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
c have the same type
T, if we know
T is a
Semigroup, we have the following law:
a <> b <> c = (a <> b) <> c = a <> (b <> c)
T is a
Monoid, which is a
Semigroup with an identity (called
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.
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
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 typeclass is evil. It’s shipped with default instances, like one
[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
? 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.
In base, there’re already several ways to express defaulted values.
mempty is a way to express a default value regarding its
binary operation (
empty provides a similar default, but for first-class
mzero provides a different default, used to absorb
everything. That’s a law of
mzero >>= f = mzero a >> mzero = mzero
Those defaults are interesting because we can reason about. If I see
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. :)