lens-family-th

Generate lens-family style lenses

http://github.com/DanBurton/lens-family-th#readme

Version on this page:0.5.0.2@rev:1
LTS Haskell 24.29:0.5.3.2
Stackage Nightly 2026-01-19:0.5.3.2
Latest on Hackage:0.5.3.2

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BSD-3-Clause licensed by Dan Burton
Maintained by [email protected]
This version can be pinned in stack with:lens-family-th-0.5.0.2@sha256:e72432d3d08671a1e8bb36f05ad1342e708e05c2818b346c0e65e195a9f54b1b,1287

Module documentation for 0.5.0.2

Used by 1 package in lts-14.12(full list with versions):
lens-simple

lens-family-th

build status

Template Haskell to generate lenses for lens-family and lens-family-core.

Usage:

{-# LANGUAGE TemplateHaskell, Rank2Types #-}

import Lens.Family2
import Lens.Family2.TH

data Foo a = Foo { _bar :: Int, _baz :: a }
         deriving (Show, Read, Eq, Ord)
$(makeLenses ''Foo)

This will create lenses bar and baz.

You can instead create these lenses by hand as explained by documentation at Lens.Family.Unchecked.

makeLenses merely generates the following definition for each field, making use of Haskell’s record update syntax:

lensName f a = (\x -> a { fieldName = x }) `fmap` f (fieldName a)

makeLenses will refuse to create lenses for data declarations with more than 1 constructor.

For data types with multiple constructors, you can use makeTraversals. For example:

{-# LANGUAGE TemplateHaskell, Rank2Types #-}

import Lens.Family2
import Lens.Family2.TH

data T a c d = A a | B | CD c d Int
$(makeTraversals ''T)

Will create traversals _A, _B, and _C in this fashion:

_A k (A a) = fmap (\a -> A a) (k a)
_A _  B    = pure B
_A _ (C c d i) = pure (C c d i)

_B _ (A a) = pure (A a)
_B k B = fmap (\() -> B) (k ())
_B _ (C c d i) = pure (C c d i)

_C _ (A a) = pure (A a)
_C _ B = pure B
_C k (C c d i) = fmap (\(c',d',i') -> C c' d' i') (k (c,d,i))