Build Status Hackage

Generically derive traversals, lenses and prisms.

Available on Hackage

This library uses GHC.Generics to derive efficient optics (traversals, lenses and prisms) for algebraic data types in a type-directed way, with a focus on good type inference and error messages when possible.

The derived optics use the so-called van Laarhoven representation, thus are fully interoperable with the combinators found in mainstream lens libraries.

Examples can be found in the examples and tests folders.

Table of contents


A typical module using generic-lens will usually have the following extensions turned on:

{-# LANGUAGE AllowAmbiguousTypes       #-}
{-# LANGUAGE DataKinds                 #-}
{-# LANGUAGE DeriveGeneric             #-}
{-# LANGUAGE DuplicateRecordFields     #-}
{-# LANGUAGE FlexibleContexts          #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE TypeApplications          #-}

Taxonomy of optics

Here is a comprehensive list of the optics exposed by generic-lens. The combinators each allow a different way of identifying certain parts of algebraic data types.


A lens identifies exactly one part of a product type, and allows querying and updating it.

By name

data Person = Person { name :: String, age :: Int } deriving (Generic, Show)

sally :: Person
sally = Person "Sally" 25

Record fields can be accessed by their label using the field lens.

>>> sally ^. field @"name"

>>> sally & field @"name" .~ "Tamas"
Person {name = "Tamas", age = 25}

Here we use visible type application to specify which field we're interested in, and use the ^. and .~ combinators from a lens library (lens, microlens, etc.) to query and update the field.

Or for standalone use, the getField and setField functions can be used instead.

>>> getField @"age" sally

>>> setField @"age" 26 sally
Person {name = "Sally", age = 26}

When a non-existent field is requested, the library generates a helpful type error:

>>> sally ^. field @"pet"
  • The type Person does not contain a field named 'pet'

For types with multiple constructors, we can still use field as long as all constructors contain the required field

data Two
 = First  { wurble :: String, banana :: Int }
 | Second { wurble :: String }
 deriving (Generic, Show)

>>> Second "woops" ^. field @"wurble"
>>> Second "woops" ^. field @"banana"
    • Not all constructors of the type Two
       contain a field named 'banana'.
      The offending constructors are:
      • Second

The type of field is

field :: HasField name s t a b => Lens s t a b

Therefore it allows polymorphic (type-changing) updates, when the accessed field mentions type parameters.

data Foo f a = Foo
  { foo :: f a
  } deriving (Generic, Show)

foo1 :: Foo Maybe Int
foo1 = Foo (Just 10)

-- |
-- >>> foo2
-- Foo {foo = ["10"]}
foo2 :: Foo [] String
foo2 = foo1 & field @"foo" %~ (maybeToList . fmap show)

This example shows that higher-kinded parameters can also be changed (Maybe -> []). We turn a Foo Maybe Int into a Foo [] String by turning the inner Maybe Int into a [String].

With DuplicateRecordFields, multiple data types can share the same field name, and the field lens works in this case too. No more field name prefixing!

By position

Fields can be accessed by their position in the data structure (index starting at 1):

data Point = Point Int Int Int deriving (Generic, Show)
data Polygon = Polygon Point Point Point deriving (Generic, Show)

polygon :: Polygon
polygon = Polygon (Point 1 5 3) (Point 2 4 2) (Point 5 7 (-2))
>>> polygon ^. position @1 . position @2

>>> polygon & position @3 . position @2 %~ (+10)
Polygon (Point 1 5 3) (Point 2 4 2) (Point 5 17 (-2))

>>> polygon ^. position @10
  • The type Polygon does not contain a field at position 10

Since tuples are an instance of Generic, the positional lens readily works:

>>> (("hello", True), 5) ^. position @1 . position @2
>>> (("hello", True, "or"), 5, "even", "longer") ^. position @1 . position @2

By type

Fields can be accessed by their type in the data structure, assuming that this type is unique:

>>> sally ^. typed @String

>>> setTyped @Int sally 26
Person {name = "Sally", age = 26}

By structure

The super lens generalises the field lens to focus on a collection rather than a single field. We can say that a record is a (structural) `subtype' of another, if its fields are a superset of those of the other.

data Human = Human
  { name    :: String
  , age     :: Int
  , address :: String
  } deriving (Generic, Show)

data Animal = Animal
  { name    :: String
  , age     :: Int
  } deriving (Generic, Show)

human :: Human
human = Human {name = "Tunyasz", age = 50, address = "London"}
>>> human ^. super @Animal
Animal {name = "Tunyasz", age = 50}

>>> upcast human :: Animal
Animal {name = "Tunyasz", age = 50}

We can apply a function that operates on a supertype to the larger (subtype) structure, by focusing on the supertype first:

growUp :: Animal -> Animal
growUp (Animal name age) = Animal name (age + 50)

>>> human & super @Animal %~ growUp
Human {name = "Tunyasz", age = 60, address = "London"}


Traversals allow multiple values to be queried or updated at the same time.

As a running example, consider a data type of weighted trees. There are two type parameters, which correspond to the type of elements and weights in the tree:

data WTree a w
  = Leaf a
  | Fork (WTree a w) (WTree a w)
  | WithWeight (WTree a w) w
  deriving (Generic, Show)

mytree :: WTree Int Int
mytree = Fork (WithWeight (Leaf 42) 1)
              (WithWeight (Fork (Leaf 88) (Leaf 37)) 2)

By type

Focus on all values of a given type.

types :: HasTypes s a => Traversal' s a
>>> toListOf (types @Int) myTree

Note that this traversal is "deep": the subtrees are recursively traversed.

By parameter

As the above example shows, the types traversal is limited in that it cannot distinguish between the two types of Ints: the weights and the values.

Instead, the param traversal allows specifying types that correspond to a certain type parameter.

param :: HasParam pos s t a b => Traversal s t a b
>>> toListOf (param @1) myTree

Here, the numbering starts from 0, with 0 being the rightmost parameter. Because param is guaranteed to focus on parametric values, it allows the type to be changed as well.

For example, we can implement Functor for WTree as simply as:

instance Functor (WTree a) where
  fmap = over (param @0)

By constraint

The most general type of traversal: we can apply a given function to every value in a structure, by requiring that all values have an instance for some type class.

constraints   :: HasConstraints c s t => Applicative g => (forall a b . c a b => a -> g b) -> s -> g t
constraints'  :: HasConstraints' c s  => Applicative g => (forall a . c a => a -> g a) -> s -> g s

Consider the Numbers type, which contains three different numeric types:

data Numbers = Numbers Int Float Double
  deriving (Show, Generic)

numbers = Numbers 10 20.0 30.0

With constraints', we can uniformly add 20 to each number in one go:

>>> constraints' @Num (\x -> pure (x + 20)) numbers
Numbers 30 40.0 50.0


A prism focuses on one part of a sum type (which might not be present). Other than querying the type, they can be "turned around" to inject the data into the sum (like a constructor).

By name

Constructor components can be accessed using the constructor's name:

type Name = String
type Age  = Int

data Dog = MkDog { name :: Name, age :: Age } deriving (Generic, Show)
data Animal = Dog Dog | Cat Name Age | Duck Age deriving (Generic, Show)

shep = Dog (MkDog "Shep" 4)
mog = Cat "Mog" 5
donald = Duck 4
>>> shep ^? _Ctor @"Dog"
Just (MkDog {name = "Shep", age = 4})

>>> shep ^? _Ctor @"Cat"

Constructors with multiple fields can be focused as a tuple

>>> mog ^? _Ctor @"Cat"
Just ("Mog",5)

>>> _Ctor @"Cat" # ("Garfield", 6) :: Animal
Cat "Garfield" 6

By type

Constructor components can be accessed using the component's type, assuming that this type is unique:

type Name = String
type Age  = Int

data Dog = MkDog { name :: Name, age :: Age } deriving (Generic, Show)
data Animal = Dog Dog | Cat Name Age | Duck Age deriving (Generic, Show)

shep = Dog (MkDog "Shep" 4)
mog = Cat "Mog" 5
donald = Duck 4
>>> mog ^? _Typed @Dog

>>> shep ^? _Typed @Dog
Just (MkDog {name = "Shep", age = 4})

>>> donald ^? _Typed @Age
Just 4

>>> mog ^? _Typed @(Name, Age)
("Mog", 5)

>>> donald ^? _Typed @Float
  • The type Animal does not contain a constructor whose field is of type Float

>>> _Typed @Age # 6 :: Animal
Duck 6


The runtime characteristics of the derived optics is in most cases identical at -O1, in some cases only slightly slower than the hand-written version. This is thanks to GHC's optimiser eliminating the generic overhead.

The inspection-testing library is used to ensure (see here) that everything gets inlined away.

TODO push benchmarks too # Contributors


  • Remove dump-core dependency
  • Relax upper bound on criterion (#42)

  • Traversals (types, param, constraints)
  • Prisms are now optimal too
  • Monomorphic versions of lenses and prisms also included

Breaking API changes

  • projectSub now returns Maybe sub instead of Either sup sub (#21)

  • Infer input type from result type (#25)
  • Allow changing of multiple type parameters (#24)
  • Allow changing of type parameters that have kinds other than * (#23)
  • Fix error message in subtype lens

  • Lenses and prisms are now type-changing.
  • More informative error messages
  • More readable type signatures in type errors and when using :t
  • Use doctest
  • Include examples in Haddock

Breaking API changes

  • The type parameters of the classes have been changed to accommodate the type-changing update:

    class HasField name a s -> class HasField name s t a b etc.

    Accordingly, field :: Lens' s a -> field :: Lens s t a b

Depends on 3 packages:
comments powered byDisqus