Define expression data type

lib/include/expr.h

@@ -3,11 +3,41 @@
 
 #define MAX_SYMBOL_LEN 32U
 
+#include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
 
 typedef struct {
 	char buf[MAX_SYMBOL_LEN];
 	size_t len;
 } symbol_t;
 
+typedef enum {
+	ATOM_TYPE_EMPTY_LIST,
+	ATOM_TYPE_INTEGER,
+	ATOM_TYPE_SYMBOL,
+} atom_type_t;
+
+typedef struct {
+	atom_type_t type;
+	union {
+		int64_t integer;
+		symbol_t symbol;
+	};
+} atom_t;
+
+struct expr;
+
+typedef struct {
+	struct expr *car, *cdr;
+} pair_t;
+
+typedef struct expr {
+	bool is_atom;
+	union {
+		atom_t atom;
+		pair_t pair;
+	};
+} expr_t;
+
 #endif