BSD-3-Clause licensed by Edward A. Kmett
Maintained by Edward A. Kmett
This version can be pinned in stack with:lens-4.14@sha256:3e28b6a161f01b027690571adf8f169c3e22ebb65fa742708c9b0d5c46977f9f,14503

Module documentation for 4.14

Lens: Lenses, Folds, and Traversals

Hackage Build Status

This package provides families of lenses, isomorphisms, folds, traversals, getters and setters.

If you are looking for where to get started, a crash course video on how lens was constructed and how to use the basics is available on youtube. It is best watched in high definition to see the slides, but the slides are also available if you want to use them to follow along.

The FAQ, which provides links to a large number of different resources for learning about lenses and an overview of the derivation of these types can be found on the Lens Wiki along with a brief overview and some examples.

Documentation is available through github (for HEAD) or hackage for the current and preceding releases.

Field Guide

Lens Hierarchy


(See wiki/Examples)

First, import Control.Lens.

ghci> import Control.Lens

Now, you can read from lenses

ghci> ("hello","world")^._2

and you can write to lenses.

ghci> set _2 42 ("hello","world")

Composing lenses for reading (or writing) goes in the order an imperative programmer would expect, and just uses (.) from the Prelude.

ghci> ("hello",("world","!!!"))^._2._1
ghci> set (_2._1) 42 ("hello",("world","!!!"))

You can make a Getter out of a pure function with to.

ghci> "hello"^.to length

You can easily compose a Getter with a Lens just using (.). No explicit coercion is necessary.

ghci> ("hello",("world","!!!"))^ length

As we saw above, you can write to lenses and these writes can change the type of the container. (.~) is an infix alias for set.

ghci> _1 .~ "hello" $ ((),"world")

Conversely view, can be used as a prefix alias for (^.).

ghci> view _2 (10,20)

There are a large number of other lens variants provided by the library, in particular a Traversal generalizes traverse from Data.Traversable.

We’ll come back to those later, but continuing with just lenses:

You can let the library automatically derive lenses for fields of your data type

data Foo a = Foo { _bar :: Int, _baz :: Int, _quux :: a }
makeLenses ''Foo

This will automatically generate the following lenses:

bar, baz :: Lens' (Foo a) Int
quux :: Lens (Foo a) (Foo b) a b

A Lens takes 4 parameters because it can change the types of the whole when you change the type of the part.

Often you won’t need this flexibility, a Lens' takes 2 parameters, and can be used directly as a Lens.

You can also write to setters that target multiple parts of a structure, or their composition with other lenses or setters. The canonical example of a setter is ‘mapped’:

mapped :: Functor f => Setter (f a) (f b) a b

over is then analogous to fmap, but parameterized on the Setter.

ghci> fmap succ [1,2,3]
ghci> over mapped succ [1,2,3]

The benefit is that you can use any Lens as a Setter, and the composition of setters with other setters or lenses using (.) yields a Setter.

ghci> over (mapped._2) succ [(1,2),(3,4)]

(%~) is an infix alias for ‘over’, and the precedence lets you avoid swimming in parentheses:

ghci> _1.mapped._2.mapped %~ succ $ ([(42, "hello")],"world")
([(42, "ifmmp")],"world")

There are a number of combinators that resemble the +=, *=, etc. operators from C/C++ for working with the monad transformers.

There are +~, *~, etc. analogues to those combinators that work functionally, returning the modified version of the structure.

ghci> both *~ 2 $ (1,2)

There are combinators for manipulating the current state in a state monad as well

fresh :: MonadState Int m => m Int
fresh = id <+= 1

Anything you know how to do with a Foldable container, you can do with a Fold

ghci> :m + Data.Char Data.Text.Lens
ghci> allOf (folded.text) isLower ["hello"^.packed, "goodbye"^.packed]

You can also use this for generic programming. Combinators are included that are based on Neil Mitchell’s uniplate, but which have been generalized to work on or as lenses, folds, and traversals.

ghci> :m + Data.Data.Lens
ghci> anyOf biplate (=="world") ("hello",(),[(2::Int,"world")])

As alluded to above, anything you know how to do with a Traversable you can do with a Traversal.

ghci> mapMOf (traverse._2) (\xs -> length xs <$ putStrLn xs) [(42,"hello"),(56,"world")]

Moreover, many of the lenses supplied are actually isomorphisms, that means you can use them directly as a lens or getter:

ghci> let hello = "hello"^.packed
ghci> :t hello
hello :: Text

but you can also flip them around and use them as a lens the other way with from!

ghci> hello^.from length

You can automatically derive isomorphisms for your own newtypes with makePrisms. e.g.

newtype Neither a b = Neither { _nor :: Either a b } deriving (Show)
makePrisms ''Neither

will automatically derive

neither :: Iso (Neither a b) (Neither c d) (Either a b) (Either c d)
nor :: Iso (Either a b) (Either c d) (Neither a b) (Neither c d)

such that

from neither = nor
from nor = neither
neither.nor = id
nor.neither = id

There is also a fully operational, but simple game of Pong in the examples/ folder.

There are also a couple of hundred examples distributed throughout the haddock documentation.

Contact Information

Contributions and bug reports are welcome!

Please feel free to contact me through github or on the #haskell IRC channel on

-Edward Kmett



  • Remove Cons and Snoc instances for NonEmpty.

  • Fixed itraverse_ and imapM_ returning bottom


  • Restore default signature for
  • Improve operations for Data.Sequence.Seq
  • Fix declarePrisms behavior on GHC 8 using GADT record syntax


  • Modified to enable the doctests to build with stack.
  • Removed .ghci.
  • Added lookupOf
  • Support GHC 8
  • Support transformers 0.5
  • Support kan-extensions 5
  • Support comonad 5
  • Better support for Closed from profunctors.


  • Pattern synonyms
  • Moved foldMapBy and foldBy into reflection 2.1
  • Added traverseByOf, sequenceByOf.
  • Reexported traverseBy and sequenceBy from reflection 2.1.
  • Modified the signatures of alaf and auf to work with a Functor rather than a Profunctor and rather drastically generalized them.
  • Removed Control.Lens.Internal.Getter.coerce in favor of the upstream phantom combinator in contravariant 1.3+
  • Renamed coerced to phantasm to get it out of the way.
  • Added Wrapped instance for Down


  • Move Review and AReview to Control.Lens.Type fixing a bug in makePrisms
  • Expose HasTypes class in Language.Haskell.TH.Lens
  • Make types of foldByOf and foldMapByOf more specific to hide implementation details
  • Add Prisms to Language.Haskell.TH for new constructors in template-haskell-2.10
  • Generalize type of _FunDep to an Iso


  • Incorporated a bug fix for foldByOf and foldMapByOf to actually let them work on folds.
  • Added a Plated instance for CofreeT


  • The Simple type alias is now poly-kinded. This lets you use Simple Field1 s a and the like in constraints.
  • Added HasTypes to Language.Haskell.TH.Lens.
  • Support for vector-0.11.0 which changes Stream to Bundle


  • reflection 2 support.


  • Give cosmosOn a more general type.


  • Added cosmos, cosmosOf, cosmosOn, cosmosOnOf to Control.Lens.Plated.
  • Added icontains, iat, iix.
  • Made various documentation improvements.
  • Added a test-templates flag.


  • Proper profunctors 5.1 support. This extended the superclass constraints for Conjoined, so it resulted in a major version bump.


  • Added elemIndexOf, elemIndicesOf, findIndexOf, and findIndicesOf.
  • Fixed Ixed instance for Tree. It no longer drops nodes prior to the traversed node.
  • bifunctors 5, profunctors 5 and semigroupoids 5 support.


  • Added _Wrapped support for NonEmpty.
  • Added _Wrapped support for Alt.
  • Fixed Rewrapped instance for Last.


  • filepath 1.4 support
  • Removed Control.Monad.Primitive.Lens and shed the primitive dependency.
  • Add missing _WithIndex instances from keys package
  • Much more code is inferred Safe rather than Trustworthy.
  • Documented the difference between unsafeSingular and singular.
  • folding now produces an actual Fold.
  • Cleaned up builds for GHC 7.10 to get rid of redundant import warnings.


  • When built with profunctors 4.4 on GHC 7.8+ we no longer need to use unsafeCoerce at all! This drastically reduces the level of trust involved in the way we have optimized lens.
  • Added fusing. This optimizes long Lens chains, by enfocing a form of fmap fusion based on the Yoneda lemma. This is particularly effective at making faster lenses the definition is recursive or complex enough that it cannot be inlined.
  • Added confusing. This optimizes long Traversal chains. As with fusing it is best used when the definition for the Traversal chain in question is recursive or complex enough that it cannot be inlined, but the implementation is much more confusing.
  • Remove deprecated stuff: Control.Lens.Loupe, headOf, makeFieldsWith, strippingPrefix, strippingSuffix
  • Added Cons and Snoc instances for NonEmpty
  • Removed Data.List.Split.Lens module
  • Reimplemented bytestring traversals to avoid internal modules
  • Added gplate, an implementation of plate for any type implementing Generic
  • Strictness revisited
    • Add generateLazyPatterns configuration flag to makeLenses rules.
    • Make the default makeLenses behavior to generate STRICT optics
    • Add strict variants of _1 .. _9 named _1' .. _9'
  • Generalized some combinators in Data.Vector.Generic.Lens and added converted


  • Migrated Control.Lens.Action to lens-action.
  • Added Data.Vector.Generic.Lens.vectorIx function for indexing vectors with only Vector constraint.
  • Added Field1 and Field2 instances for Data.Functor.Product.Product.
  • Removed the “typeclass synonym” Gettable.
  • Added new flag to makeLenses, generateUpdateableOptics, which allows the generation of only Getters and Folds. This feature is intended to be used when the constructors are hidden behind validating, “smart” constructors.
  • Fixed Template Haskell name generation when using GHC 7.10
  • Fixed Template Haskell generation of classes methods where field types used existential quantification [maintenance release]

  • Compatibility with base 4.8 [Edit: this turned out to not work for the final release of GHC 7.10]


  • Reduced Review to two arguments, like Getter.
  • Added abbreviatedFields to permit makeFieldsWith to be invoked with an argument that lets it act like it did pre-4.5 and accept arbitrary common prefixes.


  • Provide access to the typename in lensRules naming function.
  • makeFields camelcasing rules now properly support types with camelcasing. MyType with field myTypeFieldA generates fieldA now. Previously the prefix ignore capitalization and the field would need to be named mytypeFieldA.
  • makeClassy works on types even when none of the fields would generate optics.
  • Added Monad, MonadReader, MonadPlus and Bind instances for ReifiedMonadicFold
  • Added missing fixity declarations on many operators.
  • Migrated Codec.Compression.Zlib.Lens to zlib-lens package.

  • text support
  • Remove the use of the TemplateHaskell extension from the library to enable lens to be used on stage1 cross-compilers

  • Restore previous default of makeFields using the camel case field namer.


  • Internals of Template Haskell code generation rewritten. makeLenses, makeClassy, and makeFields have been unified into the same generator.
  • TH generated single constructor Lens use irrefutable pattern matching to enable construction starting with undefined.
  • TH generated traverals unify their field arguments (type synonyms not currently expanded) enabling exotic traversals to be generated.
  • Added instances for Text to Data.Aeson.Lens
  • Reimplemented makePrisms, adding support for makeClassyPrisms, infix constructrs generate periods (.) prefixed prisms.
  • Added Choice to Review so that Prism is a proper subtype of Review
  • Migrated Data.Aeson.Lens to lens-aeson package.
  • Fixed GHC.Generics.Lens.tinplate behavior on single-field data types and empty data types.


  • semigroupoids 4.2 support


  • contravariant 1.0 support


  • Added bytewise to Data.Bits


  • Switched the “direction” of the Iso argument to au to match the order generated by makePrisms and makeLenses.
  • Removed makeIsos in favor of makePrisms and makeLenses. Each of these functions will construct Isos when appropriate.
  • Removed declareIsos in favor of declarePrisms and declareLenses. Each of these functions will construct Isos when appropriate.
  • Added matching for type-changing matches with Prisms.
  • Added withPrism for recovering the functions passed to prism.
  • Added negated, the isomorphism for the negate function.


  • Added _Text isomorphisms to make the proper use with (#) more obvious and fit newer convention.
  • Added Wrapped instances for Vector types
  • Resolved issue #439. The various Prisms for string-like types in Data.Aeson.Lens are now law-abiding Prisms “up to quotient.”
  • Added selfIndex.
  • Support attoparsec 0.12.


  • When used with exceptions 0.4, throwingM will permit use with a mere MonadThrow.


  • Generalized the types of mapping, bimapping, contramapping, dimapping, lmapping, rmapping to support changing the Functor, Bifunctor, Contravariant, and Profunctor respectively.
  • Compatibility with free 4.6


  • Added Plated instances for various free monad variants.
  • Compatibility with GHC HEAD (7.9+)


  • Removed dependency on constraints. It was used in a pre-release version of 4.0 but never made it into 4.0, but the dependency had remained around complicating builds for GHC 7.4.


  • makeLenses attempt to make the accessors it can under existential quantification.
  • Added (&~).
  • Experimental support for parallel builds on GHC 7.8 with cabal install lens -fj. Due to at last one known issue with GHC, it isn’t recommended to use this option when rebuilding lens, as a race condition on at least one platform has been seen in the wild.
  • Added RoleAnnotations for GHC 7.8.1. These rule out a few user-accessible bottoms that could be caused by creative abuse of the new Coercible machinery. However, there was no unsafeCoerce exposed.
  • Removed some impossible cases that required unwritable instances from the example doctypes.


  • Added bimapping to Control.Lens.Iso
  • Restored correct behavior of makePrism on types with a single constructor.
  • makeLenses now generates Getters and Folds on universally quantified fields.


  • Made declareFields work again.


  • Fixed random segfaulting when using foldMapBy.


  • Properly bundled the modules needed for the properties test suite into the tarball for hackage.


  • Typo fixes
  • Exporting Rewrapping from Control.Lens.Wrapped.
  • Removed the dependency on cpphs.


  • Added nearly to Control.Lens.Prism.
  • Added Control.Lens.Empty, exporting _Empty.
  • We now require DefaultSignatures.
  • Added failing and ifailing to Control.Lens.Traversal.
  • Changed the signature of Data.List.Split.Lens.condensing due to the addition of DropBlankFields to Data.List.Split.CondensePolicy in split.
  • Simplified Each, Ixed, and Contains. They are no longer indexed. The previous design was actively getting in the way of user-defined instances.
  • Replaced more of our home-grown types with standard ones. They had previously been defined to help make more intelligible error messages, but when we switched to using (Contravariant f, Functor f) instead of (Gettable f), these ceased to really help. Now you can define even more lens-compatible types (e.g. Getter and Fold) without depending on lens.
    • Replaced the use of Accessor with Const.
    • Replaced the use of Mutator with Identity.
    • Replaced the use of Reviewed with Tagged.
  • Removed the deprecated Control.Lens.Simple module.
  • Repurposed Control.Lens.Combinators to re-export Control.Lens sans any operators; previous residents rehomed to Control.Lens.Lens.
  • Added Control.Lens.Operators to export just the operators. Varying your import styles between these supports many qualified usage scenarios.
  • Simplified Cons and Snoc. Now they must be a Prism.
  • Simplified Contains. This necessitated losing many instancs of Contains, but makes it much easier and more consistent to use and instantiate.
  • Simplified the various AsFoo types in Control.Exception.Lens
  • Simplified the types in System.IO.Error.Lens.
  • Merged lens-aeson into lens.
  • We’re exiling Control.Lens.Zipper to a separate package. This will let the design for it iterate faster and let us explore the trade-offs between the 3.8 style and the 3.9 style of zippers.
  • Generalized alongside, inside, both.
  • Switched to a new Typeable version of reflection for the harder combinators in Control.Exception.Lens. This enables us to comply with GHC 7.7’s ban on hand-written Typeable instances.
  • Added a _Show Prism.
  • Added Control.Lens.Extras for the combinator names we don’t have the gall to claim outright, but which are consistent with the rest.
  • Renamed the constructors for ReifiedLens, etc. to just be the name of their base type.
  • Added many many missing instances for ReifiedFold and ReifiedGetter. This permits things like runFold ((,) <$> Fold (traverse._1) <*> Fold (traverse._2)) to be a Fold and ReifiedFold can be used as a Monad, Profunctor, etc.
  • Many performance optimizations.
  • Switched to exceptions from MonadCatchIO-transformers
  • Added types for working with RelevantFold and RelevantTraversal. These are a Fold or Traversal that always has at least one target. Since Apply isn’t a superclass of Applicative, you occasionally need to convert between them, but it lets you more readily work with less unsafety.
  • Changed unwrapping and wrapping to have the same constructor-oriented order as a Prism and renamed them t _Wrapping and _Unwrapping respectively.
  • Drastically changed the way _Wrapping and _Unwrapping are built to get much better inference.
  • There are about 15,000 lines of patches over the last year, so I’m sure we missed a few big changes.

3.10.1 [maintenance release]

  • Compatibility with base 4.7 [maintenance release]

  • Compatibility with text 1.0


  • Switched to bifunctors, comonad, profunctors, and semigroupoids 4.0.


  • Generalized signatures for throwing and throwingM.


  • ‘condensingPolicy’ was updated to work with ‘split’ 0.2.2

  • Bumped dependency on generic-deriving again.

  • Bumped dependency on generic-deriving to enable building on GHC HEAD.

  • Updated the field guide image to link to imgur. Sadly the overview haddock and the haddocks are not generated in the same directory, so the haddock hook for copying the image only works locally.


  • Changed Getting to take 3 arguments instead of 5. If you need the old behavior for portability you can use Overloaded (Accessor r) s t a b instead of Getting r s t a b and it’ll work consistently back through the last few releases.
  • Added involuted to Control.Lens.Iso.
  • Factored out a common reversed definition from all the various forms of it around the library and placed it in Control.Lens.Iso.
  • Added binary, octal, decimal and hex to Numeric.Lens.
  • Added sans to Control.Lens.At.
  • Improved interoperability:
    • Reimplemented Gettable as an alias for Contravariant and Functor together to derive Getter and Fold. This means you can now implement a Getter or Fold with only a Haskell 98 dependency (contravariant).
    • Removed Reviewable. We now use Bifunctor and Profunctor together to derive Review. This means you can now implement a Review with Haskell 98 dependencies (profunctors and bifunctors).
    • These changes enables more types to be defined without incurring a dependency on the lens package. [maintenance releases]

  • Fixes to dependencies and pragmas.

3.8.6 [maintenance release]

  • Fixed an issue with DefaultSignatures being used outside of the appropriate #ifdef that caused compilation issues on GHC 7.0.2.
  • Generalized the signature of prism'
  • Added \_Void and only to Control.Lens.Prism and devoid to Control.Lens.Lens.
  • Added \_Nothing to Control.Lens.Prism.
  • Added devoid and united to Control.Lens.Lens.


  • Fixed more sporadic issues in doctests, caused by carrying flags from $setup between modules.


  • Renamed strippingPrefix to prefixed, strippingSuffix to suffixed. Left the old names as deprecated aliases.
  • Fixed issues with the test suite caused by doctests carrying flags from the $setup block between modules.
  • Benchmarks now use generic-deriving rather than ghc-prim directly, like the rest of the package.
  • Added Generics.Deriving.Lens, which is now simply re-exported from GHC.Generics.Lens.


  • Added strippingSuffix and stripSuffix to Data.Data.Lens
  • Added unpackedBytes and unpackedChars to Data.ByteString.*.Lens
  • Added unpacked to Data.Text.*.Lens
  • Added (#) as an infix form of review to ease using a Prism like a smart constructor in Control.Lens.Review.


  • Added a notion of Handleable(handler, handler_) to Control.Exception.Lens to facilitate constructing a Handler from an arbitrary Fold or Prism.
  • Added a notion of Handler and catches to and Control.Monad.Error.Lens to mirror the Control.Exception and Control.Monad.CatchIO constructions.
  • Added additional doctests and documentation.
  • Improved error messages and support for types with arguments in makeFields.


  • Fixed a bug in makeFields in hierarchical modules.

  • Fixed an issue with running the doctests test suite when an older version of semigroups is installed.


  • Overall:
    • Replaced each of the different SimpleFoo type aliases with Foo' throughout. The various Simple aliases can still be found in Control.Lens.Simple but are now deprecated.
    • Made sweeping changes to Iso and Prism and Indexed lenses internally. They are now based on profunctors. This affects how you use indexed in the resulting code and dramatically changed the meaning of Overloaded.
    • Generalized combinators to pass through indices unmodified wherever possible and added indexed variants to existing combinators. There are hundreds of these changes and they would swamp this list.
  • Control.Exception.Lens
    • This module was created to add combinators and prisms that make it possible to work with GHC’s extensible exceptions and monad transformer stacks more easily. There are knock-on changes in Data.Dynamic.Lens, System.Exit.Lens, and System.IO.Error.Lens.
  • Control.Lens.At
    • Moved At(at) and Contains(contains) and factored out Ixed(ix).
    • Deprecated _at and resultAt.
    • Removed various ordinal and ix combinators, which are subsumed by Ixed(ix).
  • Control.Lens.Cons
    • Consoldiated the various _head, _tail, _init and _last traversals that were scattered around the place into a pair of Cons and Snoc classes that provide _Cons and _Snoc prisms respectively, and combinators that build on top.
  • Control.Lens.Each
    • Generalized the signature of Each to permit it to provide an IndexedSetter for ((->) e).
    • Each now uses an Index type family that is shared with At, Ixed and Contains to indicate these operations are related.
  • Control.Lens.Equality
    • Added as a stronger form of Iso that can be used to safely cast.
    • Added the adverb simply, which can be used to simplify the types of most combinators in the library so they only take a simple lens, simple traversal, etc as their first argument instead. e.g. simply view forces a ~ b, s ~ t in the argument to view.
  • Control.Lens.Fold
    • Added foldr1Of' and foldl1Of'.
    • Added has and hasn't.
  • Control.Lens.Indexed
    • The various indexed combinators for each type were distributed to their respective modules. This module grew to encompass the remaining index-specifics.
    • Added index and indices, and removed iwhere and iwhereOf. Use itraversed.indices even and bar.indices (>3) instead.
  • Control.Lens.Internal
    • This module was exploded into more manageable component modules.
  • Control.Lens.Iso
    • Strict(strict) is now a Simple Iso.
    • Added magma and imagma which can be used to provide a ‘debugging view’ of a Traversal.
  • Control.Lens.Lens
    • Restructuring split this module out from Control.Lens.Type and merged the contents Control.Lens.IndexedLens.
  • Control.Lens.Level
    • This module was created to provide the breadth-first-search Traversals levels and ilevels which can be used to do (optionally depth-limited) breadth-first searches through arbitrary traversals reaching all leaves at finite depth in finite time. To use these in full accordance with the laws you should restrict yourself to commutative operations and finite containers, but they are useful even in the absence of these properties.
  • Control.Lens.Loupe
    • In the interest of consistency, the Loupe alias has been deprecated in favor of ALens.
    • Loupe (and ALens) are now defined in terms of Pretext rather than Context. This permits them to be cloned at a reduced cost reducing the call for ReifiedLens.
  • Control.Lens.Operators
    • Added this module for users who insist on qualified use, but want access to the operators. They can import qualified Control.Lens as Lens and import Control.Lens.Operators unqualified.
  • Control.Lens.Prism
    • Added prism' to construct SimplePrisms.
  • Control.Lens.Reified
    • Consolidated the various ReifiedFoo definitions into one module.
  • Control.Lens.Representable
    • This module was removed. Its functionality may be split out into a separate package, but currently the linear package exports is own Linear.Core module to provide this functionality. It was taking lots of useful names for little functionality and didn’t feel like the rest of the API.
  • Control.Lens.Review
    • This module now factors the review functionality out of Prism and exposes unto, which is to review what to is to view.
  • Control.Lens.Setter
    • Added contramapped and argument for mapping over inputs.
  • Control.Lens.Simple
    • Removed the infix lens aliases and repurposed the module to house the now deprecated SimpleFoo type aliases, which were replaced universally with Foo'.
  • Control.Lens.TH
    • makeLenses now generates Lens' and Traversal' where appropriate
    • Added makePrisms as a generalized makeIso that automatically generates a Prism for each constructor. makePrisms generates names with an _Foo convention. This was consolidated upon throughout the library to reduce namespace conflicts between prisms and lenses.
    • Added makeFields, which generates classes for each individual field in a data type.
    • Added makeWrapped, which automatically generates a Wrapped instance for a newtype.
  • Control.Lens.Type
    • This module was repurposed to provide a single home for all the standard lens-like type aliases used when producing lenses. You still need to go to their respective modules to find the types for consuming lens-likes if you want to generate your own lens combinators
  • Control.Lens.Wrapped
    • Added wrapped' and unwrapped' for scenarios where you need the help with type inference.
  • Control.Lens.Zipper
    • Converted Zipper to walk a magma based on the original structure and to use indices from indexed traversals when restoring from tape. This also means that when zipping around within a balanced structure with ascending keys moveTo can operate in logarithmic time, but required changing the Zipper type to add the index type.
  • Data.Bits.Lens
    • Added byteAt.
  • Data.ByteString.Lens
    • Data.ByteString.Lazy.Lens now uses Int64-based indexing.
    • The Traversal for strict ByteStrings now construct a balanced tree up to a given grain size. This permits zipper based seeking to operate in logarithmic time and speeds up many traversals.
  • Numeric.Lens
    • Created. base shows and reads integers at base-2 through base-36. integral can be used as a safe fromInteger/toInteger.

3.7.6 [maintenance release]

  • Fixed an issue with the Complex Each instance.

3.7.5 [maintenance release]

  • Fixed an errant LANGUAGE pragma

3.7.4 [maintenance release]

  • Backported the API for ALens and ALens' to support snap builds on old platforms.

3.7.3 [maintenance release]

  • Removed my intra-package dependency upper bounds for my own packages. In particular this enables us to work with semigroups 0.9.
  • Switched to transformers-compat to avoid having unbuilding modules at the top of the documentation, and to ease 3rd party compatibility.
  • Updated Setup.lhs to be compatible with Cabal 1.17

3.7.2 [maintenance release]

  • Bug fix for Magnify. It was missing functional dependencies to determine its k parameter from m or n. [maintenance release]

  • Made the doctest test suite hide all but the exact versions of packages used to build this package to avoid problems with complicated user environments.
  • Removed doctests based on :t as they are fragile and break across GHC versions.
  • Fixed GHC 7.0.4 compatibility by guarding DefaultSignatures in Control.Lens.Each. [maintenance release]

  • Removed tests that will (likely) fail in the presence of hashable 1.2


  • Added preuse, preuses to Control.Lens.Fold
  • Added Each(each) to Control.Lens.Each for indexed traversal of potentially monomorphic containers.
  • Added indexing64 and traversed64 for help with large containers.
  • Generalized the type signature of choosing.
  • Exported unwrapped from Control.Lens.Wrapped.
  • Support for hashable 1.2
  • Added (??) to Control.Lens.Combinators.

  • Fixed flagging for Safe Haskell.
  • Fixed examples.
  • Cleaned up the statement of the Prism laws.

  • Corrected bounds for hashable.
  • Fixed compatibility with Haskell Platform 2011.4.0.0 – you may have to install with –constraint=“transformers =” to avoid getting new mtl and transformer versions installed.


  • Renamed Projection to Prism.
  • Implemented a complete redesign of the way Iso and Prism are handled internally. Any Iso can now be used as a Prism.
  • The isos combinator is no longer required. iso can now be used to construct an Iso.
  • Changes to the signature of from and under were necessitated by the new design.
  • Added Control.Lens.Wrapped providing a canonical isomorphism for newtypes.
  • Repurposed ala to be closer to the original design in newtype, but added au and alaf.
  • Added _magnitude, _phase and _conjugate to Data.Complex.Lens. Renamed other lenses for consistency: _realPart, _imagPart, _polar.
  • Promoted _left and _right to prisms and moved them to Control.Lens.Prism.
  • Generalized view and views to subsume the old functionality of peruse and peruses.
  • Generalized review and reviews to both return a MonadReader and to work on a Projection.
  • Added view'/views' and use'/uses' for Simple access to the environment/state.
  • Added set', a Simple version of set.
  • Added reuse : use :: review : view and reuses : uses :: reviews : views for working a Projection from the current MonadState.
  • Removed many isomorphisms for various newtypes. _const, identity, _sum, etc. Use wrapping Const, wrapping Identity, etc.
  • Removed Data.Monoid.Lens now that its newtypes are instances of Wrapped, exporting the (<>=)-variants from Control.Lens.*.
  • Renamed via to cloneIso for consistency.
  • Moved Indexed(..) to Control.Lens.Classes.
  • Renamed index to indexed to reduce conflicts with third-party libraries.
  • Added curried and uncurried to Control.Lens.Iso.
  • Added Strict(strict) for ad hoc overloading of conversions between strict and lazy variants of ByteString and Text.
  • Bug fixes for tugTo and jerkTo.
  • These no longer traverse in the wrong direction: scanl1Of, scanr1Of, mapAccumLOf, and mapAccumROf.
  • Added anon to Control.Lens.Iso.
  • Generalized the types of the Control.Lens.Zipper combinators to work with other MonadPlus instances.
  • Added withins to Control.Lens.Zipper now that they can work better with [].
  • Added singular and unsafeSingular to Control.Lens.Traversal to assert a Traversal is a Lens, a Fold is a Getter or a MonadicFold is an Action.
  • Generalized sequenceAOf_’s type to match sequenceA_.
  • Renamed up/down/left/right to upward/downward/leftward/rightward to reduce conflicts – in particular with Control.Arrow.
  • Readded leftmost and rightmost due to the verbosity of farthest leftward/farthest rightward.
  • Added preview/previews/firstOf and deprecated headOf.
  • Added iview/iviews/iuse/iuses to Control.Lens.IndexedGetter.
  • We’ve generalized the type of Bazaar and provided generalized variants of partsOf, etc. that used it. [maintenance release]

  • Added support for test-framework 0.8 [maintenance release]

  • Added support for test-framework 0.7 [maintenance release]

  • Added more explicit dependencies to the doctest suite.
  • Disabled the ‘expected failure’ quickcheck tests that occasionally would fail with internal QuickCheck errors. [maintenance release]

  • Added explicit dependency on containers and unordered-containers to the doctest suite


  • Added upon (along with variants of it) to Data.Data.Lens, which can be used to generate a Traversal from a field accessor or any function that returns, unmodified, a single field that would be visited by template.
  • Added some missing examples/ files to the distribution.
  • Renamed Data.Bits.Lens.traverseBits to bits.
  • Removed (^!?), which was an alias for (^?!).
  • Removed the need for Trustworthy by changing the implementation of coerce for BazaarT.
  • Moved BazaarT to Control.Lens.Internal.
  • Added (<&>) to Control.Lens.Combinators.
  • element and elementOf are now indexed traversals rather than lenses and have moved to Control.Lens.IndexedTraversal. This both fixes their former partiality and lets you use chain indexed combinators with them.
  • Added elements and elementsOf as indexed traversals for ordinal indexing into regular traversals that generalize element and elementOf.
  • Renamed Data.Complex.Lens.traverseComplex to complex.
  • Changed Data.Complex.Lens.polarize to a Simple Iso, due to the RealFloat constraint causing inference problems.
  • Renamed traverseLeft and traverseRight to _left and _right respectively.
  • Renamed traverseSlice, traverseFrom, and traverseTo in Data.Sequence.Lens to sliced, slicedFrom, and slicedTo respectively.
  • Renamed traverseAt to _at in Control.Lens.IndexedTraversal.
  • Renamed traverseArray to _array in Data.Array.Lens.
  • Renamed and made the combinators in Control.Lens.Zipper more compositional to reduce third-party naming conflicts down to just left and right.
  • Renamed &= and |= to .&.= and .|.= for consistency, mutatis mutandis their related operations.
  • Added a Plated instances for Language.Haskell.TH types.
  • Renamed atIndex and atIndices in Data.Vector.Lens and Data.Vector.Generic.Lens to ordinal and ordinals to match Data.Sequence.Lens


  • Improved SafeHaskell inference.


  • Fixed a potential SafeHaskell issue where a user could use undefined to derive unsafeCoerce. You now have to import an explicitly Unsafe module and create an instance of Trustworthy for your type to cause this behavior, so if you do, it’s on your head, not mine. :)
  • Renamed EvilBazaar to BazaarT.
  • Moved a lot of internals around. Most notably, Gettable, Settable and Effective have moved to Control.Lens.Classes.
  • Exposed partsOf' and unsafePartsOf' in Control.Lens.Traversal to reduce reliance on BazaarT in Control.Lens.Zipper


  • Renamed (%) to (&) and (^%) to (^&). This avoids the conflict with Data.Ratio, which was our highest priority conflict with a third party library.
  • Switched to a more liberal type for ignored
  • Removed some “isplitting” bad combinators from Control.Lens.IndexedFold.
  • Made indexed, taking, and dropping and elementOf lazier and capable of dealing with infinite traversals and infinite folds.
  • Improved Indexing to support infinite traversals and folds.
  • Removed some of the more redundant combinators from Control.Lens.Plated, which already had existing aliases in the rest of the traversal API.
  • Moved partsOf, holesOf, and elementOf into Control.Lens.Traversal.
  • Renamed query to peruse and queries to peruses. These are much less contentious names, both contain use in their name for analogy to use and uses and the word is about reading.
  • Simpler simple.
  • Added enum and non to Control.Lens.Iso.
  • Added (^?!) to Control.Lens.Fold for unsafe access to the head of a Fold.
  • Changed _head, _tail, _init and _last to traversals in Data.List.Lens and Data.Sequence.Lens.
  • Eliminated traverseHead, traverseTail, traverseInit and traverseLast.
  • partsOf and unsafePartsOf can now also be applied to a Fold yielding a Getter or to a MonadicFold yielding an Action.


  • Redefined simple and moved it to Control.Lens.Iso. Instead of using simple l you can now compose l.simple or simple.l providing more nuanced control and a more compositional API.
  • Moved the various foo# combinators used to emit cleaner core into an unexported module, Control.Lens.Unsafe. This removes MagicHash from the public API.
  • Removed the bazaar# and runBazaar# coercions that caused issues on GHC HEAD.
  • Changed the default definition of plate to uniplate from ignored.
  • Added Data.Vector.Lens and instances for Data.Vector.
  • Added support for the split package, which is now part of the Haskell platform.
  • Removed redundant Data.List.traverseList. Use itraversed or traverse instead.
  • Moved (:<->) to Control.Lens.Simple.
  • Fixed a bug in Control.Lens.TH that was causing makeIso not to work.
  • Added lifted to Control.Lens.Setter for mapping over monads.
  • Added beside to Control.Lens.Traversal.
  • Removed the operators from Data.List.Lens, they broke the overall pattern of the rest of the API, and were terrible clutter.
  • Fixed a bug that caused resultAt to give wrong answers most of the time.
  • Changed resultAt to an IndexedLens and moved it to Control.Lens.IndexedLens
  • Changed ignored to an IndexedTraversal and moved it to Control.Lens.IndexedTraversal
  • We’ve relinquished the name value.


  • Made elementOf lazier and moved it from Control.Lens.Traversal to Control.Lens.Plated.
  • Made holesOf and partsOf lazier to deal with infinite structures.
  • Resolved issue #75. We now generate nicer core for most Setter and Fold operations, and some others.
  • Made lenses for field access like _1, _2, etc. lazier.
  • Added Control.Lens.Loupe, which provides a limited form of Lens that can be read from and written to and which can compose with other lenses, but can also be returned in a list or as a monadic result, but cannot be used directly for most combinators without cloning it first. It is easier to compose than a ReifiedLens, but slightly slower.
  • Moved (:=>) and (:->) into Control.Lens.Simple, which is not exported by Control.Lens by default to reduce name conflicts with third party libraries.


  • Simplified the type of filtered, so that it can be composed with other folds rather than be parameterized on one. Included the caveat that the new filtered is still not a legal Traversal, despite seeming to compose like one.
  • Renamed ifiltered to ifiltering, and while it still must take an indexed lens-like as an argument, I included a similar caveat about the result not being a legal IndexedLens when given an IndexedLens. The function was renamed because its signature no longer lined up with the new filtered and the gerundive ‘-ing’ suffix has come to indicate an operator that transformers another lens/traversal/etc. into a new one.
  • Added taking and dropping to Control.Lens.Traversal.


  • Alpha-renamed all combinators to a new scheme. Instead of Foo a b c d, they now follow Foo s t a b. This means that you don’t need to alpha rename everything in your head to work through the examples, simplifies exposition, and uses s and t for common state monad parameters. Thanks go to Shachaf Ben-Kiki for the grunt work of slogging through hundreds of definitions by hand and with regular expressions!
  • Restored lenses to Trustworthy status so they can be used with Safe Haskell once more.


  • Fixed a bug in rights1 and lefts1 in Control.Lens.Zipper which would cause them to loop forever when given a 0 offset.


  • Added ?~, <?~, ?= and <?= to Control.Lens.Setter for setting the target(s) of a Lens to Just a value. They are particularly useful when combined with at.


  • Refined the behavior of substType in Control.Lens.TH to match the behavior of typeVarsEx when moving under binders.


  • Added generateSignatures option to Control.Lens.TH to allow the end user to disable the generation of type signatures for the template-haskell generated lenses. This lets the user supply hand-written haddocks and more restricted signatures.


  • Added Control.Lens.Type.simple.


  • Added Control.Lens.Zipper.
  • Added <<~, a version of <~ that supports chaining assignment.
  • Added :->, :=>, and :<-> as type operator aliases for Simple Lens, Simple Traversal, and Simple Iso respectively.


  • Added <<%~, <<.~, <<%= and <<.= for accessing the old values targeted by a Lens (or a summary of those targeted by a Traversal)
  • Renamed |> to %, as %~ is the lensed version of %, and moved it to Control.Lens.Getter along with a version ^% with tighter precedence that can be interleaved with ^.
  • Upgraded to doctest 0.9, which lets us factor out common $setup for our doctests
  • Renamed merged to choosing. Added a simpler chosen operation to mirror both.
  • Added Control.Lens.Projection
  • Renamed traverseException to exception and traverseDynamic to dynamic, upgrading them to use Projection.
  • makeClassy now places each generated Lens or Traversal inside the class it constructs when possible. This makes it possible for users to just export HasFoo(..), rather than have to enumerate each lens in the export list. It can only do that if it creates the class. If the createClass flag is disabled, then it will default to the old behavior.
  • Added performs to Control.Lens.Action to mirror views in Control.Lens.Getter.


  • Restored compatibility with GHC 7.2. This required a major version bump due to making some MPTC-based default signatures conditional.

  • Added the missing Control.Lens.Combinators to exported-modules! Its absence was causing it not to be included on hackage.


  • Generalized the signature of Getting, Acting and IndexedGetting to help out with the common user code scenario of needing to read and then write to change types.
  • Documentation cleanup and additional examples.
  • Renamed au to ala, introducing further incompatibility with the newtype package, but reducing confusion.
  • Removed need for Data.Map.Lens and Data.IntMap.Lens by adding TraverseMin and TraverseMax to Control.Lens.IndexedTraversal.
  • Flipped fixity of ~: and <~:
  • Added ++~, ++=, <++~ and <++= to Data.List.Lens in response to popular demand.
  • Added |>, <$!> and <$! to Control.Lens.Combinators, which exports combinators that are often useful in lens-based code, but that don’t strictly involve lenses.
  • Added an HUnit-based test suite by @orenbenkiki


  • Fixed bugs in Traversal code-generation.


  • Added build option -f-inlining to facilitate building with the various TH 2.8 versions used by GHC 7.6 and HEAD.
  • Added build option -f-template-haskell for testing without template haskell. (Users should be able to assume TH is enabled; use this only for testing!)
  • Added support for generating a Traversal rather than a Lens when multiple fields map to the same name or some constructors are missing a field.
  • Removed _ from the lens names in System.FilePath.Lens.
  • Added iwhere, withIndices, withIndicesOf, indices and indicesOf to ease work with indexed traversals
  • Added assign as an alias for (.=) in Control.Lens.Setter.
  • Added ~:, =:, <~: and <=: to Data.List.Lens


  • Added Control.Lens.Plated, a port of Neil Mitchell’s uniplate that can be used on any Traversal.
  • Added Data.Data.Lens with smart traversals that know how to avoid traversing parts of a structure that can’t contain a given type.
  • Added Data.Typeable.Lens with _cast and _gcast like traverseData
  • Renamed IndexedStore to Context now that it is used in user-visible locations, and since I also use it as uniplate’s notion of a context.
  • Renamed Kleene to Bazaar – “a bazaar contains a bunch of stores.”
  • Added Comonad instances for Context and Bazaar, so we can use stores directly as the notion of an editable context in uniplate
  • Compatibility with both sets of template haskell quirks for GHC 7.6.1-rc1 and the GHC 7.6.1 development head.
  • Renamed children to branches in Data.Tree.Lens.
  • Added At and Contains to Control.Lens.IndexedLens.
  • Added FunctorWithIndex, FoldableWithIndex, and TraversableWithIndex under Control.Lens.WithIndex
  • Added support for unordered-containers.

  • GHC 7.6.1 development HEAD compatibility (but broke 7.6.1-rc1)

  • Haddock cleanup


  • Added the indexed Kleene store to Control.Lens.Internal
  • Moved Gettable, Accessor, Settable and Mutator to Control.Lens.Internal
  • Added cloneTraversal to Control.Lens.Traversal
  • Renamed clone to cloneLens in Control.Lens.Type
  • Generalized the type of zoom to subsume focus.
  • Removed Focus(..) from Control.Lens.Type.
  • Factored out Control.Lens.Isomorphic.
  • Moved many private types to Control.Lens.Internal
  • Added conFields to Language.Haskell.TH.Lens.
  • Added System.FilePath.Lens.


  • Added missing {-# INLINE #-} pragmas
  • Renamed meanwhile to throughout in Control.Parallel.Strategies.Lens
  • Added Magnify to Control.Lens.Getter.
  • Added Zoom to Control.Lens.Type.


  • Added <&=, <&~, <|=, and <|~
  • Moved <>~, <<>~, <>=, and <<>= to Data.Monoid.Lens
  • Template Haskell now uses eager binding to avoid adding dependencies.


  • Renamed adjust to over
  • Added au, auf and under
  • Added Data.Monoid.Lens
  • Increased lower dependency bound on mtl for cleaner installation.