Directive parsing



Preprocessor directives (#if, #define, #include, etc.) are parsed during lex.bas:lexSkipToken() by calling pp.bas:ppCheck(). After moving to the next token (or loading a new token), ppCheck() will check whether the new current token is a '#'. If so it will also check whether the previous token was an EOL. If so, it found a '#' at line begin, and directly parses the PP directive, using the same lexGetToken()/lexSkipToken() interface used by the parser. This is necessary because some PP directives result in parser functions being called, for example parser-identifier.bas:cIdentifier() is used by the #ifdef parser, to recognize variables etc.:

dim as integer i
#ifdef i
#print yes, the variable will be recognized
#endif


So, lexSkipToken() is recursive because of the PP. ppCheck() will only call pp.bas:ppParse() for toplevel calls to lexSkipToken(), but not if it was called recursively from the PP. This lets the PP parse multi-line directives like #macro ... #endmacro or skip #if ... #endif blocks without "executing" the directives those structures may contain. Note that unlike C, FB allows macros to contain PP directives.

As a result, every time the FB parser skips an EOL, lexSkipToken() might detect a '#' at line begin and handle then call the PP to let it parse that directive. It may "silently" parse more lines, and the parser stays fully unaware that the PP directives are even there. The PP parsing launched from lexSkipToken() might even encounter an #include and call fb.bas:fbIncludeFile() to parse it immediately, recursively starting a parser-toplevel.bas:cProgram() for that #include file. The parser has to be able to handle the recursion that might happen during every lexSkipToken() at EOL, but luckily that is not a big deal. The parser needs a stack to keep track of compound statements anyways.

Note that PP directives are not handled during token look ahead (lex.bas:lexGetLookAhead()). If the parser were to look ahead across EOL, it could very well see a PP directive. Luckily though looking ahead across lines is never necessary.

Macro expansion in PP directives

The beginning of directives, the keyword following the '#', is parsed without macro expansion. This means redefining PP keywords (intentionally) has no effect on the PP directives. For example:

#define define foo
#define bar baz


will not intermediately be seen as:

#foo bar baz


Directives like #if & co. make use of the PP expression parser, which does expand macros. Afterall that's the point of PP expressions. For example:
#define foo 1
#if foo = 1
#endif


The #define and #macro directives don't do macro expansion at all. A macro's body is recorded as-is.

#define/#macro parsing

pp.bas:ppDefine() first parses the macro's identifier. If there is a '(' following, without space in between, then the parameter list is parsed too.

Then the macro body is parsed. For each token, its text representation is retrieved via lexGetText(), and it is appended to the macro body text. Space is preserved (but trimmed); comments are left out; in multi-line #macros empty lines are removed.

If the macro has parameters, the macro tokens will be created (as discussed in Macro Expansion). To do that, the macro parameters are added to a temporary hash table, which associates the parameter names to their indices. Then, identifiers in the macro body are looked up, and when a parameter is recognized, a parameter(index) macro token is created, instead of appending the token to the previous text() macro token (or creating a new text() for it). After that parameter(index), if there is other text again, a new text() macro token is created.

Using # on a parameter results in the creation of a stringify_parameter(index) macro token. The PP merge operator ## is simply ommitted from the macro body, so a##b becomes ab in a text() macro token. All normal text before/after/between parameters goes into text() macro tokens.

For example:

#define add(x, y) foo bar x + y

And the actions of the #define/#macro parser will be:

'add'    - The macro's name
'(' following the name, without space in between: Parse the parameter list.
	'x'    - Parameter 0.
	','    - Next parameter.
	'y'    - Parameter 1.
	')'    - End of parameter list.
Create the macro body in form of macro tokens.
' '    - Create new text(" ").
'foo'  - Append "foo".
' '    - Append " ".
'bar'  - Append "bar".
' '    - Append " ".
'x'    - Is parameter 0, create new param(0).
' '    - Create new text(" ").
'+'    - Append "+".
' '    - Append " ".
'y'    - Is parameter 1, create new param(1).
EOL    - End of macro body.

Resulting in this macro body:

text(" foo bar "), param(0), text(" + "), param(1)


The #define parser allows macros to be redefined, if the body is the same. For example:

#define a 1
#define a 1


does not result in a duplicated definition. However this would:

#define a 1
#define a 2


Since those are pure text #defines, the comparison is the bodies is a simple string comparison. This feature is not implemented for macros with parameters currently.

PP expressions

The preprocessor has its own (but fairly small and simple) expression parser (pp-cond.bas:ppExpression()). It works much like parser-expression.bas:cExpression(), except instead of creating AST nodes, ppExpression() immediately evaluates the expressions.

PP skipping

The preprocessor uses a simple stack to manage #if/#endif blocks. Those can be nested, and there may be #includes in them, but they cannot go across files. False blocks (#if 0, or the #else of an #if 1) are immediately skipped when parsing the #if 0 or the #else (pp-cond.bas:ppSkip()), before returning to lexSkipToken().

For example:

#if 1           (push to stack: is_true = TRUE, #else not visited yet, return to lexSkipToken())
	
...     (will be parsed)

#else           1) Set the #else visited flag for the current stack node,
		   so further #else's are not allowed.
		2) Since the current stack node has is_true = TRUE,
		   that means the #else block must be skipped, -> call ppSkip().

...     (skipped in ppSkip())

#endif          (parsed from ppSkip(), skipping ends, ppSkip() returns to #else parser,
		 which returns to lexSkipToken())


Note that there are a few tricky bits about PP skipping. Since macros are allowed to contain PP directives, macro expansion must be done even during PP skipping, because an #else or #endif could be inside a multi-line macro. Also, multi-line #macro declarations are not handled during PP skipping. That means, something like this:

#if 0
#macro test()
#endif
#endmacro


will be seen as:

#if 0
 #macro test()
#endif
#endmacro


Resulting in an error (#endmacro without #macro).

So, this:

#if 0
#macro test()
	#endif
#endmacro
#endif


will not work as suggested by the indentation.

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