roconnor / lens-family-th

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

lens-family-th

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)
$(mkLenses ''Foo)

This will create lenses bar and baz.

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

mkLenses 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)

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