Advanced Usage

Déjà Fu tries to have a sensible set of defaults, but there are some times when the defaults are not suitable. There are a lot of knobs provided to tweak how things work.

Execution settings

The autocheckWithSettings, dejafuWithSettings, and dejafusWithSettings let you provide a Settings value, which controls some of Déjà Fu’s behaviour:

dejafuWithSettings mySettings "Assert the thing holds" myPredicate myAction

The available settings are:

  • “Way”, how to explore the behaviours of the program under test.
  • Length bound, a cut-off point to terminate an execution even if it’s not done yet.
  • Memory model, which affects how non-synchronised operations, such as readIORef and writeIORef behave.
  • Discarding, which allows throwing away uninteresting results, rather than keeping them around in memory.
  • Early exit, which allows exiting as soon as a result matching a predicate is found.
  • Representative traces, keeping only one execution trace for each distinct result.
  • Trace simplification, rewriting execution traces into a simpler form (particularly effective with the random testing).
  • Safe IO, pruning needless schedules when your IO is only used to manage thread-local state.

See the Test.DejaFu.Settings module for more information.

Performance tuning

  • Are you happy to trade space for time?

    Consider computing the results once and running multiple predicates over the output: this is what dejafus / testDejafus / etc does.

  • Can you sacrifice completeness?

    Consider using the random testing functionality. See the *WithSettings functions.

  • Would strictness help?

    Consider using the strict functions in Test.DejaFu.SCT (the ones ending with a ').

  • Do you just want the set of results, and don’t care about traces?

    Consider using Test.DejaFu.SCT.resultsSet.

  • Do you know something about the sort of results you care about?

    Consider discarding results you don’t care about. See the *WithSettings functions in Test.DejaFu, Test.DejaFu.SCT, and Test.{HUnit,Tasty}.DejaFu.

For example, let’s say you want to know if your test case deadlocks, but you don’t care about the execution trace, and you are going to sacrifice completeness because your possible state-space is huge. You could do it like this:

dejafuWithSettings
  ( set ldiscard
    -- "efa" == "either failure a", discard everything but deadlocks
    (Just $ \efa -> Just (if either isDeadlock (const False) efa then DiscardTrace else DiscardResultAndTrace))
  . set lway
    -- try 10000 executions with random scheduling
    (randomly (mkStdGen 42) 10000)
  $ defaultSettings
  )
  -- the name of the test
  "Never Deadlocks"
  -- the predicate to check
  deadlocksNever
  -- your test case
  testCase