Exceptions Best Practices

This is an FP Complete coding standards document written by Michael Snoyman. I'm exposing it to the outside world, but some of the prose definitely maintains the coding standard approach. This piece is highly opinionated, and I'm sure some people will have different thoughts on how to do this.

There is much debate in the Haskell community around exception handling. One commonly stated position goes something like "all exceptions should be explicit at the type level, and async exceptions are terrible." We can argue as much as we want about this point in a theoretical sense. However, practically, it is irrelevant, because GHC has already chosen a stance on this: it supports async exceptions, and all code that runs in IO can have exceptions of any type which is an instance of Exception.

As far as our coding standards go, we need to accept the world as it is, and realize that any IO code can throw any exception. (We can also discuss the theoretical benefits of the chosen setup, versus the terrible situation of checked exceptions in Java, but that's really a separate matter.) Additionally, all code must be written to be async-exception safe. How this is done is not covered in this document.

Let's identify a few anti-patterns in Haskell exception handling, and then move on to recommended practices.

The bad

ExceptT IO anti-pattern

A common (bad) design pattern I see is something like the following:

myFunction :: String -> ExceptT MyException IO Int

There are (at least) three problems with this:

It's non-composable. If someone else has a separate exception type HisException, these two functions do not easily compose.
It gives an implication which is almost certainly false, namely: the only exception that can be thrown from this function is MyException. Almost any IO code in there will have the ability to throw some other type of exception, and additionally, almost any async exception can be thrown even if no synchronous exception is possible.
You haven't limited the possibility of exceptions, you've only added one extra avenue by which an exception can be thrown. myFunction can now either throwE or liftIO . throwIO.

It is almost always wrong to wrap an ExceptT, EitherT, or ErrorT around an IO-based transformer stack.

Separate issue: it's also almost always a bad idea to have such a concrete transformer stack used in a public-facing API. It's usually better to express a function in terms of typeclass requirements, using mtl typeclasses as necessary.

Mask-them-all anti-pattern

This anti-pattern goes like this: remembering to deal with async exceptions everywhere is hard, so I'll just mask them all.

Every time you do this, 17 kittens are mauled to death by the loch ness monster.

Async exceptions may be annoying, but they are vital to keeping a system functioning correctly. The timeout function uses them to great benefit. The Warp webserver bases all of its slowloris protection on async exceptions. The cancel function from the async package will hang indefinitely if async exceptions are masked. Et cetera et cetera.

Are async exceptions difficult to work with? Sometimes, yes. Deal with it anyway. Best practices include:

Use the bracket pattern wherever possible.
If you have truly complex flow of control and non-linear scoping of resources, use the resourcet package.

The good

MonadThrow

Consider the following function:

foo <- lookup "foo" m
bar <- lookup "bar" m
baz <- lookup "baz" m
f foo bar baz

If this function returns Nothing, we have no idea why. It could be because:

"foo" wasn't in the map.
"bar" wasn't in the map.
"baz" wasn't in the map.
f returned Nothing.

The problem is that we've thrown away a lot of information by having our functions return Maybe. Instead, wouldn't it be nice if the types of our functions were:

lookup :: Eq k => k -> [(k, v)] -> Either (KeyNotFound k) v

f :: SomeVal -> SomeVal -> SomeVal -> Either F'sExceptionType F'sResult

The problem is that these types don't unify. Also, it's commonly the case that we really don't need to about why a lookup failed, we just need to deal with it. For those cases, Maybe is better.

The solution to this is the MonadThrow typeclass from the exceptions package. With that, we would write the type signatures as:

lookup :: (MonadThrow m, Eq k) => k -> [(k, v)] -> m v
f :: MonadThrow m => SomeVal -> SomeVal -> SomeVal -> m F'sResult

Versus the Either signature, we lose some information, namely the type of exception that could be thrown. However, we gain composability and unification with Maybe (as well as many other useful instances of MonadThrow, like IO).

The MonadThrow typeclass is a tradeoff, but it's a well thought out tradeoff, and usually the right one. It's also in line with Haskell's runtime exception system, which does not capture the types of exceptions that can be thrown.

Transformers

The following type signature is overly restrictive:

foo :: Int -> IO String

This can always be generalized with a usage of liftIO to:

foo :: MonadIO m => Int -> m String

This allows our function to easily work with any transformer on top of IO. However, given how easy it is to apply liftIO, it's not too horrible a restriction. However, consider this function:

bar :: FilePath -> (Handle -> IO a) -> IO a

If you want your inner function to live in a transformer on top of IO, you'll find it difficult to make it work. It can be done with lifted-based, but it's non-trivial. Instead, it's much better to express this function in terms of functions from either lifted-base or exceptions, and get one of the following more generalized type signatures:

bar :: MonadBaseControl IO m => FilePath -> (Handle -> m a) -> m a
bar :: (MonadIO m, MonadMask m) => FilePath -> (Handle -> m a) -> m a

This doesn't just apply to exception handling, but also to dealing with things like forking threads. Another thing to consider in these cases is to use the Acquire type from resourcet.

Custom exception types

The following is bad practice:

foo = do
    if x then return y else error "something bad happened"

The problem is the usage of arbitrary string-based error messages. This makes it difficult to handle this exceptional case directly in a higher level in the call stack. Instead, despite the boilerplate overhead involved, it's best to define a custom exception type:

data SomethingBad = SomethingBad
    deriving Typeable
instance Show SomethingBad where
    show SomethingBad = "something bad happened"
instance Exception SomethingBad
foo = do
    if x then return y else throwM SomethingBad

Now it's trivial to catch the SomethingBad exception type at a higher level. Additionally, throwM gives better exception ordering guarantees than error, which creates an exception in a pure value that needs to be evaluated before it's thrown.

One sore point is that some people strongly oppose a Show instance like this. This is an open discussion, but for now I believe we need to make the tradeoff at this point in the spectrum. I've proposed to the libraries mailing list to add a new method to the Exception typeclass used for user-friendly display of exceptions, which will make this less of a sore point.

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