fix(types): add str methods to dataframe types

midas/checker/types.py

@@ -160,11 +160,18 @@ class ConstraintType:
 class ColumnType:
     type: Type
 
+    def __str__(self) -> str:
+        return f"Column[{self.type}]"
+
 
 @dataclass(frozen=True, kw_only=True)
 class DataFrameType:
     columns: list[Column]
 
+    def __str__(self) -> str:
+        schema: list[str] = [f"{col.name}: {col.type}" for col in self.columns]
+        return f"Frame[{', '.join(schema)}]"
+
     @dataclass(frozen=True, kw_only=True)
     class Column:
         index: int