Explaining Type Errors by Finding the Sources of Type Conflicts

Typically a type error is reported when unication fails, even though the programmer 's actual error may have occurred much earlier in the program. The W and M inference algorithms report the site where a type conict is detected. The error message is isolated information: it is not clear from the error information the relationship between the site and the context in which the subexpression was typed. As a result, the error message may give little help to locate the source of the error. This report investigates better methods of explaining type conicts. We are trying to nd a method that may be eective even when the user has little knowledge about type checking. We have implemented an improved type error explanation system and have proposed and implemented a hybrid algorithm. 1 Introduction It is known that the place where a type error was detected may not be the place where the type error actually originated. The source of a type error may be far from where it is detected. This is b..