Starting chapter 5 now. This is the first part needed to start implementing the syntax tree.

expr.h

@@ -0,0 +1,44 @@
+#ifndef EXPRESSION_H
+#define EXPRESSION_H
+
+#include "scanner.h"
+
+typedef enum {
+    Expression,
+    Literal,
+    Grouping,
+    Unary,
+    Binary,
+    Operator
+} ExpressionType;
+
+typedef struct {
+    ExpressionType type;
+    union ex {
+        struct binary;
+        struct grouping;
+        struct literal;
+        struct unary;
+    } expression;
+} Expr;
+
+typedef struct {
+    Expr left;
+    TokenType op;
+    Expr right;
+} binary;
+
+struct grouping {
+    Expr expression;
+};
+
+struct literal {
+    char* object;
+};
+
+struct unary {
+    TokenType op;
+    Expr right;
+};
+
+#endif