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Class of transformers that don't introduce additional exit points to a computation.

Examples: StateT s, ReaderT e, IdentityT

Counter-examples: ExceptT e, ErrorT e, MaybeT, ListT

bracket acq cln use acquires the resource by running acq. If this computation aborts, the exception won't be handled and no cleanup will be performed since the resource wasn't acquired. Then use is called with the resource. Regardless if use threw an exception/aborted or finished normally, cln is called with the resource and possibly with the result of use (if it didn't abort). If there was an exception, it's rethrown: bracket is not meant to be used for exception handling.

An exception in this context is anything from actual IO exceptions for pure ones "thrown" by ExceptT or MaybeT. In case of IO, the resource acquisition and cleanup are masked from async exceptions.

Since this function can be used on almost any transformer stack, care needs to be taken that all the transformers that can throw exceptions get handled. This is why the effect isn't implicitly lifted through unknown transformers, only though ones that are instances of Unexceptional. If your transformer doesn't introduce new exit points, give it an instance of that class. There are no methods to implement.

A simpler version of bracket that doesn't use the results of the parameters.