Thursday, July 16, 2015

Don’t use Default

Don’t use Default

Background knowledge: typeclasses and laws

In Haskell, we have a lot of typeclasses. Those are very handy and – in general – come with laws. Laws are very important and give hints on how we are supposed to use a typeclass.

For instance, the Semigroup typeclass exposes an operator ((<>)) and has an associativity law. If a, b and c have the same type T, if we know that T is a Semigroup, we have the following law:

a <> b <> c = (a <> b) <> c = a <> (b <> c)

If T is a Monoid, which is a Semigroup with an identity (called mempty), we have a law for monoids:

a <> mempty = mempty <> a = a

Those laws can – have – to be used in our code base to take advantage over the structures, optimize or avoid boilerplate.

The Default typeclass

In some situations, we want a way to express default values. That’s especially true in OO languages, like in the following C++ function signature:

void foo(float i = 1);

In Haskell, we cannot do that, because we have to pass all arguments to a function. The following doesn’t exist:

foo :: Float -> IO ()
foo (i = 1) = -- […]

And there’s more. Even in C++, how do you handle the case when you have several arguments and want only the first one to be defaulted? You cannot.

So, so… Some Haskellers decided to solve that problem with a typeclass. After all, we can define the following typeclass:

class Default a where
  def :: a

We can then implement Default and have a default value for a given type.

instance Default Radians where
  def = Radians $ 2*pi

instance Default Fullscreen where
  def = Fullscreen False

However, there is an issue with that. You cannot use Default without creating newtypes to overload them. Why? Well, consider the following Default instance:

instance Default Float where
  def = -- what should we put here?

Remember that, in Haskell, an instance is defined only once and is automatically imported when you import the module holding it. That means you cannot have the following:

-- in a module A
instance Default Float where
  def = 0

-- in a module B
instance Default Float where
  def = 1

-- in a module C
import A
import B

-- What instances should we use?

Hey, that’s easy. We just have to keep the modules apart, and import the one we want to use!

Yeah, well. No. No.

Orphan instances are wrong. You should read this for further explanations.

That’s why we have to use newtypes everywhere. And that’s boring. Writing code should always have a goal. When we write as much boilerplate code as real code, we can start thinking there’s something wrong. Worse, if we have more boilerplate than real code, well, something is terribly wrong. In our case, we’re introducing a lot of newtypes for only being able to use def at a few spots. Is that even worth it? Of course not.

Default is evil

The Default typeclass is evil. It’s shipped with default instances, like one for [a], which defaults to the empty list – []. It might be clear for you but why would I want to default to the empty list? Why not to [0]? Or a more complex list? I really doubt someone ever uses def :: [a].

Another reason why Default is wrong? There’s absolutely no law. You just have a default value, and that’s all.

bar :: (Default a) => a -> Maybe String

Can you say what the default is for? Of course you cannot. Because there’s no law. The instance has no real meaning. A default value makes sense only for the computation using it. For instance, the empty list makes sense if we glue it to the list concatenation.

We already have a lot of defaults

In base, there’re already several ways to express defaulted values.

The Monoid’s mempty is a way to express a default value regarding its binary operation ((<>)).

The Alternative’s empty provides a similar default, but for first-class types.

The MonadZero’s mzero provides a different default, used to absorb everything. That’s a law of MonadZero:

mzero >>= f = mzero
a >> mzero  = mzero

Those defaults are interesting because we can reason about. If I see mzero in a monadic code, I know that whatever is following will be discarded.


So please. Stop using Default. It doesn’t bring any sense to your codebase. It actually removes some! If you want to provide functions with defaulted arguments, consider partially applied functions.

data-default is a very famous package – look at the downloads! You can now have some hindsight about it before downloading it and ruining your design. ;)

Happy hacking. :)


  1. Can you give some examples of APIs which use Default?

    Also, how would you use default in lieu of optional arguments? You alluded to that but I can't picture how it would work.

    I thought Default was typically used when you need a "don't care" value of some type (which need satisfy no law). Or for configuration data structures, where rather than every `Cfg` type in each library also exporting a "defaultCfg" binding, they can use instance Default for some uniformity in naming.

    1. the xlsx library uses them

  2. Well, just implement Monoid. mempty is your default, and mappend the way to override the configuration. :)

  3. Your blog post provides code examples of how not to do it. But as a Haskell newbie I can not imagine how to use Monoid for default values. I'd be thankful for another article about that!

    1. Sure! Typically, a Monoid is a set with a binary associative function. If you have a Configuration type, you can make it a Monoid by implementing mempty to be the « default configuration », and the binary function would take the right configuration to override the left one. :)

  4. I disagree, and think this is more of an issue of practicality. Monoid is a useful typeclass, and looks like a nice idea for config updates too, but how would that look like in real code? A default + lens (or record update syntax) for updates does the same thing without the need for a Monoid instance.