Showing posts from June, 2020

Checked exceptions break composition

A.K.A. Always Throw Runtime Exceptions or their subclasses A typical Java or C++ function can potentially come with an exception specification: for example, a method can declare that it throws exceptions of a particular type (eg. IOException, std::bad_alloc etc.) and clients need to handle that exception being thrown with a try-catch block. This seems good at the outset till we spend time thinking through what this does to the type of the function. A typical function in a happy-go-lucky world either succeeds or fails with an exception because of something beyond its control. If it succeeds, it returns with a value of the provided return type (let's call it SuccessValueType). If it fails with an exception (eg. a file read error or a mem allocation error), it throws the exception and the error handling parts of the code run. In type terms, the return type of the function is Either<SuccessValueType, RuntimeExceptionType> (where the RuntimeExceptionType is an implicit return ty