Create expression struct and memory pool

lib/include/expression.h

@@ -0,0 +1,27 @@
+#ifndef EXPRESSION_H
+#define EXPRESSION_H
+
+#include <stdbool.h>
+
+typedef enum {
+	OPERATOR_ADD,
+	OPERATOR_SUBTRACT,
+	OPERATOR_MULTIPLY,
+	OPERATOR_DIVIDE,
+} operator_t;
+
+struct _expression_t;
+typedef struct {
+	operator_t operator;
+	const struct _expression_t *operands[2];
+} application_t;
+
+typedef struct _expression_t {
+	bool is_number;
+	union {
+		application_t application;
+		int number;
+	};
+} expression_t;
+
+#endif