Module Async_durable

to_create tells the Durable how to build a fresh 'a value.

is_broken tests whether the current 'a value can be used. It should return true when you want the Durable to attempt to rebuild or recreate the 'a.

to_rebuild Is called on the broken 'a. It should return a "fixed" 'a.

When the function with_ below is called, it uses is_broken to test if the current Durable value is broken. If not, it calls either to_create or to_rebuild. If to_rebuild is None (as in the default case), the Durable will try to create a fresh value with to_create.

create does not create the Durable value. The first call to to_create will be made on the first use of with_.

create_or_fail immediately calls to_create, returning an error if that first attempt fails. This function will also return an error if the initial durable value is broken.

with_ applies the given function to the Durable value if is_broken returns false. If is_broken return true, it first calls to_create or to_rebuild. This function will return an error if either to_create or to_rebuild returns an error, or if the rebuilt or recreated durable value is broken. is_broken is checked immediately before calling f.

We will only make one attempt to rebuild a broken value. If a call to to_rebuild returns Error _, we will drop the previously built value. The next call to with_ will call to_create.

with_ will raise if f raises.