lens-family-th-0.5.0.1: README.md

lens-family-th
==============

![build status](https://api.travis-ci.org/DanBurton/lens-family-th.svg?branch=master)

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](http://hackage.haskell.org/packages/archive/lens-family-core/latest/doc/html/Lens-Family-Unchecked.html).

`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))