Implement another syntax for nullary lambdas

refs #7564

doc/15-language-reference.md

@@ -574,6 +574,16 @@ For lambdas which take exactly one argument the braces around the arguments can
 
     f = x => x * x
 
+## <a id="nullary-lambdas"></a> Abbreviated Lambda Syntax
+
+Lambdas which take no arguments can also be written using the abbreviated lambda syntax.
+
+Example:
+
+    f = {{ 3 }}
+
+This creates a new function which returns the value 3.
+
 ## <a id="variable-scopes"></a> Variable Scopes
 
 When setting a variable Icinga checks the following scopes in this order whether the variable

lib/config/config_lexer.ll

@@ -217,6 +217,8 @@ while				return T_WHILE;
 in				return T_IN;
 &&				return T_LOGICAL_AND;
 \|\|				return T_LOGICAL_OR;
+\{\{				return T_NULLARY_LAMBDA_BEGIN;
+\}\}				return T_NULLARY_LAMBDA_END;
 [a-zA-Z_][a-zA-Z0-9\_]*		{ yylval->text = strdup(yytext); return T_IDENTIFIER; }
 @[a-zA-Z_][a-zA-Z0-9\_]*	{ yylval->text = strdup(yytext + 1); return T_IDENTIFIER; }
 \<[^ \>]*\>			{ yytext[yyleng-1] = '\0'; yylval->text = strdup(yytext + 1); return T_STRING_ANGLE; }

lib/config/config_parser.yy

@@ -177,6 +177,8 @@ static void MakeRBinaryOp(Expression** result, Expression *left, Expression *rig
 %token T_ELSE "else (T_ELSE)"
 %token T_WHILE "while (T_WHILE)"
 %token T_FOLLOWS "=> (T_FOLLOWS)"
+%token T_NULLARY_LAMBDA_BEGIN "{{ (T_NULLARY_LAMBDA_BEGIN)"
+%token T_NULLARY_LAMBDA_END "}} (T_NULLARY_LAMBDA_END)"
 
 %type <text> identifier
 %type <elist> rterm_items
@@ -871,6 +873,23 @@ rterm_no_side_effect: T_STRING
 		$$ = new FunctionExpression(*$3, $5, aexpr, @$);
 		delete $3;
 	}
+	| T_NULLARY_LAMBDA_BEGIN statements T_NULLARY_LAMBDA_END
+	{
+		std::vector<Expression *> dlist;
+		typedef std::pair<Expression *, EItemInfo> EListItem;
+		int num = 0;
+		BOOST_FOREACH(const EListItem& litem, *$2) {
+			if (!litem.second.SideEffect && num != $2->size() - 1)
+				yyerror(&litem.second.DebugInfo, NULL, NULL, "Value computed is not used.");
+			dlist.push_back(litem.first);
+			num++;
+		}
+		delete $2;
+		DictExpression *aexpr = new DictExpression(dlist, @$);
+		aexpr->MakeInline();
+
+		$$ = new FunctionExpression(std::vector<String>(), NULL, aexpr, @$);
+	}
 	;
 
 rterm: rterm_side_effect