/*
 * $Id: pp.txt 12726 2009-10-19 09:54:05Z druzus $
 */

PP description
==============
By Przemyslaw Czerpak (druzus/at/priv.onet.pl)

Hi All,

I collected this text from notes I created when I was analyzing
Clipper PP and then I was updating it in few places. Sorry but
I do not have enough energy to check it and update. It's much
shorter then I planed and it does not contain many important
things I encoded in new PP code. Sorry you will have to look at
new code because now I do not want to think about PP any more.
After last days I hate PP and I'd be very happy if I could forget
about it for at least few days. I spend much more time on it then
I planed and I'm really frustrated with the brain off job I was
making in last days.

-----------------------------------------------------------------------------

1. Clipper's PP is a lexer which divides source code into tokens
   and then operates on these tokens and not on text data. This is the
   main reason why current [x]Harbour PP cannot be Clipper compatible.
   Tokenization is the fundamental condition which implicates a lot of
   Clipper PP behavior and as long as we do not replicate it then we
   will never be able to be Clipper compatible them.
   Even such simple code cannot be well preprocessed and compiler by
   current [x]Harbour PP:
      #define a   -1
      ? 1-a
   and it cannot be fixed with current code without breaking some
   other things, f.e. match markers which depends on number of spaces
   between tokens. So at start we have to forget about updating current PP.
   It will never be Clipper compatible and cannot be because it's not a
   lexer.

2. During dividing input data to tokens and later in finding match patterns
   Clipper PP always try to allocate the biggest possible set of input data
   as a given type even if it can break some possible other method of
   input data serialization. This can be seen in wild match marker <*marker*>
   behavior or optional clause in match pattern, operator tokenization, etc.
   It greatly simplify the code though introduce some limitations,
   f.e.:
      #xcommand CMD [FROM] FROM <*x*> => ? #<x>
   or:
      #xcommand CMD <x,...> , <id>  => ? #<x>
   or:
      #xcommand CMD <*x*> END => ? #<x>
   are accepted by Clipper PP but they cannot match any line.

3. Preprocessor should extract all quoted strings and create separated
   tokens from them. The string tokens contents cannot be modified later
   by any rules. Quoting by [] create string tokens when it's not just
   after keyword, macro or one of closing brackets: ) } ]
   We will have to change it to keep working already existing extensions
   like accessing string characters with [] operator so I suggest to change
   this condition and not create string token when it follows also constant
   value of any type - not only strings. It will be usable for scalar
   classes and overloading [] operator, f.e. someone can create LOGICAL
   class where:
      .T.[1] => ".T.", .T.[2] => "TRUE", .T.[3] => "YES"

   The opening square bracket '[' has to be closed with ']' in the same line.
   Such quoting has very high priority like normal string quoting. f.e:
      ? [ ; // /* ]
   should generate:
      QOUT( " ; // /* " )
   This implicates one important thing: PP has to read whole physical
   line from file, then convert it to tokens and if necessary (';' is the
   last token after preprocessing) read next line(s).
   There is also one exception to the above. When Clipper PP finds '['
   character and previous token is keyword or macro then it always checks
   for closing bracket and if in scanned text it will find odd numbers
   of other text delimiters ('") then ignore the type of previous token
   and always creates strings. This behavior breaks some valid code. F.e.
   Clipper cannot compile code like:
      x := a[ f("]") ] $ "test"
   or:
      x := a[ f( "'" ) ] $ "test"
   If it find closing ']' without odd number of other text delimiters
   then it creates differ token then for other opening square brackets '['
   open_array_index which has differ meaning in later preprocessing
   and allow to convert group of tokens inside to string by compiler.
   If something is not recognized by preprocessor as string token or
   open_array_index then it should never become string token. It doesn't
   matter how it will be preprocessed later, f.e.:
      #define O1 [
      #define O2 ]
      ? O1 b O2
   should generate:
      QOUT( [ b ] )
   not:
      QOUT( " b " )
   but:
      #command A <x> => ? <x>
      A [ b ]
   generate also:
      QOUT( [ b ] )
   and in this case Clipper compiler makes conversion to string.
   It means that only at initial line preprocessing preprocessor decides
   what can or cannot be string token. I think that we do not have to
   exactly replicate this behavior and we should allow string conversion
   also when '[' is not marked as open_array_index in final preprocessor
   pass which will create string token from the group of tokens inside '['
   and ']' tokens using the initial stringify condition which checks type
   of token before.
   In fact with new PP such operation will be done by still existing
   lexer after preprocessing and converting the preprocessed token to string
   which is then once again divided into tokens by FLEX or SIMPLEX. It's
   redundant and because neither FLEX nor SIMPLEX are MT safe and both
   have limitations like maximum line size we will not be able to fully
   benefit from the new code (read below about it).

4. # directives tokenization.
   In #define directive strings in result pattern cannot be quoted by [].
   They always will be used as array index or (in #[x]command
   and #[x]translate) as optional expression (when not quoted by '\').
   Characters like [] are not allowed in #define match pattern.
   Quoting by [] in #[x]command and #[x]translate match pattern
   produce optional clause. The left square bracket can be quoted by \ to
   disable this special meaning and in such case Clipper PP
   generates array tokens but they are not marked as open_array_index
   when in the code they are. It causes that in code like
      #command A B\[C] => QOUT("A B[C]")
      A B[C]
   A B[C] is not preprocessed because in #command match pattern '[' is
   not open_array_index and PP cannot find matching tokens.
   Anyhow it's possible to create passing match pattern which will use [].
   It's enough to create matching pattern for the code which have [] not
   translated to string and not bound
   with keyword. As I wrote above it will be possible when '[' is after
   one of the closing brackets: ')', '}' or ']', f.e.:
      #command A }\[C] => QOUT("A }[C]")
      A }[C]
   Will be perfectly translated. For me it seems to be limitation of
   Clipper PP implementation (probably it's a side effect of some internal
   solutions) or a bug. Not something intentionally designed. It's highly
   possible it's a hack to pass to compiler some additional information
   about preprocessed tokens because in Clipper PP seems to be also the
   compiler lexer. I do not think that we should try to keep strict
   compatibility in PP translation and also introduce the array ID tokens
   before preprocessing. Such operation can be done after preprocessing
   but this is differ subject.
   The important conclusion is that #directives should be preprocessed
   in differ way then normal lines. In general # as first line token disable
   using [] as string delimiters.

5. Clipper allow to quote strings using also back apostrophe (`) as
   string begin marker and normal apostrophe as string end marker, f.e:
      ? `Hello World'
   works perfectly.

6. String tokens can be part of match pattern as any other tokens, they
   are not case sensitive during preprocessing so it's important to early
   detect and convert data inside [] to string token.

7. NIL is preprocessed rather as keyword then constant value. At least it
   behaves like a keyword and I cannot find anything what can suggest
   something differ.

8. Numbers are not converted and stored as other tokens in literal
   form. It's important to not change the numbers representation or
   compiler will have problems with calculating declared size and
   decimal places. The number tokens can be in the following form:
   [0-9]*[\.[0-9]+]. Token is ended on first character which does not
   pass the above expression and this is the first character of next
   token. This behavior will interact with Harbour extensions for
   hexadecimal numbers 0x[0-9A-Z] and date constants 0d[0-9] and we
   will have to generate separated tokens for them. For strict compatibility
   we can disable it and create final tokens after preprocessing but I do
   not think we have to be such strictly compatible.

9. Logical value is a single token, .N. is translated to .F. and
   .Y. to .T.

10. Multi character operators are parsed as single token. It's important
   to keep the list of such operators and properly pars them at beginning
   or later we will have problems.

11. Clipper PP allow to use only characters in ASCII range from 32 to 125
   and some control codes with special meaning.
   '\n' is line terminator
   '\t' when not inside quoted string is converted to 4 spaces
   '\r' is always stripped, also from quoted strings
   '\0' stop line processing, like '\n' but the rest of line is ignored
   ^z   (chr(26)) works _exactly_ like '\n'
   All other characters are illegal

12. All characters which are not keyword, string, numbers and know
   operators are used as some pseudo binary operator tokens. We allow to
   use characters with ASCII code greater then 125 then I suggest to define
   for these characters new token called TEXT so they will not be pseudo
   operators and still will could use them.

13. Clipper have special macro token which marks all input data in the
   following form: [&<keyword>[.[<nextidchars>]]]+[&]
   It's a single token which has special meaning in preprocessing and
   we have to replicate it.

14. The expression is a list of keywords, macros and constant values
   separated by one or more of other tokens. If the other token is
   one of binary operators which is marked that need valid expression
   in some internal PP table then PP check if next token is keyword,
   number, string or operator marked as left unary followed by non operator
   token and if it's not then end the expression.
   AFAIK only -, --, ++, & operators are marked as left unary operators
   and +, !, @ don't what can break some expressions.
   Also the above behavior causes that '-' cannot be repeated many times
   as left unary operator (multiple negation) what can break some valid
   expressions too. The following tokens are marked as binary operators
   which needs valid expression as next token: +, -, *, /, %, ^

   The expression can be groped in (), {}, or [] and in such case
   PP looks for corresponding closing bracket but it does not respect
   other type of brackets and not update nested other bracket counters
   only for the currently processed pair. As long as the expression is not
   part of some other preprocessor rule which will change number
   of different brackets then it seems to be safe because at this level
   all strings should be separated tokens and each valid Clipper expression
   correctly closes brackets. User should only be careful with using []
   for strings quoting - see above conversion to strings. '[' works like
   a group operator only if it's not the first token. See below operators
   which cannot much regular match marker.

   The groped expression list is also ended when end of line is reach or ';'
   token. The ',' and closing brackets ), ], } tokens end the expression when
   bracket counter is 0.
   Some operators like :=, +=, -=, /=, *=, %=, ^=, **=, =, ==, and [ if
   is not marked as open_array_index cannot match regular match marker as
   first token, seems that they are marked as needing left side expression.
   Of course closing tokens ',', ';', ')', '}', ']' also cannot match
   regular match marker as first token.
   Tokens are equal only if after preprocessing they have the same type and
   the same value. It means that "!=" is equal to "<>" but not equal with "#"
   There is exception to this rule in restricted match marker and macro token

15. Match markers.

   <idMarker> regular match marker, matches non empty expression, cannot
              match single closing parenthesis and some operators, see above.
   <idMarker,...> list match marker, matches maximal number of comma separated
                  regular match markers, if the last token in parsed expression
                  is operator which need right valid expression and next token
                  is not such valid expression then it stops checking for
                  farther expressions even if the next token is comma,
                  it accepts empty separated regular expressions but cannot be
                  empty itself. It cannot also much anything starting with
                  closing bracket or some operators, see above, it's the same
                  behavior as in regular much marker.
   <idMarker:...> restricted match marker, checks if next token(s) is/are
                  exactly the same as one of the word in pattern. Words are
                  comma separated expressions. Word can be empty but as both
                  markers above it cannot match anything starting with closing
                  bracket or some chosen operators (see above). If the last
                  token in one of restricted expression in the marker is '&'
                  then it has special meaning. It will match any macro tokens.
                  But only in such case. If it's not the last token in one
                  of comma separated expressions then it will work like any
                  other ones.

   <*idMarker*> wild match marker, matches all tokens to the end of input
                line, the expression should not be stopped by ; token
                or any other ones. It's the only one marker which will match
                expressions starting with closing brackets and operators which
                need left side expression.
   <(idMarker)> extended expression match marker matches any number of tokens
                which do not have leading spaces until end of token list,
                comma (,) and or (;). Empty expressions are not allowed.
                It cannot match closing bracket: ')' and ']' but can match '}'
                and some operators. It cannot match expressions
                starting with '[' token and if the expressions start with
                '(' token then it drops the rule which check spaces but
                maps the same tokens as regular match marker.

   When the expression is created from tokens and match marker is followed
   by non optional token(s) then the expression is immediately finished when
   the first token following match marker is found and parentheses
   counter is 0. This additional stop condition does not work for wild
   match markers: <*idMarker*> which can be used only as the last part
   of match pattern or the pattern will never much anything. We can add
   here some extension to allow defining stop condition for wild match
   markers in the future. It will not interact with Clipper compatibility
   because rules which have some additional tokens in match pattern after
   wild marker do not work with Clipper PP at all.
   The non optional token which can stop the expression is passed as
   stop condition also to all nested optional match expressions which
   are just before it and this token is used instead of other stop tokens
   which can exist inside nested optional match pattern. This code
   illustrates it. Clipper does not preprocess the TR2.
        #xtranslate TR1 [<x,...> D]     => ! [#<x>] !
        #xtranslate TR2 [<x,...> D]  C  => ! [#<x>] !
        #xcommand CMD <*x*> => QOUT( #<x> )
        proc main()
        CMD $ TR1 a + b + c + d c
        CMD $ TR2 a + b + c + d c
        return

   There is also a hidden aspects of match markers defined by result
   pattern. Each match marker can have one of four possible states:
      1. ignore matched expression - when it's not part of result pattern
         We do not need any special case to implement this - it will be
         enough to not define result holder for such markers
      2. accept only one matched expression and refuse accepting any other
         - when it's used at least once inside non optional part of result
         pattern
      3. accept multiple matched expression - when it's used only in optional
         part of result pattern
      4. accept first matched expression and ignore others - in such way
         works repeated markers in #define directive with pseudo function.
         Harbour PP does not allow to repeat the same much marker in #define
         pseudo function generating error so such situation never happens.
         In new PP we can keep current behavior or simply not define result
         holder for repeated markers just like in point 1 above.
   PP tries to allocate as much expressions for each match marker as possible
   and finally checks if point 2 above was not broken and if it does then
   refuse to accept whole rule even if it was possible to find a valid match
   in differ way.

16. Result markers.
   <idMarker> Regular result marker - inserts matched result as is
              without any modifications. The first token inherits
              number of leading spaces from the result pattern.
   #<idMarker> Dumb stringify result marker - converts all matched
               tokens to single string token even if they are comma
               separated expressions. Clear number of leading spaces
               for the first token before creating string. If there
               are no matching tokens then create empty string token.
               Finally copy number of leading spaces from result
               pattern to the new string token and insert it.
   <"idMarker"> Normal stringify result marker - converts each comma
                separated expression in matched result into string tokens
                using the same rules as for dump stringify with the exception
                to macro tokens expressions starting with '&' followed by '('.
                The macro tokens are stringify in differ way. If macro
                does not have any internal '&' characters and has at most
                one '.' as last character then as result non quited keyword
                is generated. Otherwise it generate strings with stripped first
                '&' character.
                If expression starts with '&' token followed by single
                '(' then '&' token is stripped and the rest of tokens copied
                as is.
   <(idMarker)> Smart stringify result marker - converts each comma separated
                expression in matched result into string tokens using the same
                rules as for normal stringify with the exception to expressions
                which start with string or '(' token. In Such case it does not
                make any conversions to string and copy expression as is.
   <{idMarker}> Blockify result marker - converts each comma separated
                expression  in matched result into codeblock token by simple
                adding "{||" prefix and "}" suffix. The expression is not
                modified at all. Leading spaces in first '{' token are
                inherited from result pattern. If the expression starts with
                '{' token followed by '|' then Clipper PP recognize it as
                codeblock and does not add prefix and suffix.
   <.idMarker.> Logify result marker - unlike Clipper documentation says
                it only checks if match pattern passed the test and not
                is not empty and then insert logical token .T. otherwise .F.
                Leading spaces in new token are inherited from result pattern.

   The Dumb stringify result marker format is a little bit differ then all
   others. It needs a special token '#' before '<'. Clipper PP strips all
   '#' tokens which are before result marker token '<' and if the result
   marker was the regular one then it's converted to stringify dump otherwise
   the marker type is unchanged.
   When substitution is done then optional parts are repeated as many times
   as the biggest number of accepted multiple matched expressions in the match
   markers which are in the processed optional part. After each repeating
   tokens are shifted but only if marker accepted more then one value.
   This is the only one condition. The type or state of marker is unimportant.

   The above shows that there is no correlation between type of match
   marker and type of result marker. The type of conversion depends only
   on contents of marked expression(s) and type of result marker.

   Clipper does not support nested optional result patterns. I can add such
   support but I do not know if it's necessary. To keep the base rules used
   by Clipper PP the external optional pattern should be repeated as many
   times as maximum number of repeating in one of its nested optional
   patterns. It can be usable in some seldom cases for someone who knows
   what will happen but IMHO in most cases it will create problems so probably
   refusing such expressions is the best choice.
   In optional clauses you can observe one Clipper bug I do not want to
   replicate. When Clipper PP finds '[' then it will take all other tokens
   until first unquoted ']'. If it finds it then preprocess tokens inside
   as new result pattern but sets flag that other nested clauses are
   forbidden. But when it extracts tokens for new optional result pattern
   then it strips quote characters so when optional pattern is preprocessed
   then all '[' tokens even properly quoted in source code will cause C2073
   error. Clipper also does not respect the context of preprocessed tokens
   when it looks for optional pattern so it will break restricted match
   markers which contains ']' token. For me it's nothing more then to pure
   implementation which should be fixed.

   Some dipper tests shows also other bugs in Clipper PP when matched tokens
   ends with ','.
   In such case the blockify result marker does not create empty codeblock
   for the last token when for all empty expressions before they does.
   The same is with normal and smart stringify result markers but here it's
   also yet another problem when there is more commas at the end. The last
   one is converted to the string token with comma inside "," ;-)
   I do not think we should replicate such behaviors though it seems to
   be quite easy because they look like simple bugs which can appear in
   the most trivial implementation of some conditions.
   In general I think that many of Clipper PP behaviors even the documented
   ones was not intentionally designed. Just simply someone in the past
   created preprocessor and then the same person or probably someone else
   documented - more or less precisely - some side effects and even bugs
   of this implementation as expected behavior.

17. Storing real expression strings for later stringify operation in PP
    output and stringify result patterns.

   * Tabs are replaced by 4 spaces.
   * Only one leading space is left from the lines concatenated with ;
   * Each token should have counter with number of leading spaces
   * When result pattern is created all repeated spaces are replaced by
     a single one.
     In #define pseudo functions there is small difference to the above.
     In result pattern number of spaces before parameter(s) and token before
     is significant and stored with pattern definition. The maximum number of
     spaces between keywords is not 1 but 2.
   * During result markers substitution the original number of leading
     spaces in match marker token should overwrite number of leading
     spaces in first substituted token

18. TEXT [TO [PRINTER | FILE <(fileName)>]] / ENDTEXT
   It enables in Clipper PP special stream output. It work in differ way
   then our implementation. Clipper PP preprocess whole lines. When it
   finds:
      TEXT <keyword>,<keyword>
   command then he set special mode for next lines so they will not be
   divided to tokens in standard way but whole lines will
   be converted to string toke until special marker (ENDTEXT at the beginning
   of line) will not be found. But if the line with TEXT token has some other
   commands after ; then they are preprocessed in normal way. The new mode
   will effect _ONLY_ the next lines which will be read from file not
   currently preprocessed one. So we are not Clipper compatible here and I
   will change it. The above means that Clipper PP already supports the
   starting function Ryszard implemented in #pragma __text. Just simply
   it's enough to add it TEXT <keyword>,<keyword> after ';' token.

19. The optional match patterns can be nested and each nested submatch
   pattern is fully functional match pattern and only operates on the
   same markers as parent pattern. If optional match pattern is followed
   by another ones then they can match expressions which are any
   combination of these patterns which will pass aggressive allocation
   (see point 2 above) with one exception. Clipper PP tries to detect
   optional match patterns which contain only match markers and always
   gives them the lowest priority and if it detect more then one of
   such patterns in the series of not separated optional patterns generate
   an error.
   The optional match patterns are one of the weakest point of current PP.
   Even such simple code:
        #xcommand CMD <x> [IN [GET] [PUT]] => ? #<x>
        CMD sth IN PUT GET
   Is not well preprocessed.

20. rule have to begin with non empty token or the rule will never be used.
   Generate warning for such rules? or maybe add support for such rules
   to implement some language extensions, f.e. clasfunc{p1,p2,p3}

21. translation algorithm used by Clipper PP

   Initiate token list

   Do
      get line stripping comments and dividing line to tokens
   While last token in list is ;


   Do While not empty token list

      Do

         If the first token is # then
            parse # directive and remove all line tokens
            break
         EndIf

         Do
            Do
               For each keyword token check if it match:
                  #define
                     If token(s) can be substituted then substitute
               Next
            While anything substituted

            Do
               For each token check if it match:
                  #[x]translate
                     If token(s) can be substituted then substitute
               Next
            While anything substituted
            If anything substituted
               continue

            Do While 1-st token match some #[x]command pattern
               substitute
            EndDo

         While anything substituted

         Output processed token until the last one or ; token

         If 1-st token is '#'
            continue

         Remove all tokens in the list until the last one or ; token
         break

      While True

   EndDo

   Output EOL

   The above algorithm is differ then the one used by [x]Harbour and this is
   the next reason why we are not Clipper compatible in substitution precedence.
   This code illustrate the problem:

      #define    RULE( p )    ? "define value", p
      #translate RULE(<p>) => ? "translate value", <p>
      #command   RULE(<p>) => ? "command value", <p>

      #define    DEF( p )     RULE( p )
      #translate TRS(<p>)  => RULE(<p>)
      #command   CMD(<p>)  => RULE(<p>)

      proc main()
      DEF("def")
      TRS("trs")
      CMD("cmd")
      return

   Compile it by Clipper and [x]Harbour and compare the results.

   Next important thing is that Clipper preprocess all indirect #directive body.
   It means that in Clipper is not possible to execute indirect #undef DEFNAME
   because if DEFNAME is already defined then it will be preprocessed and as
   result we will have #undef <DEFNAME_value> before PP execute this #
   directive. We can replicate this behavior but personally I do not like it.
   for me it's a limitation not a feature and I do not want to replicate it.
   So as I would like to define additional stop condition for line tokens
   preprocessing: ';' followed by '#'.
   I do not want to make all ';' the stop condition like in current [x]Harbour
   PP because the same stop condition has to be used in wild match marker.
   In Clipper it matches the text to the end if line. In new PP it will match
   the text to the end of line or next # directive. I think it will give
   reasonable compatibility level and the body of indirect # directive will
   not be preprocessed. Please note that programmer still will be able to
   force preprocessing of indirect # directive body using additional
   preprocessor rule(s) and even control the preprocessing level f.e.:
      #define   PREPROCESS_DIRECTIVE      DO_DIRECTIVE
      #xcommand HASH_DIRECTIVE [<*x*>] => PREPROCESS_DIRECTIVE <x>
      #xcommand DO_DIRECTIVE   [<*x*>] => \# <x>

      #define NEWCMD    MYCMD
      #xcommand CREATEDIRECTIVE => HASH_DIRECTIVE xcommand NEWCMD \<x> => ;;
                                   QOUT( "INDIRECT # DIRECTIVE", #\<x> )
      CREATEDIRECTIVE
      MYCMD Hello

   The second problem is stop condition in # directive body. When PP finds #
   as first token then always remove all tokens to the end of line and take
   them as part of # directive or ignore. It does not respect ';' token as
   command separator. This also causes pleasure side effects, f.e. it's
   not possible to insert indirect # directive without breaking commands
   after it because they will be always used as part of the inserted #
   directive by PP. Here I strongly prefer to define the following behavior:
   direct #define, #[x]translate and #[x]command always accept tokens to the
   end of line ignoring ';'. Just like in Clipper. All other #directive will
   respect ';' as end of # directive - It will cause that ';' cannot be used
   in #error and #stdout. If it's a problem then I can define add support for
   quoting ';' by '\' for this command and be default keep Clipper
   compatibility for not quoted ';' and end rule for quoted ones or by default
   use unquoted ';' as end of command and display others. Current Harbour
   PP always stop #error and #stdout on ; what is not Clipper compatible and
   so far I haven't seen that people reported it as bug so probably it's not
   big problem.
   Indirect #define, #[x]translate and #[x]command will also respect ';'
   as end of command. If user will need to use multiple commands in result
   pattern of indirect # directive then it will be enough to define ';' as
   some other preprocessor rule, .f.e:
      #define EOC ;;

      #xcommand CREATECMD     => #xcommand NEWCMD => QOUT("1") EOC QOUT("2")
      CREATECMD
      NEWCMD

   This will give programmer full control on preprocessed data when in
   Clipper the indirect # directive seems to be a hack added later and
   can be used only in very limited way. F.e. in Class(y) as workaround
   for Clipper PP behavior #include is used to execute# directive directly
   from included files.

22. #define exception. I do not understand why Clipper has it.
   during substitution if substituted token is:
        '#' 'define' <otherTokens,...>
   then it replaces all tokens from current position to the end of line.
   It's quite possible that it's a work around for some side effects with
   indirect directive in Clipper PP described above. Anyhow it's not
   complex solution so I will not replicate it.

23. Conditional compilation.
   a. #if[n]def directive pushes on the conditional statement stack current
      conditional compilation flag and create new one. If the flag already
      disabled preprocessing then the new flag have condition which cannot
      be changed by #else.
   b. #endif directive pops this value. If conditional statement stack was
      empty then error is generated: "Error C2069  #endif does not match #endif".
   c. #else directive reverts the current conditional statement flag if its
      status allow modifications by #else
      If conditional statement stack was empty then error is generated:
      "Error C2070  #else does not match #ifdef"
   If the conditional compilation flag is set then Clipper PP ignores all
   parsed tokens except #if, #endif, #else directive.
   The conditional compilation flag and stack are global for all included
   files so one can set the new condition and other change or pop it.
   In Clipper #if[n]def <define> has to be separate statement in a line
   additional token or line concatenators (;) are not allowed. #endif
   and #else have to be the first token in the line all tokens after are
   ignored to the end of line, command separator ';' is also ignored.

24. NOTE is not an instruction keyword but whole like comments and has
   to be stripped at beginning. It has higher priority then /* */
   It does not have its special meaning when is used after ;

25. Suggested extensions:
   - Higher priority for multi line comments /* */ stripping then in Clipper
     where line concatenation (;) is interpreted before /* */. Just like now.
     IMHO we should not replicate exact Clipper behavior here.
   a. Add #pragma operator directive to define new multi character operators.
      Such feature will allow to remove from FLEX/SIMPLEX the hack for ::
      translation to Self and define safely other operators which will be
      used as single token. Now it's impossible so things like @: matches
         @ <any_number_of_blank_characters> :
   b. token concatenation with new PP operator/marker or automatic in some
      chosen cases, f.e. no spaces between two tokens and both tokens are
      valid keywords
   c. Something to stop result pattern definition in # directives and begin
      new command. ; does not interrupt it but it's included to result pattern.
      The result pattern works like wild match marker: <*resultPattern*>
      It should work also for #define. Maybe it should be global token to
      stop all wild markers. We can add special status for ; token. Such
      token will have to also break loops with #define and #[x]translate
      processing or we will never be able to make from #undef SOME_DEFINE
      indirect PP rule used when SOME_DEFINE exists - just simply SOME_DEFINE
      will be preprocessed earlier to the defined value.
      Such special status can be added automatically when ; token is followed
      by # or ; is quoted by \
   d. already existing xHarbour extensions:
         #[x]unTrasnslate, #[x]unCommand
      but modified to locate match pattern which can cover exactly the same
      data.
         #if
      but working with integer to allow using 64bit ones which are broken
      due to conversion to double. The semantic for expressions will be
      similar to C one with the exception to ! (not) operator precedence.
      I do not think that Clipper/xbase users are familiar with the exact
      not operator precedence in C which is differ then the one in xbase
      world.
   e. modified version of Harbour's
         #pragma {__text, __stream, __cstream, __endtext}
   f. other, see in the code.

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Other things you can see in the new PP code. I was adding comments or
using HB_CLP_STRICT macro to mark the most important things. In few places
I had to break Clipper compatibility to keep FLEX working. Just simply
I cannot generate preprocessed line in exactly the same form as Clipper
does because FLEX or SIMPLEX will not be able to decode it.

Update:
New Harbour lexer is simple translator between PP tokens and Harbour
grammar terminal symbols so it's not necessary to convert preprocessed
code to strings to pass them to FLEX or SIMPLEX. It works faster and it's
fully compatible Clipper behavior what fixes above problem.

best regards,
Przemek
2006-11-08

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