# The Preprocessor

TADS 3 provides a macro preprocessor that's essentially equivalent to the standard ANSI C preprocessor.

The preprocessor is effectively a separate phase of processing that runs before the compiler: the preprocessor's output is the compiler's input. The preprocessor works entirely at the textual level; it's not part of the compiler per se, and it doesn't make any attempt to understand the underlying TADS language or program structure.

## Lexical structure

The preprocessor uses all the same lexical rules as the TADS language: strings are enclosed in single or double quotes; symbols consist of an alphabetic or underscore character followed by zero or more alphanumeric or underscore characters; numbers consist of strings of digits, with periods to indicate decimal points and 'e' or 'E' to indicate exponents in floating-point values; and various punctuation marks and combinations of punctuation marks are used as operator tokens.

Block comments are specified with /* ... */ sequences, and // begins a comment that runs to the end of the line. Each comment is replaced with single space character before any other work is done.

In addition to the basic TADS lexical structure, the preprocessor has a couple of special features of its own:

- If the last character of a source line is "\" (a backslash), the preprocessor replaces the backslash and the newline that follows with a single space character. This effectively joins two physical lines of the source file into one "logical" line. This is done before directives are interpreted, which allows you to write a single directive that spans multiple physical lines. Note that this only takes effect when a "\" is the very last character on the line; if anything else follows, even whitespace, the backslash is treated as just part of the source text, and doesn't join the line with the next one.
- "#" at the beginning of a line introduces a preprocessor directive. Whitespace can optionally precede the "#" and can come between the "#" and the directive name. Each directive is exactly one logical line long, and runs to the end of the logical line.

## Directives

All preprocessor directives have compilation unit scope. A few, such as #pragma newline_spacing, are further limited to the current include file; exceptions to compilation unit scope are noted with the individual directive descriptions.

### #charset

#charset specifies the file's character set. If this directive is used, it must be the very first thing in the file. It can't be preceded by anything, not even a comment or whitespace.

The syntax is:

    #charset "name"

where name is the character set name. You can use one of the built-in character sets ("us-ascii", "latin1") or any of the available mappings. On most platforms, the TADS 3 distribution includes a large set of common mappings, including the ISO-8859-n character sets, many of the 8-bit Windows code pages, and the Macintosh sets.

### #define

This directive defines a macro. It has two forms: one that defines a simple constant symbol, and one that defines a function-like macro with parameters.

    #define simpleMacro text
    #define funcMacro( [ param1 [ , param2 ... ]  ]  ) textWithParameters

The first form defines a "simple" macro, which simply associates a symbol name with some replacement text. Each time the macro symbol name appears in subsequent source text, it's simply replaced with the expansion text.

The second form defines a "function-like" macro. This means that the macro is syntactically like a function, in that a list of arguments, enclosed in parentheses, must appear after the macro name each time it's used. The parameter list can include zero, one, or more parameters, separated by commas; each parameter name must be a symbol token, and a name cannot be repeated within the list. Whenever the macro name appears in the source text, it must be followed by a parenthesized list of values; these values will be substituted into the replacement text where the names of the parameters appear in the replacement text.

Note that if you define a parameter list, there must be no whitespace between the macro symbol name and the open parenthesis. The parenthesis must appear immediately after the macro name.

Macro processing is described in more detail below.

### #error

This macro lets you generate a compiler error with your own custom message. The message is displayed on the console in the same format as an actual compiler message. This is most useful with conditional compilation, such as to flag build configurations or module combinations that you don't wish to allow.

The syntax is:

    #error token [ token2 ... ]

The token list can contain any number of tokens of any kind. However, note that macro expansion is performed on the line, so in most cases, you'll want to enclose your message text in quotes so that it's displayed literally, rather than being expanded into any macros that happen to match the words in the message text.

Here's an example that checks to make sure an included library header has a suitable version number:

    #include "MyLib.h"
    #if MYLIB_VSN < 5
    #error "This module requires MyLib version 5 or higher."
    #endif

### #if, #ifdef, #elif, #else, #endif

These directives allow you to include or exclude code conditionally.

The syntax of these macros is as follows:

    #if expression
    #ifdef macroName
    #elif expression
    #else
    #endif

The macro of an #ifdef is simply a macro symbol.

The expression of an #if or #elif can evaluate to a string constant, an integer constant, or the tokens `true` or `nil`. You can use the standard arithmetic operators (+, -, *, /, %, <<, >>, !, &, |, &&, ||, ?:) within integer and boolean expressions. In addition, the special operator `defined(macro)` evalutes to 1 if the given macro symbol is currently defined, 0 if not. An #if or #elif expression is considered true if it evaluates to a non-zero integer, or to any string value, or to the special value `true`.

Each #if or #ifdef must be matched by exactly one #endif.

Within a #if...#endif or #ifdef...#endif block, you can include any number of #elif directives, and you can optionally include a single #else directive that follows any #elif directives. #elif stands for "else if"; it takes effect only if its expression evaluates to true and the controlling #if or #ifdef evaluated to false and all preceding #elif's in the same block evaluated to false.

The preprocessor handles an #if...#endif block as follows. It first expands any macros in the expression, then evaluates the result as a constant expression (taking into account arithmetic operators). Then:

- If the expression evaluates to true (as defined above), the source text between the #if and the matching #elif, #else, or #endif - whichever comes first - is retained; all text after the matching #elif or #else is discarded and replaced with whitespace, up to the matching #endif.
- If, on the other hand, the expression evaluates to false, the source text between the #if and the matching #elif, #else, or #endif is discarded and replaced with whitespace. If the next thing is an #elif, its condition is evaluated, and the process is repeated as though it were the original #if. If the next thing is instead an #else, the text between the #else and the #endif is retained.

#ifdef is handled much the same way, except that its condition is simply the existence or absence of a definition for the given macro symbol. The #ifdef condition is true if the given macro symbol has been defined, false if not.

Note that there's no such thing as "#elifdef" - that is, else-if-defined - but you can get the same effect by using "#elif defined(macro)".

#if and #ifdef blocks can be nested to any depth.

Here's an example Note that the defined() preprocessor operator can be used within an expression in these directives to test to determine if a preprocessor symbol is defined. This allows for tests of combinations of defined symbols: #if defined(MSDOS) || defined(AMIGA) || defined(UNIX)

### #include

This directive inserts the contents of another source file into the compilation unit at the current point. The contents of the included file effectively replace the #include directive in the preprocessor output.

#include has two forms.

    #include "filename"

This form searches for a file with the given name, starting the search in the directory containing the including file, then in the directory containing the file that included the including file, and so on up the list of includers until reaching the original source file. We search in this order and stop at the first matching file we find. If we still can't find the file after looking in the directory containing the top-level source file, then we proceed as for an angle-bracketed file.

    #include <filename>

This form searches for the given file in each directory in the "include path," as specified in the compiler command options. For the command-line t3make program, this is the list of directories specified with -I options. The directories are searched in the order of the -I (or equivalent) options, and we stop and use the first matching file we find. If we exhast the include path without finding the file, we search in the compiler's install directory - this is where the standard system headers are usually located.

In both cases above, if the file still can't be found after searching all of the directories described, and the filename appears to be an "absolute" path (according to local path naming conventions, which vary by system), the compiler tries again, this time treating the name as an absolute path. An absolute path is one that fully specifies the location of the file; on Unix, for example, this is a path starting with a "/", and on Windows it's a path starting with a drive specifier (such as "C:").

The file can be specified with a macro symbol or a combination of macro symbols. If you use a macro expression, the expansion must match one of the formats shown above.

#### Specifying relative directory paths in include file names

Filenames in #include directives can refer to relative paths (such as to subdirectories) using relative URL-style notation. (A URL is a "uniform resource locator," which is a World Wide Web standard mechanism for specifying names of things like files.) In particular, you can use a forward slash, "/", as the path separator character, regardless of your local system's filename conventions. Even if you're running on Windows (which uses "\" as the path separator) or Macintosh (which uses ":" as the separator and has rather different rules than Unix-style systems), you can use "/" to specify relative paths, and your source will compile correctly on all systems.

For example, suppose you're using a Macintosh, and you have your source files in a folder called "My HD:My TADS Games:Caverns of Gloom". Now, suppose you decide that you'd like to keep your include files in a subfolder of this folder called "Include Files". You could write your #include lines like so:

    #include ":Include Files:defs.h"

But if you did this, and then you gave a copy of your source code to a friend running Windows, your friend wouldn't be able to compile the code without changing that #include directive to match Windows path name conventions.

The solution to this problem is to use URL-style path names. A URL-style path name works the same on every system, so your friend on Windows will be able to compile your code without changes if you use this notation. To use URL notation, use "/" as the path separator, and - regardless of local conventions - place slashes only between path elements, never at the beginning of the filename. So, we'd rewrite the example above like this:

    #include "Include Files/defs.h"

Even though this doesn't look a thing like a Mac path name, the Mac version of the TADS 3 compiler will happily find your file in the correct directory, because the Mac version knows how to convert URL-style path names to the correct Mac conventions, just like the Windows and Unix versions know how to convert URL-style names to their own local conventions.

In all include file searches, when the filename looks like a URL-style path, the compiler takes the following steps to look for the file. It takes these steps at each point in the include path search before moving on to the next directory in the list:

- First, the compiler tries treating the filename as a URL-style path. This means that the compiler converts all "/" characters to the local path separator characters, and performs any other transformations required to make the name appear relative. For example, on the Mac, the compiler would convert "Include Files/defs.h" to ":Include Files:defs.h", then would try looking for that file relative to the current directory being searched.
- If the first step fails to find a file, and the file does not use an absolute path (according to the local file system rules), the compiler tries again, this time treating the name as a native name, rather than URL path. That is, the compiler simply uses the name exactly as given, without any syntax conversions, and looks for a file with that name in the directory currently being searched.

Even though the compiler accepts local filenames, we strongly encourage using URL-style filenames for all #include files that specify paths, since this will ensure that your source code will compile without changes on other platforms.

### #line

As the preprocessor reads through the source and include files, it automatically keeps track of where it is in the file - the filename and the current line number in the file. This lets the compiler tell you where it was looking whenever it encounters a syntax error or other problem in your code, to make it easier to pinpoint the source of a problem so you can fix it.

This directive lets you override the compiler's internal notion of its current position in the source text. The syntax is:

    #line number 'filename'

This tells the compiler that it should pretend that it's reading from the file named file at line number number.

Note that this directive doesn't change where the compiler is actually reading from. In particular, it doesn't force the compiler to jump ahead to a different line in the file, or change to a different file. The directive merely overrides the location that the compiler will report on subsequent error messages.

The #line directive is mainly of interest to people writing their own pre-compiler tools. A pre-compiler is a tool that runs before the compiler, reading from a user-prepared source file and writing out a temporary file, and then passing the temporary file to t3make as the TADS 3 source file. For tools like this, the #line directive lets the tool apprise the TADS compiler of the original source code location associated with the generated code in the temporary file. This in turn causes the TADS compiler to report error messages at locations that will be meaningful to the user. Since the user is working with the original pre-compiler input file, it will be a lot more helpful to report errors in terms of the original input file than in terms of the temporary file.

### #pragma message

This directive lets you display a message on the console while the compiler is running. The syntax is:

    #pragma message ( token ... )

Each token can be a single-quoted string, a double-quoted string, a symbol token, or an integer. Any macro symbols you use are expanded as in normal program text.

### #pragma newline_spacing

Unlike C and Java, a string in TADS 3 doesn't have to fit all on one line. You can instead write a string that spans several lines:

    "This is a string
    that spans several
    lines of source code. ";

By default, the compiler treats this as though you had written it all on one line. It converts each line break into a single space character. If a line break is immediately followed by one or more whitespace characters (spaces, tabs, and so on), the compiler collapses the line break and all of the subsequent spaces into a single space character; so each line break turns into a single space, no matter how many spaces are at the start of the next line. This is important because most people like to use indentation to make their source code easier to read; ignoring the leading whitespace on each extra line within a string means that you don't have to worry about indentation affecting the appearance of a string.

This default handling is called "collapse" mode, because each newline and subsequent run of whitespace is collapsed into a single space. You can explicitly set this mode by writing this directive in your source file:

    #pragma newline_spacing(collapse)

(Prior to TADS 3.1.3, this mode's name was "on" instead of "collapse". The compiler still accepts #pragma newline_spacing(on) as equivalent, so existing code will work without any changes.)

In some written languages, such as Chinese, there isn't typically any whitespace between words. In such languages, the normal "collapse" mode described above isn't convenient, since it introduces unwanted spacing between words wherever you happen to put a line break in the source code. To accommodate languages without inter-word spacing, the compiler has another newline spacing mode, called "delete" mode:

    #pragma newline_spacing(delete)

(The pre-3.1.3 name for this mode was "off", which is also still accepted and is equivalent to "delete" mode.)

In "delete" mode, when the compiler encounters a line break within a string, it simply deletes the newline and all immediately subsequent whitespace. This mashes the text on the two lines together without any spacing.

There's one additional mode, "preserve" mode, for times when you want precise control over the contents of strings.

    #pragma newline_spacing(preserve)

In "preserve" mode, the compiler keeps the string exactly as you wrote it, with each line break turning into a \n (newline) character, and all subsequent spacing left intact. This can be useful if you're constructing multi-line strings for special purposes, such as storing in external files.

The newline_spacing pragma only affects strings following the pragma in the source file, so you can change the mode at any time within the same source file. Whenever you change the mode, the new mode is in effect until you change it with another pragma, or until the end of the current source file. This means that if you change the mode within a header file, the mode will revert to its previous setting at the end of the header file. This lets you #include a file without worrying about mode changes, because even if the included file changes the mode, the mode will revert back once the compiler finishes with the included file.

### #pragma sourceTextGroup

This directive lets you control the generation of sourceTextGroup property values. You can also control this with the compiler's "-Gstg" command-line option, but the #pragma gives you finer-grained control, since it lets you turn generation on and off within particular modules, or even within sections of a module.

To turn on sourceTextGroup generation, include this directive in your source code:

    #pragma sourceTextGroup(on)

This tells the compiler to generate sourceTextGroup values for objects defined after this point in the current module's source code.

You can turn sourceTextGroup generation off with this directive:

    #pragma sourceTextGroup(off)

This disables sourceTextGroup generation for subsequent object definitions.

Note that the effect of `#pragma sourceTextGroup` lasts only within the source module where it appears. The directive doesn't carry over to other modules that are part of the same build - at the start of each new module, the compiler resets to the status specified in the command line makefile (off by default, or on if the "-Gstg" option is specifed).

### #pragma once

This directive adds the current file (i.e., the file in which the #pragma directly appears) to the "once-only" list. After a file is in the once-only list, subsequent #include directives that attempt to include the file will be ignored.

This is sometimes useful to ensure that a header file isn't included redundantly. For example, suppose you are creating a library that has several header files for different subsystems in the library, and all of these main subsystem headers in turn depend on a common header for the overall library. Each of the subsystem headers would need to #include the common header. However, you can't predict in advance which of the subsystem headers a user might need to #include in her own program: the user might want to include only one of your subsystem headers, but also might be using several of your subsystems and thus would need to include several of the headers. That's where the problem comes up: if the user includes three of your subsystem headers, and each of the subsystem headers includes the common header, the common header will end up being included three times. This could cause compiler errors due to the repeated inclusions of the definitions in the common header. You can avoid this by putting a "#pragma once" directive in the common header; if you do this, it will ensure that only the first inclusion of the file will have any effect, and the subsequent redundant inclusions will be harmlessly ignored.

Note that C/C++ programmers usually solve this same problem by enclosing the entire contents of each header file in a protective series of #ifdef-type directives. Each header file follows this pattern:

    #ifndef MY_HEADER_NAME_H
    #define MY_HEADER_NAME_H

    // the rest of the contents of the header go here

    #endif

The idea is that, the first time this file is included, the preprocessor symbol MY_HEADER_NAME_H (which is usually based on the name of the header file: for myheader.h, we'd use MYHEADER_H) will be undefined, so the #ifndef ("if not defined") would succeed. Thus, everything between the #ifndef and the matching #endif would be compiled, including the #define for the same symbol. If the same file is included again, MY_HEADER_NAME_H would be defined this time, because of the #define that got compiled the first time around; so the #ifndef would fail, so the compiler would skip everything up to the matching #endif.

### #pragma all_once

This directive turns "all-once" mode on or off. "#pragma all_once +" turns all-once mode on, and "#pragma all_once -" turns it off.

By default, all-once mode is off. When all-once mode is on, the preprocessor automatically adds each included file to the once-only list at the moment when the #include for the file is encountered.

Once activated, all-once mode remains in effect until explicitly turned off.

### #undef

This directive deletes - "undefines" - a macro that was previously defined. This is useful because it lets you redefine a macro with a new value. The syntax is:

    #undef macroName

It's perfectly legal to undefine a macro that was never defined. This is silently ignored; it doesn't even generate a warning.

## Macros

### Substitution parameters

When you define a macro with a parameter list, any occurrences of the parameter names in the macro's expansion text are replaced with the actual values in each usage of the macro.

For example, suppose we define this macro:

    #define ERROR(msg)  tadsSay('An error occurred: ' + msg + '\n')

Now suppose we write this in our code somewhere:

    ERROR('invalid value');

During compilation, the preprocessor will expand this macro invocation, substituting the actual parameter value when msg appears in the replacement text. The resulting expansion is:

    tadsSay('An error occurred: ' + 'invalid value' + '\n');

(It is worth pointing out that the compiler will subsequently compute the constant value of this string concatenation, so this will not result in any string concatenation at run-time.)

### Macros referencing macros

It's perfectly legal for a macro's expansion text to refer to other macros.

Macro references within a macro's definition are expanded when a macro is expanded, rather than when it's defined. This is important, because it means that a macro's full, final expansion isn't set in stone at the moment the macro is defined. It's not until the macro is actually used that the full implications of its expansion can be known. What's more, different occurrences of the same macro could have radically different expansions due to changes in the other macros that the first macro references.

Here's how this works: each time the preprocessor expands a macro - that is, substitutes the macro's definition for an occurrence of the macro name in the source text - the preprocessor "re-scans" the expansion text to see if it contains any macros that need to be expanded.

For example, consider the following:

    #define SUM(a, b)  (VAL(a) + VAL(b))
    #define VAL(a)     (a)

    local x = SUM(1, 2);

    #undef VAL
    #define VAL(a)     (-(a))

    local y = SUM(1, 2);

There are two important things to note about this example. The first is that our macro SUM references a macro named VAL that isn't even defined yet at the point where SUM is defined. This is perfectly okay, because of the delayed expansion behavior: since we won't try to expand macros in SUM's definition until SUM is actually used, it doesn't matter whether or not VAL is defined at the point where SUM is defined. The second thing to notice is that we redefine VAL before using SUM for the second time. Since SUM's expansion is re-scanned for other macros each and every time it's used, this means that we've effectively changed the definition of SUM when we change the definition of VAL.

The code above will expand as follows:

    local x = ((1) + (2));
    local y = ((-(1)) + (-(2)));

### Recursive macros

Despite the re-scanning rule for macro expansion, the preprocessor doesn't expand macros recursively - that is, if a macro's expansion contains a mention of the macro's own name, the recursive self-reference is not expanded. Similarly, the preprocessor won't expand circular references, where macro A's expansion contains macro B, and macro B's expansion contains macro A.

There's no rule against writing a recursive-looking macro. Rather, the prohibition on recursion is embodied in the ANSI C rules of macro expansion. Basically, when a macro is being expanded, the preprocessor temporarily "forgets" the macro, then remembers it again as soon as the expansion is finished. So, for example, suppose you write this:

    #define hello(x) hello((x)+1)

This macro looks like it would expand forever. If we wrote hello(1), we'd naively expect this to expand to hello((1)+1), which would in turn expand to hello(((1)+1)+1), then to hello((((1)+1)+1)+1), and so on ad infinitum. However, the rule against recursive macros prevents this. Instead, the preprocessor simply forgets about the definition of hello() while expanding it, so we simply get hello((1)+1) - and then we're done.

The TADS 3 preprocessor uses the detailed ANSI C rules with regard to recursive and circular macro definitions. These rules are quite complex and only rarely come into play in real-world situations, so we won't try to explain them in full here. Instead, we invite authors who feel the need to know the full gory details to refer to an ANSI C programming book.

### Macros with variable-length argument lists

A macro can be defined to take a varying number of arguments, which is especially useful when the macro calls a function or method with a varying number of parameters. Although the 1999 ANSI C specification includes a varying macro argument feature, the ANSI C version is quite limited. TADS 3 diverges here from the ANSI definition to provide a more powerful facility.

To define a macro with varying arguments, place an ellipsis ("...") immediately after the last parameter in the macro's formal parameter list:

    #define ERROR(msg, args...) displayError('Error:', msg, args)

The "..." after the last argument tells the preprocessor that the macro allows zero or more arguments in place of the last parameter, so the ERROR() macro defined above will accept one or more arguments. We call this the "varying argument" - this single parameter name stands in for zero, one, or more actual arguments each time the macro is used.

#### Simple expansion with the variable list parameter

During expansion, the parameter name of the varying argument will be replaced by the entire varying part of the argument list, including the commas between adjacent arguments, but not including the comma before the first varying argument. For example:

    #define VAR(a, b...) { b }

This macro will expand as follows:

    VAR(1) expands to { }
    VAR(1,2) expands to { 2 }
    VAR(1,2,3,4) expands to { 2,3,4 }

#### Expansion with no variable arguments, and deleting the extra comma

If the varying part of the list contains zero arguments, note that it is replaced by nothing at all. In some cases, this can be problematic; for example, in the ERROR macro defined above, consider this expansion:

    ERROR('syntax error') -> displayError('Error:', 'syntax error', )

Note the extra comma after the last argument to displayError - the comma is from the original expansion text in the macro definition, not from the parameter "args", which is empty in this case because no varying arguments were supplied. The extra comma will cause a syntax error when the function call is compiled, so the macro as written is not compatible with an empty varying argument list, even though the preprocessor will allow it.

To correct this problem, we can use a special feature of the token-pasting operator (described in more detail below), "##". This operator has a special meaning when it appears after a comma and before a varying-argument macro parameter: when (and only when) the varying list is empty, the "##" operator deletes the preceding comma. This only works with commas - if anything else precedes the "##" operator, the operator works as it would in normal (non-varying arguments) cases. We can use this feature to rewrite the ERROR macro:

    #define ERROR(msg, args...) displayError('Error:', msg, ## args)

Now when we expand this macro with no additional arguments, the extra comma is deleted:

ERROR('syntax error') expands to displayError('Error:', 'syntax error')
ERROR('token error', 1) expand to displayError('Error:', 'token error', 1)

(In case you're wondering where this bizarre kludge came from, the idea is borrowed from the Gnu C compiler, which has the same feature. It's not some random thing we invented, even though it might look that way. It's a reasonably simple solution for the most common case, and it doesn't seem to create any significant complications for other cases, so we included support for it. Read on for our more general alternative approach, though.)

#### Iterative expansion with #foreach

The comma-deleting feature of the "##" operator is useful as far as it goes, but sometimes it's necessary to construct more elaborate expansions from varying arguments. For example, suppose we wanted to concatenate the arguments to the ERROR macro together - in other words, we'd like the expansion to look like this:

ERROR('token error', 1, 2) expands to displayError('Error:' + 'token error' + 1 + 2)

This is clearly beyond the scope of what we've seen so far. Fortunately, the TADS 3 preprocessor has another feature that makes this sort of construction possible: the #foreach operator. This operator must immediately follow - with no intervening spaces - the varying argument name, and must be immediately followed with a "delimiter" character. Following the delimiter is the main iteration expansion, which ends at the next instance of the delimiter character. Following the second delimiter is the "interim" expansion, which itself ends at the next instance of the delimiter.

You can choose any non-symbol character for the delimiter, as long as it doesn't appear in any of the expansion text - a non-symbol character is anything that can't appear in a symbol, specifically alphabetic characters, numerals, and underscores. The point of letting you choose your own delimiter is to allow you to use anything in the expansion text by choosing a delimiter that doesn't collide with the expansion.

Note that you should be careful if you choose a forward slash ("/") as the delimiter - the preprocessor removes comments before processing macros, so if you have an empty section, the compiler will completely remove two consecutive slashes because it will think it indicates a comment. You're probably better off avoiding using "/" as the delimiter.

This sounds a bit complicated, so let's see an example:

    #define ERROR(msg, arg...) displayError('Error: ' + msg arg#foreach: +arg ::)

The first part of the macro is simple:

    displayError('Error: ' + msg

This part expands in the familiar way. Now we come to this sequence:

    arg#foreach: +arg ::

Remember that the #foreach operator must appear immediately after the varying argument name, as we see here. After the #foreach operator, we have the delimiter; in this case, we've chosen ":", since we don't need any colons in our expansion text. We could just as well have chosen any other character; all that matters is that we don't need the character anywhere in our expansion, since the next appearance of this character terminates the expansion.

So, we have two sub-parts, delimited by colons. The first subpart is " +arg ", and the second subpart is empty.

The first subpart is the main iteration expansion. The preprocessor expands this part once for each actual varying argument, expanding the varying argument name in this part to merely the current argument in the varying list. In the rest of the macro, remember that the varying argument name expands to the full varying list; in a #foreach, though, the varying argument name expands merely to the single, current argument.

The second subpart is the interim iteration expansion. The preprocessor expands this part once for each actual varying argument except for the last one. This is why we call it the "interim" expansion - it's expanded between iterations.

Let's look at how the macro expands. Consider this invocation:

    ERROR('syntax error')

In this case, we have no varying arguments at all, so the entire #foreach sequence - from the "arg#foreach" part to the final colon - is iterated zero times, and hence expands to nothing at all. The expansion is thus:

    displayError('Error:' + 'syntax error' )

Note that we don't have any problem handling the zero varying arguments, since the entire iteration simply occurs zero times in this case.

Now consider what happens when we include some arguments:

    ERROR('token error', 1, 2)

This time, the #foreach sequence is iterated twice. The first time, "arg" expands to "1", since that's the first varying argument, and the second time, "arg" expands to "2". The two iterations are expanded like this:

    +1
    +2

These are concatenated together, so the result looks like this:

    displayError('Error: ' + 'token error' +1 +2)

The "interim" portion is useful for solving the same kinds of problems as the "##" comma deletion feature, but is more general. Since the interim portion appears only between each adjacent pair of varying arguments, it is useful for building lists of zero or more arguments. For example, suppose we want to write a macro that adds zero or more values:

    #define ADD(val...) val#foreach:val:+:

If we call this with no arguments, the expansion will be empty, because we'll iterate the #foreach zero times. If we call this with one argument, the result will simply be the argument: we'll iterate the #foreach one time, but we won't include the interim expansion at all, because we skip the interim expansion after the last argument. With two arguments, we'll expand the interim once, between the two. Here are some sample expansions:

    ADD() expands to nothing
    ADD(1) expands to 1
    ADD(1,2) expands to 1+2
    ADD(1,2,3) expands to 1+2+3

#### Conditional expansion with #ifempty and #ifnempty

In some cases, it's necessary to include a block of text immediately before or after the variable arguments, but only when the argument list is non-empty. In other cases, it's necessary to provide some text instead of the variable arguments when the variable argument list is empty. A pair of operators, #ifempty and #ifnempty, provide these types of conditional expansion.

The #ifempty and #ifnempty operators are similar in syntax to #foreach: these operators must appear in macro expansion text directly after the name of the variable argument formal parameter, with no intervening spaces, and the operator is immediately followed by a delimiter character. After the delimiter comes the conditional expansion text, which is terminated by another copy of the delimiter character.

#ifempty includes its expansion text in the macro's expansion only when the variable argument list is empty, and #ifnempty includes the text only when the variable argument list is non-empty.

For example, suppose you want to define a macro that expands its variable arguments into a concatenated list, and then passes the concatenated list as the second argument to another function. We might try defining this using #foreach:

    #define CALL_CONCAT(firstArg, args...) \
       myFunc(firstArg, args#foreach#args#+#)

However, this has a problem: if the varying argument part of the list is empty, we have an unnecessary comma in the expansion:

    CALL_CONCAT(test) -> myFunc(test, )

This is similar to the problem that we mentioned earlier in describing the "##" operator, but we can't use the "##" operator to delete the comma in this case, because the "##" comma deletion works only when the variable list argument appears directly after the ", ##" sequence.

This is where the #ifempty and #ifnempty operators come in. In this case, we want to include the comma after firstArg in the expansion only when the argument list isn't empty, so we can change the macro like this:

    #define CALL_CONCAT(firstArg, args...) \
       myFunc(firstArg args#ifnempty#,# args#foreach#args#+#)

This does what we want: when the variable argument list is empty, the #ifnempty expansion text is omitted, so we have no extra comma; when we have one or more varying arguments, the #ifnempty expansion is included, so the comma is included in the expansion.

#### Getting the variable argument count with #argcount

There's one more feature for varying argument lists: you can obtain the number of varying arguments with the #argcount operator. Like #foreach, the #argcount operator must appear immediately after the name of the varying parameter, without any spaces. This operator expands to a token giving the number of arguments in the varying list. For example:

    #define MAKELIST(ret, val...) ret = [val#argcount val#foreach#,val##]

This expands as follows:

MAKELIST(lst) expands to lst = [0]
MAKELIST(lst, 'a') expands to lst = [1,'a']
MAKELIST(lst, 'a', 'b') expands to lst = [2,'a','b']

Note that #argcount expands to the number of arguments in the varying part of the list only, and doesn't count any fixed arguments.

### Stringizing

It's sometimes useful to write a macro that uses the actual text of a substitution parameter as a string constant. This can be accomplished using the "stringizing" operators. The # operator, when it precedes the name of a macro formal parameter in macro expansion text, is replaced by the text of the actual argument value enclosed in double quotes. The #@ operator has a similar effect, but encloses the text in single quotes. For example, suppose we wanted to write a debugging macro that displays the value of an arbitrary expression:

    #define printval(val) tadsSay(#@val + ' = ' + toString(val))

We could use this macro in our code like this:

    printval(MyObject.codeNum);

This would expand as follows:

    tadsSay('MyObject.codeNum' + ' = ' + toString(MyObject.codeNum));

### Token Pasting

In some cases, it's useful to be able to construct a new symbol out of different parts. This can be accomplished with "token pasting," which constructs a single token from what were originally several tokens. The token pasting operator, ##, when it appears in a macro's expansion text, takes the text of the token to the left of the operator and the text of the token to the right of the operator and pastes them together to form a single token. If the token on either side is a formal parameter to the macro, the operator first expands the formal parameter, then performs pasting on the result.

For example, suppose we wanted to construct a method call based on a partial method name:

    #define callDo(verb, actor)  do##verb(actor)

We could use the macro like this:

    dobj.callDo(Take, Me);

This would expand into this text:

    dobj.doTake(Me);

The preprocessor scans a pasted token for further expansion, so if the pasted token is itself another macro, the preprocessor expands that as well:

    #define PASTE(a, b) a##b
    #define FOOBAR 123
    PASTE(FOO, BAR)

The macro above expands as follows. First, the preprocessor expands the PASTE macro, pasting the two arguments together to yield the token FOOBAR. The preprocessor then scans that and finds that it's another macro, so it expands it. The final text is simply 123.

Token pasting only works within macro expansion text; the token pasting operator is ignored if it appears anywhere outside of a #define.

#### String Concatenation

When you use the ## operator to paste two tokens together, the preprocessor checks to see if both of the tokens being pasted together are strings of the same kind (i.e., they both have the same type of quotes). If they are, the preprocessor combines the strings by removing the closing quote of the first string and the opening quote of the second string.

If either operand of the ## operator is itself modified by the # operator, the preprocessor first applies the # operator or operators, and then applies the ## operator. So, if you paste together two stringized parameters, the result is a single string.

Here are some examples:

    #define PAREN_STR(a) "(" ## a ")"
    #define CONCAT(a, b) a ## b
    #define CONCAT_STR(a, b) #a ## #b
    #define DEBUG_PRINT(a) "value of " ## #a ## " = <<a>>"

    1: PAREN_STR("parens")
    2: CONCAT("abc", "def")
    3: CONCAT_STR(uvw, xyz)
    4: DEBUG_PRINT(obj.prop[3])

After preprocessing, the file above would appear as follows:

    1: "(parens)"
    2: "abcdef"
    3: "uvwxyz"
    4: "value of obj.prop[3] = <<obj.prop[3]>>"

Note that string concatenation is a TADS extension, and is not found in ANSI C preprocessors. The C preprocessor doesn't provide a way of combining string tokens because the C language (not the preprocessor, but the language itself) has a different way of accomplishing the same thing: in C, two adjacent string tokens are always treated as a single string formed by concatenating the two strings together. The TADS language doesn't allow this kind of implicit string pasting, because (unlike in C) there are times when it is valid to use two or more adjacent string tokens, such as in dictionary property lists. The TADS preprocessor therefore provides its own mechanism for concatenating string tokens.

#### Pre-defined Macros

The compiler provides several pre-defined macros that you can use to get information about the compilation environment. Note that these macros are defined by the compiler - they don't necessarily tell you anything about the run-time environment, since they're fixed at compile time, and don't change while the program is running.

__DEBUG is defined as 1 for debug-mode builds, and is undefined for release-mode builds. This means that you can use `#ifdef __DEBUG` to conditionally include code for debug builds only, excluding the code for versions that you release to users. The Adv3 library uses this feature to include some debugging verbs in games only in debug builds, so that the verbs are available while you're testing the game but won't be included in releases.

__TADS_SYSTEM_NAME is defined as a symbol giving the name of the operating system where the compiler is running. (Note that this only tells you where you compiled your program - it doesn't tell you which system the program is actually executing on at run-time.) This varies by operating system; on Windows, it's WIN32.

__TADS_SYS_xxx is defined as 1, where xxx is the current operating system symbol defined for __TADS_SYSTEM_NAME. For example, when compiling on Windows, __TADS_SYS_WIN32 is defined as 1.

__TADS_VERSION_MAJOR is the major version number of the compiler - this will always be 3 for TADS 3.

__TADS_VERSION_MINOR is the minor version number. This is the second element of the dotted version string for the compiler: for example, for TADS 3.0.15, this is 0; for 3.11.22, this would be 11.

__TADS3 is defined as 1. This provides an easy way to detect that the compiler is a TADS 3 compiler (as opposed to a TADS 2 compiler, say). (This is essentially redundant with __TADS_SYSTEM_MAJOR, except that if for some reason you wanted to write single-sourced code that compiled under TADS 2 and TADS 3, using conditional compilation to handle the syntax differences in the languages, you wouldn't have been able to use __TADS_SYSTEM_MAJOR, because TADS 2 doesn't have an equivalent of the #if feature for macro expression evaluation.)

__DATE__ is defined as a single-quoted string giving the date at which the compilation started, in the format 'MMM dd yyyy', where MMM is the three-letter month abbreviation, dd is the day of the month in a two-digit format, and yyyy is the four-digit year. For example, August 15, 2008 would be represented as 'Aug 15 2008'.

__TIME__ is defined as a single-quoted string giving the time at which compilation started, in the format 'hh:mm:ss', where hh is the two-digit hour on the 24-hour clock (midnight is 00, noon is 12, eleven PM is 23, etc.), mm is the two-digit minute of the hour, and ss is the two-digit second within the minute. For example, half past noon is represented as '12:30:00'.

__LINE__ is a always defined as the current line number within the current source file that the compiler is scanning. The line numbering starts at 1 in each file, and increases by 1 at each newline (usually a carriage-return, line-feed, or combination of the two, depending on local conventions).

__FILE__ is always defined as a single-quoted string giving the name of the source file that the compiler is scanning.

__FILE__ and __LINE__ sometimes come in handy for debugging purposes, since they let you flag the source file location of a particular bit of code for examination at run-time. This is especially useful within macros, because __FILE__ and __LINE__ are always expanded at the last possible moment - if you use them within a macro, they won't be expanded until the containing macro is itself expanded, which means they'll yield the source location of the code that ultimately invoked the macro rather than the location of the definition of the macro.

TADS 3 System Manual
Table of Contents | The Language > The Preprocessor
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