.. proposal-number:: 11.. trac-ticket:: `#14324 <https://ghc.haskell.org/trac/ghc/ticket/14324>`_.. implemented:: Not yetContents
The invariant-checking mechanism provided by GHC's stm library is buggy and
appears to be largely unused. Let's remove it.
GHC's STM subsystem has long had the ability to run user-specified invariant
checks when committing transactions, embodied by the always and
alwaysSucceeds functions of Control.Monad.STM.
However, if Hackage is any indication this feature has seen very little use of the past ten years. In fact, it has very likely been quite broken (see #14310) for this entire duration yet no one noticed. Moreover, the mechanism is likely mediocre at achieving its goal of catching broken invariants due to odd interleavings of transactions as its implementation is extremely sychronization-heavy. Additionally, the mechanism currently fails to handle some corner-cases correctly (#7930).
In short, the feature is complex, buggy, and not pulling its weight.
For GHC 8.4.1 we mark Control.Monad.STM.always and
Control.Monad.STM.alwaysSucceeds with DEPRECATED pragmas. These
interfaces will be removed three major releases later.
This will allow us to remove the invariants implementation from the runtime system.
Current users of the interface have a few options to adapt their code to manually call invariant their checks where necessary.
The removal of the interfaces will break any existing users.
Not removing the interface.
How many users does this interface have? Is it crucial to their programs' functions?
I, Ben Gamari, will implement this.