improve error message

examples/evalexpr.py

@@ -7,6 +7,7 @@ import sys
 
 sys.path.append(f"{dirname(__file__)}/../src")
 from pyalibert import regex, Parser, just, Declare
+from pyalibert.result import ParseError
 
 # ast modeling
 
@@ -79,9 +80,12 @@ def repl():
     print("> ", end="")
     sys.stdout.flush()
     for line in sys.stdin:
-        result = parser.parse(line.strip())
-        print(result)
-        print(f"= {result.eval()}")
+        try:
+            result = parser.parse(line.strip())
+            print(result)
+            print(f"= {result.eval()}")
+        except ParseError as err:
+            print(err)
         print("> ", end="")
         sys.stdout.flush()