implemented infix operator parsing

src/parser/precedence.cpp

@@ -0,0 +1,22 @@
+#include "precedence.hpp"
+
+namespace parser {
+    precedence precedence_for(token::type type) {
+        switch (type) {
+        case token::type::EQ:
+        case token::type::NEQ:
+            return precedence::EQUALS;
+        case token::type::LT:
+        case token::type::GT:
+            return precedence::LESS_GREATER;
+        case token::type::PLUS:
+        case token::type::MINUS:
+            return precedence::SUM;
+        case token::type::ASTERISK:
+        case token::type::SLASH:
+            return precedence::PRODUCT;
+        default:
+            return precedence::LOWEST;
+        }
+    }
+} // namespace parser