Beautiful Failures

A Haskell library that helps you keep your users as happy as possible when things go wrong.

Overview

This library intends to make it as easy as possible for you to preserve failure context, render it to a display-agnostic form, and finally serialize it beautifully to a variety of outputs -- a terminal, a window in your GUI application, or even a LaTeX document.

failure conversions

There are two standard ways to handle failures -- one is treating the failure as a value, and returning it in a disjunction, like Either. The other is exceptions, which don't appear in the type and have non-local effects. In general, we prefer the Either approach, but sometimes we don't really have a choice.

throwLeft and throwIOLeft convert the disjunction into an exception. This should only be used when necessary -- e.g., your function has external constraints that make it impossible for an Either to cross the boundary.

ad-hoc exception creation

If a failure type was created by an external library, it may not provide an Exception instance, which means you can't throw it. We provide types both for turning "regular" types into Exceptions and for adding CallStacks to Exceptions.