diff --git a/base/components/interface.hpp b/base/components/interface.hpp
index d18937e..01d3815 100644
--- a/base/components/interface.hpp
+++ b/base/components/interface.hpp
@@ -336,6 +336,7 @@ CodeBody parse_file( Str path );
 
 GEN_API CodeClass       parse_class        ( Str class_def       );
 GEN_API CodeConstructor parse_constructor  ( Str constructor_def );
+GEN_API CodeDefine      parse_define       ( Str define_def      );
 GEN_API CodeDestructor  parse_destructor   ( Str destructor_def  );
 GEN_API CodeEnum        parse_enum         ( Str enum_def        );
 GEN_API CodeBody        parse_export_body  ( Str export_def      );
diff --git a/base/components/interface.parsing.cpp b/base/components/interface.parsing.cpp
index d96e0d6..eea7def 100644
--- a/base/components/interface.parsing.cpp
+++ b/base/components/interface.parsing.cpp
@@ -82,6 +82,21 @@ CodeConstructor parse_constructor( Str def )
 	return result;
 }
 
+CodeDefine parse_define( Str def )
+{
+	check_parse_args( def );
+
+	TokArray toks = lex( def );
+	if ( toks.Arr == nullptr )
+		return InvalidCode;
+
+	_ctx->parser.Tokens = toks;
+	push_scope();
+	CodeDefine result = parser_parse_define();
+	parser_pop(& _ctx->parser);
+	return result;
+}
+
 CodeDestructor parse_destructor( Str def )
 {
 	check_parse_args( def );
diff --git a/base/components/lexer.cpp b/base/components/lexer.cpp
index 5a0a807..78c414b 100644
--- a/base/components/lexer.cpp
+++ b/base/components/lexer.cpp
@@ -123,19 +123,19 @@ s32 lex_preprocessor_define( LexContext* ctx )
 		name.Text.Len++;
 	}
 
-	Specifier spec   = str_to_specifier( name.Text );
-	TokType   attrib = str_to_toktype( name.Text );
+	Specifier spec    = str_to_specifier( name.Text );
+	TokType   attrib  = str_to_toktype( name.Text );
 	b32 not_specifier = spec   == Spec_Invalid;
 	b32 not_attribute = attrib <= Tok___Attributes_Start;
 
-	PreprocessorMacro  macro            = { name.Text, MT_Statement, (MacroFlags)0 };
+	PreprocessorMacro  macro            = { name.Text, MT_Expression, (MacroFlags)0 };
 	PreprocessorMacro* registered_macro = lookup_preprocess_macro(name.Text);
 
 	if ( registered_macro == nullptr && not_specifier && not_attribute ) {
-		log_fmt("Warning: '%S' was not registered before the lexer processed its #define directive, it will be registered as a statement macro\n"
+		log_fmt("Warning: '%S' was not registered before the lexer processed its #define directive, it will be registered as a expression macro\n"
 			, name.Text 
 		);
-		GEN_DEBUG_TRAP();
+		// GEN_DEBUG_TRAP();
 	}
 	array_append( _ctx->Lexer_Tokens, name );
 
@@ -145,7 +145,7 @@ s32 lex_preprocessor_define( LexContext* ctx )
 			log_fmt("Warning: %S registered macro is not flagged as functional yet the definition detects opening parenthesis '(' for arguments\n"
 				, name.Text
 			);
-			GEN_DEBUG_TRAP();
+			// GEN_DEBUG_TRAP();
 		}
 		else {
 			macro.Flags |= MF_Functional;
@@ -526,35 +526,41 @@ void lex_found_token( LexContext* ctx )
 	}
 
 	PreprocessorMacro* macro = lookup_preprocess_macro( ctx->token.Text );
-	if ( macro )
+	b32 has_args          = ctx->left && (* ctx->scanner) == '(';
+	b32 resolved_to_macro = false;
+	if (macro) {
+		ctx->token.Type   = macrotype_to_toktype(macro->Type);
+		b32 is_functional = macro_is_functional(* macro);
+		resolved_to_macro = has_args ? is_functional : ! is_functional;
+	}
+	if ( resolved_to_macro )
 	{
-		ctx->token.Type = macrotype_to_toktype(macro->Type);
-
 		// TODO(Ed): When we introduce a macro AST (and expression support), we'll properly lex this section.
 		// Want to ignore any arguments the define may have as they can be execution expressions.
-		if ( ctx->left && (* ctx->scanner) == '(' )
+		if ( has_args )
 		{
 			ctx->token.Flags |= TF_Macro_Functional;
 
-			move_forward();
-			ctx->token.Text.Len++;
+			// move_forward();
+			// ctx->token.Text.Len++;
 
-			s32 level = 0;
-			while ( ctx->left && ((* ctx->scanner) != ')' || level > 0) )
-			{
-				if ( (* ctx->scanner) == '(' )
-					level++;
+			// s32 level = 0;
+			// while ( ctx->left && ((* ctx->scanner) != ')' || level > 0) )
+			// {
+			// 	if ( (* ctx->scanner) == '(' )
+			// 		level++;
 
-				else if ( (* ctx->scanner) == ')' && level > 0 )
-					level--;
+			// 	else if ( (* ctx->scanner) == ')' && level > 0 )
+			// 		level--;
 
-				move_forward();
-				ctx->token.Text.Len++;
-			}
+			// 	move_forward();
+			// 	ctx->token.Text.Len++;
+			// }
 
-			move_forward();
-			ctx->token.Text.Len++;
+			// move_forward();
+			// ctx->token.Text.Len++;
 		}
+
 		//if ( (* ctx->scanner) == '\r' && ctx->scanner[1] == '\n' )
 		//{
 		//	move_forward();
@@ -1277,7 +1283,7 @@ TokArray lex( Str content )
 		{
 			lex_found_token( ctx );
 			TokType last_type = array_back(_ctx->Lexer_Tokens)->Type;
-			if ( last_type == Tok_Preprocess_Macro_Stmt )
+			if ( last_type == Tok_Preprocess_Macro_Stmt || last_type == Tok_Preprocess_Macro_Expr )
 			{
 				Token thanks_c = { { c.scanner, 0 }, Tok_Invalid, c.line, c.column, TF_Null };
 				c.token = thanks_c;
diff --git a/base/components/parser.cpp b/base/components/parser.cpp
index 749661c..40a5713 100644
--- a/base/components/parser.cpp
+++ b/base/components/parser.cpp
@@ -93,7 +93,15 @@ bool lex__eat(TokArray* self, TokType type )
 		self->Idx ++;
 	}
 
-	if ( at_idx.Type != type )
+	b32 not_accepted  = at_idx.Type != type;
+	b32 is_identifier = at_idx.Type == Tok_Identifier;
+	if ( not_accepted )
+	{
+		PreprocessorMacro* macro = lookup_preprocess_macro(at_idx.Text);
+		b32 accept_as_identifier = macro && bitfield_is_set(MacroFlags, macro->Flags, MF_Allow_As_Identifier );
+		not_accepted             = type == Tok_Identifier && accept_as_identifier ? false : true;
+	}
+	if ( not_accepted )
 	{
 		Token tok = * lex_current( self, lex_skip_formatting );
 		log_failure( "Parse Error, TokArray::eat, Expected: ' %s ' not ' %.*s ' (%d, %d)`\n%s"
@@ -188,7 +196,7 @@ internal CodeComment        parse_comment                      ();
 internal Code               parse_complicated_definition       ( TokType which );
 internal CodeBody           parse_class_struct_body            ( TokType which, Token name );
 internal Code               parse_class_struct                 ( TokType which, bool inplace_def );
-internal CodeDefine         parse_define                       ();
+internal CodeDefine         parser_parse_define                ();
 internal Code               parse_expression                   ();
 internal Code               parse_forward_or_definition        ( TokType which, bool is_inplace );
 internal CodeFn             parse_function_after_name          ( ModuleFlag mflags, CodeAttributes attributes, CodeSpecifiers specifiers, CodeTypename ret_type, Token name );
@@ -902,7 +910,7 @@ CodeBody parse_class_struct_body( TokType which, Token name )
 				break;
 			}
 			case Tok_Preprocess_Define: {
-				member = cast(Code, parse_define());
+				member = cast(Code, parser_parse_define());
 				// #define
 				break;
 			}
@@ -1286,9 +1294,14 @@ Code parse_complicated_definition( TokType which )
 }
 
 internal inline
-CodeDefine parse_define()
+CodeDefine parser_parse_define()
 {
 	push_scope();
+	if ( check(Tok_Preprocess_Hash)) {
+		// If parse_define is called by the user the hash reach here.
+		eat(Tok_Preprocess_Hash);
+	}
+
 	eat( Tok_Preprocess_Define );
 	// #define
 
@@ -1698,7 +1711,7 @@ CodeBody parse_global_nspace( CodeType which )
 			break;
 
 			case Tok_Preprocess_Define:
-				member = cast(Code, parse_define());
+				member = cast(Code, parser_parse_define());
 				// #define ...
 			break;
 
@@ -2933,6 +2946,32 @@ Code parse_simple_preprocess( TokType which )
 	// <Macro>
 
 	PreprocessorMacro* macro = lookup_preprocess_macro( full_macro.Text );
+	if ( which != Tok_Preprocess_Unsupported && macro == nullptr ) {
+		log_failure("Expected the macro %S to be registered\n%S", full_macro,  parser_to_strbuilder(_ctx->parser));
+		return NullCode;
+	}
+
+	// TODO(Ed) : Parse this properly later (expression and statement support)
+	if ( macro && macro_is_functional(* macro) )
+	{
+		eat( Tok_Capture_Start );
+
+		s32 level = 0;
+		while ( left && ( currtok.Type != Tok_Capture_End || level > 0 ) )
+		{
+			if ( currtok.Type == Tok_Capture_Start )
+				level++;
+
+			else if ( currtok.Type == Tok_Capture_End && level > 0 )
+				level--;
+
+			eat( currtok.Type );
+		}
+		eat( Tok_Capture_End );
+		// <Macro> ( <params> )
+
+		full_macro.Text.Len = ( (sptr)prevtok.Text.Ptr + prevtok.Text.Len ) - (sptr)full_macro.Text.Ptr;
+	}
 
 	if ( macro && macro_expects_body(* macro) && peektok.Type == Tok_BraceCurly_Open )
 	{
@@ -3619,7 +3658,7 @@ CodeEnum parser_parse_enum( bool inplace_def )
 		if ( str_contains( tok_to_str(currtok), enum_underlying_macro.Name) )
 		{
 			use_macro_underlying = true;
-			underlying_macro     = parse_simple_preprocess( Tok_Preprocess_Macro_Typename );
+			underlying_macro     = parse_simple_preprocess( Tok_Preprocess_Macro_Expr );
 		}
 	}
 
@@ -3660,7 +3699,7 @@ CodeEnum parser_parse_enum( bool inplace_def )
 				break;
 
 				case Tok_Preprocess_Define:
-					member = cast(Code, parse_define());
+					member = cast(Code, parser_parse_define());
 					// #define
 				break;
 
@@ -4916,22 +4955,32 @@ CodeTypedef parser_parse_typedef()
 #if GEN_PARSER_DISABLE_MACRO_TYPEDEF
 	if ( false )
 #else
-	if ( check( Tok_Preprocess_Macro_Typename ))
+	b32 valid_macro = false;
+	valid_macro |= left && currtok.Type == Tok_Preprocess_Macro_Typename;
+	valid_macro |= left && currtok.Type == Tok_Preprocess_Macro_Stmt;
+	// if (currtok.Type == Tok_Preprocess_Macro_Stmt)
+	// {
+		// PreprocessMacro* macro = lookup_preprocess_macro(currtok.Text);
+		// valid_macro |= macro && macro_expects_body(* macro));
+	// }
+
+	if ( valid_macro )
 #endif
 	{
-		type = cast(Code, t_empty);
-		name = currtok;
+		type          = cast(Code, t_empty);
+		Code macro    = parse_simple_preprocess(currtok.Type);
+		name          = currtok;
+		name.Text.Len = macro->Content.Len;
 		_ctx->parser.Scope->Name = name.Text;
-		eat( Tok_Preprocess_Macro_Typename );
 		// <ModuleFalgs> typedef <Preprocessed_Macro>
 
 		if ( currtok.Type == Tok_Identifier )
 		{
-			type = untyped_str(name.Text);
+			type = macro;
 			name = currtok;
 			eat(Tok_Identifier);
+			// <ModuleFalgs> typedef <Preprocessed_Macro> <Identifier>
 		}
-		// <ModuleFalgs> typedef <Preprocessed_Macro> <Identifier>
 	}
 	else
 	{
@@ -5178,7 +5227,7 @@ CodeUnion parser_parse_union( bool inplace_def )
 				break;
 
 				case Tok_Preprocess_Define:
-					member = cast(Code, parse_define());
+					member = cast(Code, parser_parse_define());
 				break;
 
 				case Tok_Preprocess_If:
diff --git a/base/components/parser_types.hpp b/base/components/parser_types.hpp
index ad15bf4..4421615 100644
--- a/base/components/parser_types.hpp
+++ b/base/components/parser_types.hpp
@@ -129,8 +129,8 @@ struct ParseContext
 
 enum MacroType : u16
 {
-	MT_Statement,      // A macro is assumed to be a statement if not resolved.
-	MT_Expression,
+	MT_Expression,     // A macro is assumed to be a expression if not resolved.
+	MT_Statement,      
 	MT_Typename,
 	MT_Attribute,      // More of a note to the parser than anythign else (attributes should be defined in the user attribues def).
 	MT_Specifier,      // More of a note to the parser than anythign else (specifiers should be defined in the user attribues def).
@@ -175,8 +175,10 @@ Str macrotype_to_str( MacroType type )
 
 enum EMacroFlags : u16
 {
-	MF_Functional     = bit(0), // Macro has parameters (args expected to be passed)
-	MF_Expects_Body   = bit(1), // Expects to assign a braced scope to its body.
+	MF_Functional          = bit(0), // Macro has parameters (args expected to be passed)
+	MF_Expects_Body        = bit(1), // Expects to assign a braced scope to its body.
+	MF_Allow_As_Identifier = bit(2), // lex__eat wil treat this macro as an identifier if the parser attempts to consume it as one.
+                                     //  ^^^ This is a sort of kludge because we don't support push/pop macro programs rn. ^^^
 
 	MF_Null           = 0,
 	MF_UnderlyingType = GEN_U16_MAX,
diff --git a/base/dependencies/memory.hpp b/base/dependencies/memory.hpp
index 9304b21..c28cb09 100644
--- a/base/dependencies/memory.hpp
+++ b/base/dependencies/memory.hpp
@@ -225,7 +225,7 @@ forceinline ssize         size_remaining(Arena& arena, ssize alignment) { return
 // This id is defined by Unreal for asserts
 #pragma push_macro("check")
 #undef check
-forceinline void check(Arena& arena) { return arena_check(& arena); };
+forceinline void check(Arena& arena) { return arena_check(& arena); }
 #pragma pop_macro("check")
 #endif
 
diff --git a/gen_c_library/c_library.cpp b/gen_c_library/c_library.cpp
index 8c7e2b3..9ed579b 100644
--- a/gen_c_library/c_library.cpp
+++ b/gen_c_library/c_library.cpp
@@ -65,19 +65,123 @@ int gen_main()
 	Context ctx {};
 	gen::init(& ctx);
 
-	ctx.PreprocessorDefines.append(txt("GEN_API_C_BEGIN"));
-	ctx.PreprocessorDefines.append(txt("GEN_API_C_END"));
-	ctx.PreprocessorDefines.append(txt("Array("));
-	ctx.PreprocessorDefines.append(txt("HashTable("));
-	ctx.PreprocessorDefines.append(txt("GEN_NS_PARSER"));
-	ctx.PreprocessorDefines.append(txt("GEN_NS_PARSER_BEGIN"));
-	ctx.PreprocessorDefines.append(txt("GEN_NS_PARSER_END"));
-	ctx.PreprocessorDefines.append(txt("Using_Code("));
-	ctx.PreprocessorDefines.append(txt("Using_CodeOps("));
-	ctx.PreprocessorDefines.append(txt("GEN_OPTIMIZE_MAPPINGS_BEGIN"));
-	ctx.PreprocessorDefines.append(txt("GEN_OPITMIZE_MAPPINGS_END"));
-	ctx.PreprocessorDefines.append(txt("GEN_PARAM_DEFAULT"));
-	//PreprocessorDefines.append(txt("GEN_EXECUTION_EXPRESSION_SUPPORT"));
+	register_preprocess_macros( args(
+		(PreprocessorMacro { txt("bit"),                          MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("bitfield_is_set"),              MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("GEN_C_LIKE_CPP"),               MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("cast"),                         MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("ccast"),                        MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("rcast"),                        MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("pcast"),                        MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("scast"),                        MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("stringize_va"),                 MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("stringize"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("do_once"),                      MT_Expression, MF_Functional }), 
+		(PreprocessorMacro { txt("do_once_defer"),                MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("do_once_start"),                MT_Statement,  MF_Null       }), 
+		(PreprocessorMacro { txt("do_once_end"),                  MT_Statement,  MF_Null       }), 
+		(PreprocessorMacro { txt("labeled_scope_start"),          MT_Statement,  MF_Null       }), 
+		(PreprocessorMacro { txt("labeled_scope_end"),            MT_Statement,  MF_Null       }), 
+		(PreprocessorMacro { txt("compiler_decorated_func_name"), MT_Expression, MF_Null       }), 
+		(PreprocessorMacro { txt("num_args_impl"),                MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("num_args"),                     MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("count_of"),                     MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("clamp"),                        MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("is_between"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("size_of"),                      MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("min"),                          MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("max"),                          MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("offset_of"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("static_assert"),                MT_Statement,  MF_Functional }),
+		(PreprocessorMacro { txt("typeof"),                       MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("GEN_API_C_BEGIN"),              MT_Statement,  MF_Null       }),
+		(PreprocessorMacro { txt("GEN_API_C_END"),                MT_Statement,  MF_Null       }),
+		(PreprocessorMacro { txt("nullptr"),                      MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("GEN_REMOVE_PTR"),               MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("GEN_PARAM_DEFAULT"),            MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("struct_init"),                  MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("GEN_OPTIMIZE_MAPPINGS_BEGIN"),  MT_Statement,  MF_Null       }),
+		(PreprocessorMacro { txt("GEN_OPITMIZE_MAPPINGS_END"),    MT_Statement,  MF_Null       }),
+		(PreprocessorMacro { txt("Array"),                        MT_Typename,   MF_Functional }),
+		(PreprocessorMacro { txt("HashTable"),                    MT_Typename,   MF_Functional }),
+		(PreprocessorMacro { txt("Using_Code"),                   MT_Statement,  MF_Functional }),
+		(PreprocessorMacro { txt("Using_CodeOps"),                MT_Statement,  MF_Functional }),
+		(PreprocessorMacro { txt("kilobytes"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("megabytes"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("gigabytes"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("terabytes"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("GEN__ONES"),                    MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("GEN__HIGHS"),                   MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("GEN__HAS_ZERO"),                MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("zero_item"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("zero_array"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("GEN_DEFAULT_MEMORY_ALIGNMENT"), MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("GEN_DEFAULT_ALLOCATOR_FLAGS"),  MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("alloc_item"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("alloc_array"),                  MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("malloc"),                       MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("mfree"),                        MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("GEN_PRINTF_MAXLEN"),            MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("cast_to_str"),                  MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("current"),                      MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("txt"),                          MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("GEN_FILE_OPEN_PROC"),           MT_Statement,  MF_Functional | MF_Expects_Body }),
+		(PreprocessorMacro { txt("GEN_FILE_READ_AT_PROC"),        MT_Statement,  MF_Functional | MF_Expects_Body }),
+		(PreprocessorMacro { txt("GEN_FILE_WRITE_AT_PROC"),       MT_Statement,  MF_Functional | MF_Expects_Body }),
+		(PreprocessorMacro { txt("GEN_FILE_SEEK_PROC"),           MT_Statement,  MF_Functional | MF_Expects_Body }),
+		(PreprocessorMacro { txt("GEN_FILE_CLOSE_PROC"),          MT_Statement,  MF_Functional | MF_Expects_Body }),
+		(PreprocessorMacro { txt("log_failure"),                  MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("operator"),                     MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("InvalidCode"),                  MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("NullCode"),                     MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("Verify_POD"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("gen_main"),                     MT_Statement,  MF_Null       })
+	));
+	register_preprocess_macros( args(
+		(PreprocessorMacro { txt("name"),                         MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("code"),                         MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("args"),                         MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("code_str"),                     MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("code_fmt"),                     MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("parse_fmt"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("token_fmt"),                    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check_member_val"),             MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check_member_str"),             MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check_member_content"),         MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check_member_ast"),             MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check_params"),                 MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check_param_eq_ret"),           MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("specs"),                        MT_Expression, MF_Functional | MF_Allow_As_Identifier }),
+		(PreprocessorMacro { txt("name_check"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("null_check"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("def_body_start"),               MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("def_body_code_array_start"),    MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("move_forward"),                 MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("skip_whitespace"),              MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("end_line"),                     MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check_parse_args"),             MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("currtok_noskip"),               MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("currtok"),                      MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("peektok"),                      MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("prevtok"),                      MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("nexttok"),                      MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("nexttok_noskip"),               MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("eat"),                          MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("left"),                         MT_Expression, MF_Null       | MF_Allow_As_Identifier }),
+		(PreprocessorMacro { txt("def_assign"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("CHECK_WAS_DEFINED"),            MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("check_noskip"),                 MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("check"),                        MT_Expression, MF_Functional | MF_Allow_As_Identifier }),
+		(PreprocessorMacro { txt("push_scope"),                   MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("cut_length"),                   MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("cut_ptr"),                      MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("pos"),                          MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("move_fwd"),                     MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("Entry"),                        MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("CheckEndParams"),               MT_Expression, MF_Functional }),
+		(PreprocessorMacro { txt("UseTemplateCapture"),           MT_Expression, MF_Null       }),
+		(PreprocessorMacro { txt("check_current"),                MT_Expression, MF_Functional })
+	));
 
 	Code push_ignores           = scan_file( path_base "helpers/push_ignores.inline.hpp" );
 	Code pop_ignores            = scan_file( path_base "helpers/pop_ignores.inline.hpp" );
@@ -139,8 +243,8 @@ int gen_main()
 			CodeTemplate tmpl = cast(CodeTemplate, entry);
 			if ( tmpl->Declaration->Name.contains(txt("swap")))
 			{
-				log_fmt("SWAPPED");
-				CodeBody macro_swap = parse_global_body( txt(R"(
+				register_preprocess_macro({ txt("swap"), MT_Expression, MF_Functional });
+				CodeDefine macro_swap = parse_define( txt(R"(
 #define swap( a, b )        \
 do                          \
 {                           \
@@ -241,7 +345,8 @@ do                          \
 		{
 			if ( str_contains(entry->Name, txt("Msg_Invalid_Value")))
 			{
-				CodeDefine define = def_define(entry->Name, entry->Value->Content);
+				Opts_def_define opts = { {}, entry->Value->Content };
+				CodeDefine define = def_define(entry->Name, MT_Expression, opts );
 				header_printing.append(define);
 				continue;
 			}
@@ -407,7 +512,8 @@ do                          \
 			CodeVar var = cast(CodeVar, entry);
 			if (var->Specs.has(Spec_Constexpr) > -1)
 			{
-				CodeDefine define = def_define(entry->Name, entry->Value->Content);
+				Opts_def_define opts = { {}, entry->Value->Content };
+				CodeDefine define = def_define(entry->Name, MT_Expression, opts);
 				header_filesystem.append(define);
 				continue;
 			}
@@ -811,7 +917,8 @@ R"(#define AST_ArrSpecs_Cap \
 					ast.append(def);
 					break;
 				}
-				CodeDefine def = def_define(var->Name, var->Value.to_strbuilder());
+				Opts_def_define opts = { {}, var->Value.to_strbuilder() };
+				CodeDefine def = def_define(var->Name, MT_Expression, opts );
 				ast.append(def);
 				break;
 			}
@@ -832,6 +939,7 @@ R"(#define AST_ArrSpecs_Cap \
 		txt("CodeClass"),
 		txt("CodeConstructor"),
 		txt("CodeDefine"),
+		txt("CodeDefineParams"),
 		txt("CodeDestructor"),
 		txt("CodeEnum"),
 		txt("CodeExec"),
@@ -1209,8 +1317,10 @@ R"(#define <interface_name>( code ) _Generic( (code), \
 			if (var->Specs)
 			{
 				s32 constexpr_found = var->Specs.remove( Spec_Constexpr );
-				if (constexpr_found > -1) {
-					CodeDefine define = def_define(entry->Name, entry->Value->Content);
+				if (constexpr_found > -1)
+				{
+					Opts_def_define opts = { {},  entry->Value->Content };
+					CodeDefine define = def_define(entry->Name, MT_Expression, opts );
 					header_end.append(define);
 					continue;
 				}
diff --git a/gen_c_library/c_library.refactor b/gen_c_library/c_library.refactor
index c743e5c..569be07 100644
--- a/gen_c_library/c_library.refactor
+++ b/gen_c_library/c_library.refactor
@@ -325,6 +325,7 @@ word CodeComment,        gen_CodeComment
 word CodeClass,          gen_CodeClass
 word CodeConstructor,    gen_CodeConstructor
 word CodeDefine,         gen_CodeDefine
+word CodeDefineParams,   gen_CodeDefineParams
 word CodeDestructor,     gen_CodeDestructor
 word CodeEnum,           gen_CodeEnum
 word CodeExec,           gen_CodeExec
diff --git a/gen_c_library/components/containers.array.hpp b/gen_c_library/components/containers.array.hpp
index 2317375..e2fd8b3 100644
--- a/gen_c_library/components/containers.array.hpp
+++ b/gen_c_library/components/containers.array.hpp
@@ -22,9 +22,9 @@ CodeBody gen_array_base()
 	Code get_header   = untyped_str( txt( "#define array_get_header( self ) ( (ArrayHeader*)( self ) - 1)\n" ));
 	Code type_define  = untyped_str( txt( "#define Array(Type) gen_Array_##Type\n"));
 
-	Code array_begin = def_define(txt("array_begin(array)"),       code( (array) ));
-	Code array_end   = def_define(txt("array_end(array)"),         code( (array + array_get_header(array)->Num ) ));
-	Code array_next  = def_define(txt("array_next(array, entry)"), code( (entry + 1) ));
+	Code array_begin = def_define(txt("array_begin(array)"),       MT_Expression, { {}, code( (array) ) } );
+	Code array_end   = def_define(txt("array_end(array)"),         MT_Expression, { {}, code( (array + array_get_header(array)->Num ) ) } );
+	Code array_next  = def_define(txt("array_next(array, entry)"), MT_Expression, { {}, code( (entry + 1) ) } );
 
 	return def_global_body( args(
 		fmt_newline,
diff --git a/gen_c_library/components/memory.fixed_arena.hpp b/gen_c_library/components/memory.fixed_arena.hpp
index 262f06d..c9fd7ab 100644
--- a/gen_c_library/components/memory.fixed_arena.hpp
+++ b/gen_c_library/components/memory.fixed_arena.hpp
@@ -89,7 +89,14 @@ CodeBody gen_fixed_arenas()
 	result.append(arena_interface_2mb);
 	result.append(arena_interface_4mb);
 
-	CodeDefine def = def_define(txt("fixed_arena_allocator_info(fixed_arena)"), txt("( (AllocatorInfo) { arena_allocator_proc, & (fixed_arena)->arena } )"));
+	register_preprocess_macros( args(
+		( PreprocessorMacro { txt("fixed_arena_allocator_info"), MT_Expression, MF_Functional }),
+		( PreprocessorMacro { txt("fixed_arena_init"),           MT_Expression, MF_Functional }),
+		( PreprocessorMacro { txt("fixed_arena_free"),           MT_Expression, MF_Functional }),
+		( PreprocessorMacro { txt("fixed_arena_size_remaining"), MT_Expression, MF_Functional })
+	));
+
+	CodeDefine def = parse_define(txt("#define fixed_arena_allocator_info(fixed_arena) ( (AllocatorInfo) { arena_allocator_proc, & (fixed_arena)->arena } )\n"));
 	result.append(def);
 	result.append(fmt_newline);
 
diff --git a/gen_c_library/components/misc.hpp b/gen_c_library/components/misc.hpp
index 727d2b6..b288663 100644
--- a/gen_c_library/components/misc.hpp
+++ b/gen_c_library/components/misc.hpp
@@ -7,7 +7,7 @@ void convert_cpp_enum_to_c( CodeEnum to_convert, CodeBody to_append )
 {
 #pragma push_macro("enum_underlying")
 #undef enum_underlying
-	StrCached type = to_convert->UnderlyingType ? to_convert->UnderlyingType : to_convert->Name
+	StrCached type = to_convert->UnderlyingType ? to_convert->UnderlyingType.to_strbuilder().to_str() : to_convert->Name;
 	CodeTypedef tdef = parse_typedef(token_fmt("type", type, "name", to_convert->Name, stringize( typedef enum <type> <name>; )));
 	if (to_convert->UnderlyingType)
 	{