Haxl.Core.Monad

The implementation of the Haxl monad. Most users should import Haxl.Core instead of importing this module directly.

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)

MonadThrow (GenHaxl u)

Since: 0.3.1.0

MonadCatch (GenHaxl u)

Since: 0.3.1.0

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.

withEnv :: Env u -> GenHaxl u a -> GenHaxl u a

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

Env

data Env u

The data we carry around in the Haxl monad.

Constructors

Env

Fields

cacheRef :: IORef (DataCache ResultVar)

memoRef :: IORef (DataCache (MemoVar u))

flags :: Flags

userEnv :: u

statsRef :: IORef Stats

states :: StateStore

Data sources and other components can store their state in here. Items in this store must be instances of StateKey.

type Caches u = (IORef (DataCache ResultVar), IORef (DataCache (MemoVar u)))

caches :: Env u -> Caches u

initEnvWithData :: StateStore -> u -> Caches u -> IO (Env u)

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

initEnv :: StateStore -> u -> IO (Env u)

Initializes an environment with StateStore and an input map.

emptyEnv :: u -> IO (Env u)

A new, empty environment.

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

Catch an exception in the Haxl monad

catchIf :: Exception e => (e -> Bool) -> GenHaxl u a -> (e -> GenHaxl u a) -> GenHaxl u a

Catch exceptions that satisfy a predicate

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

Returns Left e if the computation throws an exception e, or Right a if it returns a result 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. Exceptions thrown by the IO operation (except for asynchronous exceptions) are propagated into the Haxl monad and can be caught by catch and try.

cachedComputation :: forall req u a. (Eq (req a), Hashable (req a), Typeable (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.