semicolon (;):

ntCaseSelector 
├─ ntCaseLabels 
│  ╰─ ntCaseLabel 
│     ╰─ ntExpression 
│        ╰─ ntLiteral Value="7" @anType="numeric"		(TValuedSyntaxNode)
╰─ ntEmptyStatement 

But the same is not true for an if statement. The following is not allowed to have an empty statement:

if expression then                           // ntIf ntExpression ntThen
   {nop};                                    // Syntax error!
else                                         // ntElse
begin                                        // ntStatements
   // statements
end;

That semi-colon is not allowed there. It's not an ntEmptyStatement; it is a syntax error.

I vote to close.