This **partially** addresses
https://github.com/hwchase17/langchain/issues/1524, but it's also useful
for some of our use cases.
This `DocstoreFn` allows to lookup a document given a function that
accepts the `search` string without the need to implement a custom
`Docstore`.
This could be useful when:
* you don't want to implement a `Docstore` just to provide a custom
`search`
* it's expensive to construct an `InMemoryDocstore`/dict
* you retrieve documents from remote sources
* you just want to reuse existing objects
