Re: Proposal: Remove the bogus MonadFail instance for ST

That seems reasonable. But I wonder if pattern matching failure in IO do should be allowed to slip by silently, or whether we should exclude the otherwise-reasonable instance to catch more mistakes. On Mar 14, 2018 10:31 AM, "Michael Snoyman" wrote:

One possible "well behaved" intuition could be "cannot result in an exception thrown from pure code without usage of unsafe functions." By this definition:

* Maybe's fail is well behaved: using `fail "foo"` results in a total Nothing value * List's: same thing, but with an empty list * IO: runtime exception, but the exception is _not_ in pure code, but rather from within IO, where exceptions are always to be expected * ST: `runST (fail "foo")` results in a pure value which, when evaluated, throws a runtime exception, breaking the well behaved definition * Identity: `Identity (fail "foo")` can only be a pure value which throws an exception, and is therefore not well behaved

Note that I added the requirement of "without usage of unsafe functions," since `unsafePerformIO (fail "foo")` can result in a pure bottom value.

On Wed, Mar 14, 2018 at 4:25 PM, Ryan Scott wrote:

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Thanks, that makes more sense. I'm inclined to agree that MonadFail instances should fail in a "well-behaved" way. (I wish I knew how to make the phrase "well-behaved" more formal, but I don't.) It might be worth adding this intuition to the Haddocks for MonadFail.

That being said, one thing to consider before removing this instance is that there will be some breakage. Ben Gamari added this instance in [1] because apparently the regex-tdfa package needed it. Other than that, though, I don't have any real objections to removing this instance.

Ryan S. ----- [1] https://phabricator.haskell.org/D3982

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I expect a MonadFail instance to have a well-behaved notion of failure within the monad. An exception from "pure" code (which is what ST simulates) is not that. On the other hand, perhaps you're right and the instance should be removed for IO as well; I don't have as strong a sense of revulsion, but maybe users should be forced to be explicit with throwIO.

On Wed, Mar 14, 2018 at 9:46 AM, Ryan Scott wrote:

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OK. You used the phrase "utterly contrary to the purpose of MonadFail", so I'm trying to figure out exactly what you mean here. Prima facie, the purpose of MonadFail (at least, as explained in its Haddocks) is to provide a type class–directed way of desugaring partial pattern matches in do-notation. With this in mind, the current MonadFail instance for ST doesn't seem too offensive.

However, I think you have some additional property in mind that you feel the MonadFail ST instance runs afoul of. Do you mind explaining in further detail what this is? (I'm not trying to be snarky here—I genuinely don't know what you're getting at.)

Ryan S.

On Wed, Mar 14, 2018 at 9:41 AM, David Feuer wrote:

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I am not. I think that instance is fairly legitimate, as it raises an IO exception that can be caught in IO. IO's Alternative instance is a bit shadier, but that's not a topic for this proposal either. ST is an entirely different story, and I'm sorry I accidentally mixed it in.

On Wed, Mar 14, 2018 at 9:05 AM, Ryan Scott wrote:

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It's worth noting that the MonadFail instance for IO [1] also simply

On Wed, Mar 14, 2018 at 9:58 AM, David Feuer wrote: throws

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an error (by way of failIO). Are you proposing we remove this instance as well?

Ryan S. ----- [1] http://git.haskell.org/ghc.git/blob/cb6d8589c83247ec96d5faa8 2df3e93f419bbfe0:/libraries/base/Control/Monad/Fail.hs#l80

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