sum types and pattern matching are great. For me, I'm anti-exception.
"Point on the doll where the exceptions hurt you." I worked on a project with exceptions as control flow (cough, twisted python, cough), and the error handling was caught several classes and mixins up and over in the file directory. In some far away file, your exception triggered a callback or an errback, and if that excepted, similar magic happened. It got to the point that several engineering choices had to be made around "well, this really would be nice to have an exception and some standard handling, but the framework will take it and do strange things."
I _love_ handling my errors where they are created.
"Point on the doll where the exceptions hurt you." I worked on a project with exceptions as control flow (cough, twisted python, cough), and the error handling was caught several classes and mixins up and over in the file directory. In some far away file, your exception triggered a callback or an errback, and if that excepted, similar magic happened. It got to the point that several engineering choices had to be made around "well, this really would be nice to have an exception and some standard handling, but the framework will take it and do strange things."
I _love_ handling my errors where they are created.