Haxl.Core.Monad

The implementation of the Haxl monad.

The monad

newtype GenHaxl u a

The Haxl monad, which does several things:

• It is a reader monad for Env and IORef RequestStore, The latter is the current batch of unsubmitted data fetch requests.
• 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.

Constructors

GenHaxl

Fields

unHaxl :: Env u -> IORef (RequestStore u) -> IO (Result u a)

Instances

Monad (GenHaxl u)

Functor (GenHaxl u)

Applicative (GenHaxl u)

Fractional a => Fractional (GenHaxl u a)

Num a => Num (GenHaxl u a)

IsString a => IsString (GenHaxl u a)

~ * u1 u2 => IfThenElse (GenHaxl u1 Bool) (GenHaxl u2 a)

runHaxl :: Env u -> GenHaxl u a -> IO a

Runs a Haxl computation in an Env.

env :: (Env u -> a) -> GenHaxl u a

Extracts data from the Env.

Exceptions

throw :: Exception e => e -> GenHaxl u a

Throw an exception in the Haxl monad

catch :: Exception e => GenHaxl u a -> (e -> GenHaxl u a) -> GenHaxl u a

try :: Exception e => GenHaxl u a -> GenHaxl u (Either e a)

tryToHaxlException :: GenHaxl u a -> GenHaxl u (Either HaxlException 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.

Data fetching and caching

dataFetch :: (DataSource u r, Request r a) => r a -> GenHaxl u a

Performs actual fetching of data for a Request from a DataSource.

uncachedRequest :: (DataSource u r, Request r a) => r a -> GenHaxl u a

A data request that is not cached. This is not what you want for normal read requests, because then multiple identical requests may return different results, and this invalidates some of the properties that we expect Haxl computations to respect: that data fetches can be aribtrarily reordered, and identical requests can be commoned up, for example.

uncachedRequest is useful for performing writes, provided those are done in a safe way - that is, not mixed with reads that might conflict in the same Haxl computation.

cacheRequest :: Request req a => req a -> Either SomeException a -> GenHaxl u ()

Inserts a request/result pair into the cache. Throws an exception if the request has already been issued, either via dataFetch or cacheRequest.

This can be used to pre-populate the cache when running tests, to avoid going to the actual data source and ensure that results are deterministic.

cacheResult :: Request r a => r a -> IO a -> GenHaxl u a

Transparently provides caching. Useful for datasources that can return immediately, but also caches values.

cachedComputation :: forall req u a. Request req a => req a -> GenHaxl u a -> GenHaxl u a

cachedComputation memoizes a Haxl computation. The key is a request.

Note: These cached computations will not be included in the output of dumpCacheAsHaskell.

dumpCacheAsHaskell :: GenHaxl u String

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))

Unsafe operations

unsafeLiftIO :: IO a -> GenHaxl u a

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

unsafeToHaxlException :: GenHaxl u a -> GenHaxl u a

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.