There should be a patch release of dejafu after this, since the
doctest examples changed.
Also, need to bump to a nightly snapshot in `stack.yaml` for
stylish-haskell, which looks to have skipped GHC 9.0 support.
In principle, we could now drop support for GHC 8.0, 8.2, and 8.4, but
supporting those isn't causing any problems yet.
This lets dejafu test CoCo-style properties, and introduces a
dependency on [leancheck][].
For example, consider this statement about MVars: "using readMVar is
better than a takeMVar followed by a putMVar because the former is
atomic but the latter is not."
This module can test properties like that:
```
sig e = Sig
{ initialise = maybe newEmptyMVar newMVar
, observe = \v _ -> tryReadMVar v
, interfere = \v s -> tryTakeMVar v >>
maybe (pure ()) (void . tryPutMVar v) s
, expr = e
}
> check $ sig (void . readMVar) `equivalentTo`
sig (\v -> takeMVar v >>= putMVar v)
*** Failure: (seed Just ())
left: [(Nothing,Just ())]
right: [(Nothing,Just ()),(Just Deadlock,Just ())]
```
The two expressions are not equivalent, and we get given the
counterexample!
In addition to "equivalentTo", there is also "refines" and
"strictlyRefines" (and negated variants)
[leancheck]: https://hackage.haskell.org/package/leancheck
The new 'concurrency' package is starting at version 1.0.0.0 because the
API is already very mature (copied from base).
This breaks the dejafu-0.2 compatibility of async-dejafu.
Closes#51.
