refactor: evaluator incomplete.

SO, i forgot to implement nud and led correctly and the parser cant tell
apart from - as unary and - as binary (+ as well), i need to correct
that, move Node * to TreeResult so to use NodeResult with nud and led

include/evaluator.h

@@ -5,7 +5,22 @@
 #include "parser.h"
 #include <stdint.h>
 
-int64_t evaluate(ParseResult context);
-int64_t evaluate_tree(ASTNode *tree);
+typedef enum {
+    EVALUATOR_OK,
+    EVALUATOR_MATH_ERR,
+    EVALUATOR_INVALID_PARSING,
+    EVALUATOR_INVALID_TREE, // just to shut up the compiler with the swithces
+} EvaluatorErr;
+
+typedef struct {
+    bool is_valid;
+    union {
+        int64_t val;
+        EvaluatorErr err;
+    };
+} EvaluatorResult;
+
+EvaluatorResult evaluate(ParserResult context);
+EvaluatorResult evaluate_tree(Node *tree);
 
 #endif // !EVALUATOR_H