The class of data sources, parameterised over the request type for that data source. Every data source must implement this class.

A data source keeps track of its state by creating an instance of StateKey to map the request type to its state. In this case, the type of the state should probably be a reference type of some kind, such as IORef.

For a complete example data source, see Examples.

A convenience only: package up Eq, Hashable, Typeable, and Show for requests into a single constraint.

A BlockedFetch is a pair of

• The request to fetch (with result type a)
• A ResultVar to store either the result or an error

We often want to collect together multiple requests, but they return different types, and the type system wouldn't let us put them together in a list because all the elements of the list must have the same type. So we wrap up these types inside the BlockedFetch type, so that they all look the same and we can put them in a list.

When we unpack the BlockedFetch and get the request and the ResultVar out, the type system knows that the result type of the request matches the type parameter of the ResultVar, so it will let us take the result of the request and store it in the ResultVar.

A data source can fetch data in one of four ways.

Hints to the scheduler about this data source

A sink for the result of a data fetch in BlockedFetch

Like putResult, but used to get correct accounting when work is being done in child threads. This is particularly important for data sources that are using BackgroundFetch, The allocation performed in the child thread up to this point will be propagated back to the thread that called runHaxl.

Note: if you're doing multiple putResult calls in the same thread ensure that only the last one is putResultFromChildThread. If you make multiple putResultFromChildThread calls, the allocation will be counted multiple times.

If you are reusing a thread for multiple fetches, you should call System.Mem.setAllocationCounter 0 after putResultFromChildThread, so that allocation is not counted multiple times.

Common implementation templates for fetch of DataSource.

Example usage:

fetch = syncFetch MyDS.withService MyDS.retrieve
  $ \service request -> case request of
    This x -> MyDS.fetchThis service x
    That y -> MyDS.fetchThat service y

A version of asyncFetch (actually asyncFetchWithDispatch) that handles exceptions correctly. You should use this instead of asyncFetch or asyncFetchWithDispatch. The danger with asyncFetch is that if an exception is thrown by withService, the inner action won't be executed, and we'll drop some data-fetches in the same round.

asyncFetchAcquireRelease behaves like the following:

asyncFetchAcquireRelease acquire release dispatch wait enqueue =
  AsyncFetch $ \requests inner ->
    bracket acquire release $ \service -> do
      getResults <- mapM (submitFetch service enqueue) requests
      dispatch service
      inner
      wait service
      sequence_ getResults

except that inner is run even if acquire, enqueue, or dispatch throws, unless an async exception is received.

Function for easily setting a fetch to a particular exception