In Haskell, type classes define operations with their type signatures. Many of them also define laws which must be satisfied by each of their instances. As laws can’t be checked by the compiler, writing new instances is a potentially dangerous operation. We have various options for assuring our handwritten instances are lawabiding such as penandpaper reasoning, and propertybased testing.
A compelling alternative is to have the compiler write the instances for us:
this is known as deriving instances. To this end, GHC implements several strategies
such as DeriveFunctor
and DeriveAnyClass
. The most recent addition DerivingVia
is
based on the concept of isomorphisms. The idea is simple: to derive an
instance for type A
, look at some isomorphic type B
and reuse its instance,
inserting conversions between A
and B
where they are needed.
By design, DerivingVia
operates only on a single isomorphism: coerce
, which
converts between types with the same runtime representation.
The paper introducing the DerivingVia
extension shows
how to generalize the technique slightly and derive through the isomorphisms
generated by GHC Generics. We will take this technique a step further and show
how to derive instances through arbitrary isomorphisms. We refer to this as
isoderiving.
In this post I’ll present isoderiving and show some examples of isoderiving in action. I’ve packaged up all the types and classes used here in the isoderiving package.
Isomorphisms
We say that
two types A
and B
are isomorphic if there is a pair of functions f :: A > B
and g :: B > A
satisfying f . g = id
and g . f = id
.
For example:
ByteString
is isomorphic to[Word8]
usingpack
andunpack
Integer
is isomorphic to itself using(+ 2)
and( 2)
(a, b)
is isomorphic to(b, a)
usingswap
andswap
The following type class captures the fact that a
and b
are isomorphic.
class Isomorphic a b where
inj :: a > b
prj :: b > a
Instances should satisfy inj . prj = id
and prj . inj = id
.
Passing around an isomorphism
To isoderive instances for a
, we will need to pick some isomorphic type b
and pass
it along in the deriving
clause. We will define a newtype wrapper for this:
type As :: Type > Type > Type
newtype As a b = As b
A value of type As a b
is represented at runtime as b
, but can be converted
to a
(and back) with no loss of information.
We will also need such wrappers for higherkinded types:
type As1 :: (k > Type) > (k > Type) > k > Type
newtype As1 f g a = As1 (g a)
type As2 :: (k1 > k2 > Type) > (k1 > k2 > Type) > k1 > k2 > Type
newtype As2 f g a b = As2 (g a b)
We will need a special “wrapper” instance for each class we want to derive
isomorphically. Much like the instances generated by DerivingVia
, these all
follow a strict mechanical pattern. Here is the instance for Num
:
instance (Isomorphic a b, Num a) => Num (As a b) where
(As x) + (As y) = As $ inj @a @b $
prj @a @b x + prj @a @b y
(As x) * (As y) = As $ inj @a @b $
prj @a @b x * prj @a @b y
signum (As x) = As $ inj @a @b $
signum (prj @a @b x)
abs (As x) = As $ inj @a @b $
abs (prj @a @b y)
fromInteger x = As $ inj @a @b $
fromInteger x
We’re omitting some methods for brevity, but they follow the same pattern. Note
the use of XTypeApplications
to make the choice of inj
and prj
unambiguous^{1}.
The generation of these instances could be automated, but as we’ll only ever need one
instance per class, it suffices to put them into the library. Here are some of the instances
exported by isoderiving
:
instance (Isomorphic a b, Eq a)
=> Eq (As a b)
instance (Isomorphic a b, Ord a)
=> Ord (As a b)
instance (Isomorphic a b, Semigroup a)
=> Semigroup (As a b)
instance (Isomorphic a b, Monoid a)
=> Monoid (As a b)
instance (forall x . Isomorphic (f x) (g x), Functor f)
=> Functor (As1 f g)
instance (forall x . Isomorphic (f x) (g x), Applicative f)
=> Applicative (As1 f g)
instance (forall x . Isomorphic (f x) (g x), Monad f)
=> Monad (As1 f g)
instance (forall x y . Isomorphic (f x y) (g x y), Category f)
=> Category (As2 f g)
Now that we know how isoderiving works, let’s take a look at some examples.
Points and Vectors
Here is a type representing 2D points:
data Point a = Point { x :: a, y :: a }
What instances might we want to derive? GHC will happily derive Eq
, Show
and Functor
for us. However it won’t be able to give us Num
or
Applicative
.
To derive Num
and Applicative
, we will use an isomorphism between
Point a
and a familiar type. While (a, a)
is the obvious choice,
it wouldn’t let us derive Applicative
either. Instead, we will use
Bool > a
.
We will also use Ap
from
Data.Monoid
.
Ap
defines a Num
instance lifted through a given applicative
functor. That is (+)
is implemented as liftA2 (+)
, negate
as fmap negate
, and so on. We will give our wrapper the name
Squared
to be able to refer to it later.
data Point a = Point { x :: a, y :: a }
deriving (Eq, Show, Functor)
deriving Num
via (Squared a `As` Point a)
deriving (Applicative, Monad)
via (Squared `As1` Point)
type Squared = Ap ((>) Bool)
The compiler will ask us to fill in the missing isomorphism, which is straightforward.
instance Isomorphic (Squared a) (Point a) where
prj (Point x y) = Ap $ \p > if p then x else y
inj (Ap f) = Point (f True) (f False)
We still need to convince ourselves that we have provided a valid isomorphism. Fortunately, this is usually much simpler than verifying all the instance laws.
These instances behave in the expected way. Applicative
works pointwise on the
elements and Num
is lifted through the Applicative
.
>>> pure 2 :: Point Double
Point {x = 2, y = 2}
>>> Point 1 2 + Point 10 100
Point {x = 11, y = 102}
We can even do scalar multiplication:
>>> Point 1 2 * 2
Point {x = 2, y = 4}
Note how we were able to derive several instances for the price of just one isomorphism.
Counting elements
Let’s say we want a monoid describing whether a collection is empty or not.
data NoneOrMore
= None
 ^ No elements
 OneOrMore
 ^ At least one element
This monoid is akin to Any, mapping None
to False
and OneOrMore
to True
.
data NoneOrMore = None  OneOrMore
deriving (Semigroup, Monoid)
via (Any `As` NoneOrMore)
instance Isomorphic Any NoneOrMore where
inj (Any False) = None
inj (Any True) = OneOrMore
prj None = Any False
prj OneOrMore = Any True
The default element is None
and composition behaves like disjunction.
>>> mempty :: NoneOrMore
None
>>> (None, []) <> (OneOrMore, [1,2])
(OneOrMore, [1,2])
>>> mconcat [None, None]
None
>>> mconcat [None, OneOrMore, None]
OneOrMore
There is an isomorphism not
between Any
and All
that maintains
monoid structure, so we could also have defined NoneOrMore
in terms
of All
.
Monads from transformers
The These
type can be used to represent either or both of two values (but not neither).
data These a b = This a  That b  These a b
deriving (Functor)
The these
package defines an
instance Semigroup a => Monad (These a)
. To develop an understanding for this
instance, consider the isomorphism:
These a b
= Either a (b, Maybe a)
= WriterT (Maybe a) (Either a) b
The Monad
instance for These a
is indeed the same as that of
WriterT (Maybe a) (Either a)
. Therefore we can isoderive Monad (These a)
.
data These a b = This a  That b  These a b
deriving stock (Functor)
deriving (Applicative, Monad)
via (TheseMonad a `As1` These a)
type TheseMonad a = WriterT (Maybe a) (Either a)
instance Isomorphic (TheseMonad a b) (These a b) where
prj (This a) = WriterT (Left a)
prj (That b) = WriterT (Right (b, Nothing))
prj (These a b) = WriterT (Right (b, Just a))
inj (WriterT (Left a)) = This a
inj (WriterT (Right (b, Nothing))) = That b
inj (WriterT (Right (b, Just a))) = These a b
A nice thing about this is that the implementation details involving transformers are hidden away. Users of our type will see:
data These a b = This a  These a b  That b
instance Functor (These a)
instance Semigroup a => Applicative (These a)
instance Semigroup a => Monad (These a)
And it works in the expected way:
>>> pure 2 :: These () Integer
That 2
>>> do { a < These "warning!" 1; b < That 2; pure (a + b) }
These "warning!" 3
>>> do { a < This "error!"; b < That 2; pure (a + b) }
This "error!"
Performance
Isoderiving inserts inj
and prj
in the definition of type class
methods. The performance of isoderived instances depends on the
performance of inj
and prj
. Sometimes, GHC will optimise inj
and prj
away, but you shouldn’t count on it. If performance is an issue,
you want your instances to be handwritten instead.
Conclusion
Isoderiving is a lightweight technique for deriving lawful instances of any type class with minimal effort. It allows very quick exploration of the space of potential instances, especially when performance is not critical. Please try it out for yourself and let me know what use cases you find for isoderiving!

Certain type classes don’t need both sides of the bijection. For example
↩Show
,Eq
andOrd
could work with just the projection, whileRead
works with just the injection. The isoderiving package defines superclasses capturing this.