The Haxl monad, which does several things:

• It is a reader monad for Env, which contains the current state of the scheduler, including unfetched requests and the run queue of computations.
• It is a concurrency, or resumption, monad. A computation may run partially and return Blocked, in which case the framework should perform the outstanding requests in the RequestStore, and then resume the computation.
• The Applicative combinator <*> explores both branches in the event that the left branch is Blocked, so that we can collect multiple requests and submit them as a batch.
• It contains IO, so that we can perform real data fetching.

The result of a computation is either Done with a value, Throw with an exception, or Blocked on the result of a data fetch with a continuation.

A data representation of a Haxl continuation. This is to avoid repeatedly traversing a left-biased tree in a continuation, leading O(n^2) complexity for some pathalogical cases - see the "seql" benchmark in tests/MonadBench.hs. See "A Smart View on Datatypes", Jaskelioff/Rivas, ICFP'15

A synchronisation point. It either contains a value, or a list of computations waiting for the value.

The contents of a full IVar. We have to distinguish exceptions thrown in the IO monad from exceptions thrown in the Haxl monad, so that when the result is fetched using getIVar, we can throw the exception in the right way.

A completed request from a data source, containing the result, and the IVar representing the blocked computations. The job of a data source is just to add these to a queue (completions) using putResult; the scheduler collects them from the queue and unblocks the relevant computations.

The data we carry around in the Haxl monad.

Initialize an environment with a StateStore, an input map, a preexisting DataCache, and a seed for the random number generator.

Initializes an environment with StateStore and an input map.

A new, empty environment.

Extracts data from the Env.

Returns a version of the Haxl computation which always uses the provided Env, ignoring the one specified by runHaxl.

A list of computations together with the IVar into which they should put their result.

This could be an ordinary list, but the optimised representation saves space and time.

Throw an exception in the Haxl monad

Catch an exception in the Haxl monad

Catch exceptions that satisfy a predicate

Returns Left e if the computation throws an exception e, or Right a if it returns a result a.

Like try, but lifts all exceptions into the HaxlException hierarchy. Uses unsafeToHaxlException internally. Typically this is used at the top level of a Haxl computation, to ensure that all exceptions are caught.

Dump the contents of the cache as Haskell code that, when compiled and run, will recreate the same cache contents. For example, the generated code looks something like this:

loadCache :: GenHaxl u ()
loadCache = do
  cacheRequest (ListWombats 3) (Right ([1,2,3]))
  cacheRequest (CountAardvarks "abcabc") (Right (2))

Dump the contents of the cache as Haskell code that, when compiled and run, will recreate the same cache contents.

Takes the name and type for the resulting function as arguments.

Under ordinary circumstances this is unnecessary; users of the Haxl monad should generally not perform arbitrary IO.

Convert exceptions in the underlying IO monad to exceptions in the Haxl monad. This is morally unsafe, because you could then catch those exceptions in Haxl and observe the underlying execution order. Not to be exposed to user code.