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4.26.3 Preprocessor Macros

M4 Macro: boolean defined (identifier)

The identifier must be the name of an optional abstract argument to the function. This macro must be used only within a function definition. It expands to the MFL expression that yields true if the actual parameter is supplied for identifier. For example:

func rcut(string text; number num)
  returns string
  if (defined(num))
    return substr(text, length(text) - num)
    return text

This function will return last num characters of text if num is supplied, and entire text otherwise, e.g.:

rcut("text string") ⇒ "text string"
rcut("text string", 3) ⇒ "ing"

Invoking the defined macro with the name of a mandatory argument yields true

M4 Macro: printf (format, …)

Provides a printf statement, that formats its optional parameters in accordance with format and sends the resulting string to the current log output (see Logging and Debugging). See String formatting, for a description of format.

Example usage:

printf('Function %s returned %d', funcname, retcode)
M4 Macro: string _ (msgid)

A convenience macro. Expands to a call to gettext (see NLS Functions).

M4 Macro: string_list_iterate (list, delim, var, code)

This macro intends to compensate for the lack of array data type in MFL. It splits the string list into segments delimited by string delim. For each segment, the MFL code code is executed. The code can use the variable var to refer to the segment string.

For example, the following fragment prints names of all existing directories listed in the PATH environment variable:

string path getenv("PATH")
string seg

string_list_iterate(path, ":", seg, `
     if access(seg, F_OK)
       echo "%seg exists"

Care should be taken to properly quote its arguments. In the code below the string str is treated as a comma-separated list of values. To avoid interpreting the comma as argument delimiter the second argument must be quoted:

string_list_iterate(str, `","', seg, `
     echo "next segment: " . seg')
M4 Macro: N_ (msgid)

A convenience macro, that expands to msgid verbatim. It is intended to mark the literal strings that should appear in the .po file, where actual call to gettext (see NLS Functions) cannot be used. For example:

/* Mark the variable for translation: cannot use gettext here */
string message N_("Mail accepted")

prog envfrom
  /* Translate and log the message */
  echo gettext(message)

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