Oleg Grenrus wrote:
I cannot see any justification for that instance
-- | @since 4.11.0.0 instance Fail.MonadFail (ST s) where fail s = errorWithoutStackTrace s
It's certainly not in the spirit of the MonadFail proposal. Arguably, *all* implementations of `fail` in terms of `error*` are wrong, even the one for IO. (I suspect dropping the instance for IO would cause a lot of pain though...) I'm neutral on the proposal itself.
If someone wants to fail pattern matches in `ST s`, please do that explicitly.
However, I don't like this framing. In practice, there's at least one other use of MonadFail for historical reasons: generic code that may fail. Before MFP, one could write (and people have written) code like fooM :: Monad m => a -> m b fooM ... = fail "unsupported case" It is easy enough to change the signature of such a `fooM` to require MonadFail, but then the function becomes impossible to use for monads that don't have a MonadFail instance. So after the MFP, one should provide two functions instead, one using `error` and another one using `fail`...
Note: compatibility package https://hackage.haskell.org/package/fail-4.9.0.0/docs/Control-Monad-Fail.htm... doesn't have that instance, so I suspect not-that many use(d) that instance.
Or perhaps nobody cares about ghc-7.10 anymore. Cheers, Bertram