I'm also in favor of removing this instance. Since there is currently no
deprecation mechanism for instances, I think it should simply be removed in
GHC 8.12.
On Mon, Nov 25, 2019 at 7:14 AM Oleg Grenrus
As replied by Henning, fail @IO = throwIO . userError, which isn't throwing a pure exception. The IO instance is in the spirit of MonadFail proposal, it behaves well.
If you write GHC-7.10..GHC-8.x code, you **should** depend on `fail` package, to get MonadFail class for GHC-7.x. See
https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail#Adaptingoldcode
On 25.11.2019 12.28, Bertram Felgenhauer via Libraries 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
Oleg Grenrus wrote: 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,
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