Typed Errors

April 2019

by Daniel Winograd-Cort

I read a post by Matt Parsons called The Trouble with Typed Errors that talks about the difficulties we face in Haskell from not having open variant types. Matt says:

Haskell doesn’t have open variants, and the attempts to mock them end up quite clumsy to use in practice.

But, I disagree. I think row-types handles the typed error case quite nicely.


As this is a Literate Haskell file, let’s get the imports and pragmas out of the way first…

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
module TypedErrors where

import Data.Row


I’ll try to set up the situation similar to how Matt sets it up in his blog. Let’s start with two functions, foo and bar, that may each fail.

data FooErr = FooErr Int
  deriving (Show)

data BarErr = BarErr String
  deriving (Show)

foo :: Either FooErr Int
foo = Left (FooErr 3)

bar :: Either BarErr Int
bar = Left (BarErr "Oops")

Of course, the problem with this code is that there’s no good way to deal with these two errors together. Matt explains in his blog the various problems, but in short:

A row-types solution

Generating Errors

With row-types, we have open variants easily available to us, which means we can do the following:

foo :: (AllUniqueLabels r, r .! "fooErr"  Int)    => Either (Var r) Int
foo = Left (IsJust #fooErr 3)

bar :: (AllUniqueLabels r, r .! "barErr"  String) => Either (Var r) Int
bar = Left (IsJust #barErr "Oops")

baz :: (AllUniqueLabels r, r .! "bazErr"  Bool)   => Either (Var r) Int
baz = Left (IsJust #bazErr True)

  :: ( AllUniqueLabels r
     , r .! "fooErr"  Int
     , r .! "barErr"  String
     , r .! "bazErr"  Bool)
  => Either (Var r) Int
foobarbaz = bar *> foo *> bar *> baz

In foo, we create error data with the expression IsJust #fooErr 3. This creates a new row-types variant at the label "fooErr" with the value 3. The context indicates that the error type may have other possibilities: specifically, AllUniqueLabels r is some boilerplate that guarantees that no two possibilities have the same name, and r .! "fooErr" ≈ Int declares that the fooErr possibility has a payload of type Int.

We can do the same for bar/barErr and baz/bazErr, and then if we want to compose them together, we can easily do so as in foobarbaz. Furthermore, although we provide the type signatures here, GHC will infer them just fine (with NoMonomorphismRestriction).

Handling Errors

We can handle these errors in multiple ways.

First off, it’s easy enough to show our value (so long as the data in the errors is Showable):

printFoobarbaz :: String
printFoobarbaz = show specificFoobarbaz
  where specificFoobarbaz :: Either (Var ("fooErr" .== Int
                                       .+ "barErr" .== String
                                       .+ "bazErr" .== Bool)) Int
        specificFoobarbaz = foobarbaz

All row-types variants implement an obvious Show instance, but do note that to show foobarbaz, we must specify its type. This is because foobarbaz is defined polymorphically over any variant that has appropriate entries for fooErr, barErr, and bazErr, but to show it, we must pick a concrete type to use for the Show instance. In this case, we pick the minimum variant.

We can also deal with a single error at a time using the trial function. This function lets us pluck a particular possibility out of a variant, allowing us to handle that possibility or be left with the leftovers of the variant. In the following case, we handle the fooErr possibility, using the Int value it contains as our return value. If foobarbaz is not a fooErr, then we’re left with a Left error value that cannot be a fooErr.

handleFoo :: forall r.
  ( AllUniqueLabels r
  , r .! "fooErr"  Int
  , r .! "barErr"  String
  , r .! "bazErr"  Bool)
  => Either (Var (r .- "fooErr")) Int
handleFoo =
  case foobarbaz of
    Left err  -> case trial @_ @r err #fooErr of
      Left  i     -> Right i
      Right other -> Left other
    Right i   -> Right i

The type signature of handleFoo is a little disappointing but necessary because we’re keeping our variant type entirely polymorphic. However, if we were willing to monomorphize our error to a concrete type, the constraints (and the type applications on trial) would no longer be necessary. This is a tradeoff that one needs to make based on the situation.

Finally, we have the option of handling all errors at once using switch.

handleAll :: String
handleAll =
  case foobarbaz of
    Left err -> switch err $
         #fooErr .== (\n -> "FooErr of " ++ show n)
      .+ #barErr .== (\s -> "BarErr of " ++ s)
      .+ #bazErr .== (\b -> "BazErr of " ++ show b)
    Right i  -> "Got the result " <> show i

Specifically, switch allows us to define a case for every possibility of a variant, allowing us to reduce the variant to an ordinary result. In this case, type annotations are not needed because the variant must match exactly the form of the switch’s cases. Because we have exactly 3 cases, one for each of our errors, GHC monomorphizes the error component of foobarbaz to Var ("fooErr" .== Int .+ "barErr" .== String .+ "bazErr" .== Bool) automatically.

Achievements and Limitations

Using variants, we are able to create and handle typed errors without dealing with weird nesting of Eithers and without losing any static guarantees. Furthermore, variant typed errors can be easily defined with constraints (as we did here with the constraints like r .! "fooErr" ≈ Int) with minimal boilerplate: no extra data declarations necessary! And, once monomorphized, two variants with the same possibilities always share the same type, regardless of the order that the possibilities are described in the type. I also didn’t discuss diversify, which allows one to expand the possibilities in a variant, which, for typed errors, allows one to use a limited (perhaps already monomorphized) error type in a more general setting.

However, there are downsides to variants typed errors. A little bit of boilerplate does remain in the form of the AllUniqueLabels constraint, which just about always needs to be used. Also, GHC has trouble inferring all the types and constraints when we want to remain as polymorphic as possible, which means writing out some annoying types and occasionally using type annotations (as seen in handleFoo above). Lastly, the switch expression seems a lot like an ordinary Haskell case expression, but it isn’t, which means the user is forced to learn what amounts to a special syntax just for dispatching the errors.

There are specific considerations for any project, but I think row-types variants are a great choice for typed errors.