Updated the project structure, and modified the Makefile to hopefully be future proof so I can freely make sub-folders in the src/ folder and not have to worry about breaking the Makefile.

src/expr.h

@@ -0,0 +1,48 @@
+#ifndef EXPRESSION_H
+#define EXPRESSION_H
+
+#include "scanner.h"
+#include <stdarg.h>
+
+typedef enum {
+    EXPRESSION,
+    LITERAL,
+    GROUPING,
+    UNARY,
+    BINARY,
+    OPERATOR
+} ExpressionType;
+
+typedef struct Expr Expr;
+
+struct binary {
+    struct Expr* left;
+    Token* op;
+    struct Expr* right;
+};
+
+struct grouping {
+    struct Expr* expression;
+};
+
+struct unary {
+    Token* op;
+    struct Expr* right;
+};
+
+struct Expr {
+    ExpressionType type;
+    union ex {
+        struct binary Binary;
+        struct grouping Grouping;
+        Token* Literal;
+        struct unary Unary;
+    } expression;
+};
+
+// void VisitBinary(struct binary);
+// void VisitGrouping(struct grouping);
+// void VisitLiteral(Token);
+// void VisitUnary(struct unary);
+
+#endif