Handle nesting and subtraction in reader

lib/reader.c

@@ -3,6 +3,10 @@
 #include <ctype.h>
 #include <stddef.h>
 
+static int parse_expression(
+    memory_pool_t *pool, const char *input, int remaining,
+    expression_t *out);
+
 static int parse_number(
     memory_pool_t *pool, const char *input, int remaining, expression_t *out)
 {
@@ -35,14 +39,25 @@ static int parse_application(
 	remaining -= result;
 	input += result;
 
-	if (remaining <= 0 || *input != '+')
+	if (remaining <= 0)
 		return 0;
+	operator_t operator;
+	switch (*input) {
+	case '+':
+		operator= OPERATOR_ADD;
+		break;
+	case '-':
+		operator= OPERATOR_SUBTRACT;
+		break;
+	default:
+		return 0;
+	}
 	++used;
 	--remaining;
 	++input;
 
 	expression_t *operand1 = allocate_expression(pool);
-	result = parse_number(pool, input, remaining, operand1);
+	result = parse_expression(pool, input, remaining, operand1);
 	if (0 == result)
 		return 0;
 	used += result;
@@ -50,7 +65,7 @@ static int parse_application(
 	input += result;
 
 	out->is_number = false;
-	out->application.operator = OPERATOR_ADD;
+	out->application.operator= operator;
 	out->application.operands[0] = operand0;
 	out->application.operands[1] = operand1;