changed semantic of [def] statements:

- [def] is used to define a variable (like before)
  but if we assign a string such as [def var "string"] the string is evaluated to bool
  in a different way: empty string is false, not empty string is true
- added [def?] statement - similar like [def] but define a variable only if such
  a variable is not already defined
- added [let] statement - similar like [def] but with lazy evaluation (it is an alias to a function)
  if it is called to assign a string e.g. [let var "string"] then this has the
  same meaning like [def] - an alias is only created when we assign a function e.g. [let var my_function]
  both [def] and [let] are using the same Vars object
- added [let?] - make an alias but only if such a variable is not already defined
- a std::map of variables moved outside of Generator
  added method void SetVariables(Vars & variables);
- fixed: a result status was not correctly propagated when evaluating [def] statements,
  this was in Call(...) function: last_res from variables was set in Find()
  but later was overwritten by Call(...) called for parameters (recursively)

src/patternparser.cpp

@@ -5,7 +5,7 @@
  */
 
 /* 
- * Copyright (c) 2007-2018, Tomasz Sowa
+ * Copyright (c) 2007-2021, Tomasz Sowa
  * All rights reserved.
  * 
  * Redistribution and use in source and binary forms, with or without
@@ -486,7 +486,7 @@ bool PatternParser::IsNameChar(wchar_t c)
 	return ((c>='a' && c<='z') ||
 			(c>='A' && c<='Z') ||
 			(c>='0' && c<='9') ||
-			c=='_' || c=='-' || c=='.' || c=='#');
+			c=='_' || c=='-' || c=='.' || c=='#' || c=='?');
 }
 
 
@@ -784,10 +784,85 @@ void PatternParser::ReadDirectiveInclude(Item & item)
 void PatternParser::ReadDirectiveDef(Item & item)
 {
 	item.type = Item::item_def;
-	ReadFunction(item);
+
+	if( ReadFunction(item) )
+	{
+		if( item.function.parameters.size() > 1 )
+			item.type = Item::item_err;
+	}
 }
 
 
+void PatternParser::ReadDirectiveDefIfNotSet(Item & item)
+{
+	item.type = Item::item_def_if_not_set;
+
+	if( ReadFunction(item) )
+	{
+		if( item.function.parameters.size() > 1 )
+			item.type = Item::item_err;
+	}
+}
+
+
+void PatternParser::ReadDirectiveLet(Item & item)
+{
+	item.type = Item::item_let;
+
+	if( ReadFunction(item) )
+	{
+		std::vector<Item::Function*> & parameters = item.function.parameters;
+
+		if( parameters.size() > 1 )
+		{
+			item.type = Item::item_err;
+			item.function.Clear();
+		}
+		else
+		if( parameters.size() == 1 && parameters[0]->is_function && !parameters[0]->parameters.empty() )
+		{
+			/*
+			 * if the first parameter in [let] is a function e.g. [let variable function_name] (here the first parameter is function_name)
+			 * then the function cannot have parameters itselt (because it is not evaluated here)
+			 * this is only an alias
+			 */
+			item.type = Item::item_err;
+			item.function.Clear();
+		}
+	}
+}
+
+
+void PatternParser::ReadDirectiveLetIfNotSet(Item & item)
+{
+	item.type = Item::item_let_if_not_set;
+
+	if( ReadFunction(item) )
+	{
+		std::vector<Item::Function*> & parameters = item.function.parameters;
+
+		if( parameters.size() > 1 )
+		{
+			item.type = Item::item_err;
+			item.function.Clear();
+		}
+		else
+		if( parameters.size() == 1 && parameters[0]->is_function && !parameters[0]->parameters.empty() )
+		{
+			/*
+			 * if the first parameter in [let] is a function e.g. [let variable function_name] (here the first parameter is function_name)
+			 * then the function cannot have parameters itselt (because it is not evaluated here)
+			 * this is only an alias
+			 */
+			item.type = Item::item_err;
+			item.function.Clear();
+		}
+	}
+}
+
+
+
+
 void PatternParser::ReadDirectiveFilter(Item & item)
 {
 	item.type = Item::item_filter;
@@ -825,11 +900,16 @@ void PatternParser::ReadDirectiveOut(Item & item)
 void PatternParser::ReadDirectiveBlock(Item & item)
 {
 	item.type = Item::item_block;
-	ReadFunction(item);
 
-	// only one function without arguments
-	if( !item.function.parameters.empty() )
-		item.type = Item::item_err;
+	if( ReadFunction(item) )
+	{
+		// only one function without arguments
+		if( !item.function.parameters.empty() )
+		{
+			item.type = Item::item_err;
+			item.function.Clear();
+		}
+	}
 }
 
 
@@ -859,6 +939,9 @@ std::wstring name;
 	else if( name == L"for" )		ReadDirectiveFor(item);
 	else if( name == L"include" )	ReadDirectiveInclude(item);
 	else if( name == L"def" )		ReadDirectiveDef(item);
+	else if( name == L"def?" )		ReadDirectiveDefIfNotSet(item);
+	else if( name == L"let" )		ReadDirectiveLet(item);
+	else if( name == L"let?" )		ReadDirectiveLetIfNotSet(item);
 	else if( name == L"filter" )	ReadDirectiveFilter(item);
 	else if( name == L"ezc" )		ReadDirectiveEzc(item);
 	else if( name == L"out" )		ReadDirectiveOut(item);