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Oops 🍌

When we're writing functional code involving errors, we often find ourselves reaching for a type like Either (usually ExceptT): we put our "success type" on the Right, and our "error type" on the Left. When our code gets more complicated, however, we're going to find ourselves introducing multiple error types (see Matt Parsons' blog for a nice introduction to this practice). This is great, but the solution is also a new problem: our error types are not all the same! In order to use the monad instance, we need all our results to have the same Left type. How do we have both?

One solution is the nested Either type. As our error catalogue grows, so does our type signature:

Possible errors Type
1 ExceptT a IO ()
2 ExceptT (Either a b) IO ()
3 ExceptT (Either a (Either b c)) IO ()
4 ExceptT (Either a (Either b (Either c d))) IO ()
... ...

This is fine: we can use some type synonyms to hide all this noise (type Errors = Either ...), or maybe even alias Either (type (+) = Either) to something smaller. Both are acceptable, but it comes with a big maintenance burden. The structure of the Either type is quite fragile, and adding more errors to the catalogue will invariably break other code (what was once added with Right . Right . Right is now Right . Right . Left). Add to that the fact that it's just noisy. What if we had...

Either Variant
a Variant '[a]
Either a b Variant '[a, b]
Either a (Either b c) Variant '[a, b, c]
Either a (Either b (Either c d)) Variant '[a, b, c, d]

With the Variant type, we declare (in the type) the list of possible values, just as we do with Either. The only real difference at this point is that the syntax is nicer! Still, there must be more to it; what can we do with a Variant?

The library also defines VariantF, which works in the same way, but the type also mentions a type constructor, and the list of types are applied to it. For example, VariantF IO '[Int, String] is actually either IO Int or IO String. We can think of Variant as the special case of VariantF Identity.

Typically, a module involving a Variant may need some of the following extensions, depending on what you're doing with it:

    , FlexibleContexts
    , MonoLocalBinds
    , RankNTypes
    , ScopedTypeVariables
    , TypeApplications
    , TypeOperators #-}


throw :: xs `CouldBe` x => x -> Variant xs

Given some variant of types xs (e.g. '[Int, String, Bool]), if we have some type x in that variant, we say that the variant could be x. throw lets us lift any type into a variant that could be that type! In other words:

eg0 :: Int -> Variant '[Int]
eg0 = throw

eg1 :: Bool -> Variant '[Bool, String]
eg1 = throw

eg2 :: IO () -> Variant '[Int, IO (), Bool]
eg2 = throw

Now, why do we call it throw?


catch :: Catch x xs ys => Variant xs -> Either (Variant ys) x

The catch function effectively "plucks" a type out of the constraint. In other words, if I catch @String on a Variant '[Int, String, Bool], the result is Either (Variant '[Int, Bool]) String. This allows us to remove errors from the catalogue as we go up up the call stack.

The name is a reference to the throw/catch exception systems in other languages. In Java, I may see a definition like this:

public static void whatever() throws ExceptionA, ExceptionB

The equivalent in Haskell using this library would be:

  :: ( e `CouldBe` ExceptionA
     , e `CouldBe` ExceptionB
  => String -> Either e ()

"Throwing" and "Catching"

The interesting thing about the above two functions is that you should almost never see the Catch constraint in one of your signatures. Let's see an example:

data NetworkError      = NetworkError
data UserNotFoundError = UserNotFoundError

  :: ( e `CouldBe` NetworkError
     , e `CouldBe` UserNotFoundError
  => String
  -> ExceptT (Variant e) IO String

getUser = \case
  "Alice" -> throwM NetworkError
  "Tom"   -> pure "Hi, Tom!"
  _       -> throwM UserNotFoundError

We've got ourselves a fresh (and extremely contrived) bit of business logic! Notice that, according to the constraints, a couple things could go wrong: we could have a network error, or fail to find the user!

Now, let's say we're calling this from another function that does some more contrived business logic:

import Control.Monad.Oops

renderProfile :: ()
  => e `CouldBe` NetworkError
  => Text
  -> ExceptT (Variant e) IO ()
renderProfile username = do
  name <- catch @UserNotFoundError (getUser username) $ \_ -> do
    liftIO (putStrLn "ERROR! USER NOT FOUND. Defaulting to 'Alice'.")
    pure "Alice"

  liftIO (putStrLn name)

Here, we've tried to call getUser, and handled the UserNotFoundError explicitly. You'll notice that, as a result, this signature doesn't mention it! Thanks to some careful instance trickery, a CouldBe and a Catch constraint will actually cancel each other out!

{-# LANGUAGE BlockArguments #-}

import Control.Monad.Oops

renderProfile :: ()
  => Monad m
  => es `CouldBe` NetworkError
  => es `CouldBe` InvalidPassword
  => Text
  -> Text
  -> ExceptT (Variant es) IO ()
renderProfile username password = do
  name <- loginUser username password
    & do catch @UserNotFoundError \_ -> do
          liftIO (putStrLn "ERROR! USER NOT FOUND. Defaulting to 'Alice'.")
          pure "Alice"
    & do catch @InvalidPassowrd \e -> do
          liftIO (putStrLn "ERROR! INVALID PASSWORD.")
          throwM e

  liftIO (putStrLn name)

This library gives us all the benefits of Haskell's type system, forcing us to be explicit about all the possible errors we encounter, but doesn't force us to stick to a concrete error stack throughout. Our code is less fragile, our functions are decoupled, and error-handling is actually bearable!


Many examples can be found in the oops-example package.


Type inference

Type-inference generally works, however the types inferred may not be the simplest or may be more generic than your needs.

For example the following:

readInt :: (MonadError (Variant e) m, CouldBeF e Text) => String -> m Int

Can be simplified to one of the following:

readInt :: (MonadError (Variant e) m, CouldBe e Text) => String -> m Int
readInt :: (MonadError (Variant e) m, e `CouldBe` Text) => String -> m Int
readInt :: (MonadError (Variant e) IO, e `CouldBe` Text) => String -> IO Int
readInt :: e `CouldBe` Text => String -> ExceptT (Variant e) m Int
readInt :: e `CouldBe` Text => String -> ExceptT (Variant e) IO Int

Understanding error messages

If you get the following error:

• Could not deduce (OO.CouldBeF e MyErrorType)
    arising from a use of ‘OO.throw’
  from the context: (MonadError (Variant e) m, OO.CouldBeF e Text)
    bound by the type signature for:
               readIntV1 :: forall (e :: [*]) (m :: * -> *).
                            (MonadError (Variant e) m, OO.CouldBeF e Text) =>
                            String -> m Int
    at /Users/jky/wrk/haskell-works/oops/examples/src/Examples.hs:(27,1)-(31,10)

It means the function body is throwing MyErrorType and the function doesn't have the constraint to declare the error propagates to the caller.

In this case you have two choices:

  • Add the constraint to the function's type signature to propage the error to the caller.
  • Catch the exception in the function body and handle it. The handler can return a fallback value or throw an error of another type.

If you get the following error:

• Uh oh! I couldn't find MyErrorType inside the variant!
  If you're pretty sure I'm wrong, perhaps the variant type is ambiguous;
  could you add some annotations?

It means the expression under runOops or similar throws an error that is not handled.

In this case you have two choices:

  • Swap runOops or similar for something else that catches the uncaught error.

  • Catch the exception in the function body and handle it. The handler can return a fallback value or throw an error of the type caught by the runOops equvalent. Note runOops itself catches no errors, so in this case all errors must be handled.


For examples of oops code, see the Examples.hs file.

For examples of compile errors when using oops and how to fix them, see the troubleshooting page.

Projects using oops


This library is heavily based on the original oops library by Tom Harding.


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