diff --git a/lib/include/expression.h b/lib/include/expression.h
new file mode 100644
index 0000000..e31a448
--- /dev/null
+++ b/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
diff --git a/lib/include/memory_pool.h b/lib/include/memory_pool.h
new file mode 100644
index 0000000..48e12f3
--- /dev/null
+++ b/lib/include/memory_pool.h
@@ -0,0 +1,16 @@
+#ifndef MEMORY_POOL_H
+#define MEMORY_POOL_H
+
+#include "expression.h"
+
+#define MEMORY_POOL_SIZE 128U
+
+typedef struct {
+	expression_t buffer[MEMORY_POOL_SIZE];
+	expression_t *free_pointer;
+} memory_pool_t;
+
+void init_memory_pool(memory_pool_t *pool);
+expression_t *allocate_expression(memory_pool_t *pool);
+
+#endif
diff --git a/lib/memory_pool.c b/lib/memory_pool.c
new file mode 100644
index 0000000..d1f8d4a
--- /dev/null
+++ b/lib/memory_pool.c
@@ -0,0 +1,15 @@
+#include "memory_pool.h"
+
+#include <stddef.h>
+
+void init_memory_pool(memory_pool_t *pool)
+{
+	pool->free_pointer = &pool->buffer[0];
+}
+
+expression_t *allocate_expression(memory_pool_t *pool)
+{
+	if (pool->free_pointer > pool->buffer + MEMORY_POOL_SIZE)
+		return NULL;
+	return pool->free_pointer++;
+}
diff --git a/scripts/build.sh b/scripts/build.sh
index 594d166..890313d 100644
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -2,12 +2,18 @@ cd "$(git rev-parse --show-toplevel)"
 
 CFLAGS="$CFLAGS -std=c11 -pedantic -Wall -Wextra"
 CFLAGS="$CFLAGS -Og -ggdb"
+CFLAGS="$CFLAGS -Ilib/include"
 
 mkdir -p build
 
+# Build library
+clang $CFLAGS -c -o build/memory_pool.o lib/memory_pool.c
+clang $CFLAGS -c -o build/reader.o lib/reader.c
+ar -crs build/lib.a build/memory_pool.o build/reader.o
+
 # Build tests
 clang $CFLAGS -Itests -c -o build/testing.o tests/testing.c
 
 # Build application
 clang $CFLAGS -c -o build/main.o app/main.c
-clang $CFLAGS -o build/infix-calculator build/main.o
+clang $CFLAGS -o build/infix-calculator build/main.o build/lib.a