    {-# LANGUAGE PackageImports #-}
    {-# LANGUAGE AllowAmbiguousTypes    #-}
    {-# LANGUAGE FlexibleInstances      #-}
    {-# LANGUAGE FunctionalDependencies #-}
    {-# LANGUAGE MonoLocalBinds         #-}
    {-# LANGUAGE MultiParamTypeClasses  #-}
    {-# LANGUAGE PolyKinds              #-}
    {-# LANGUAGE ScopedTypeVariables    #-}
    {-# LANGUAGE TypeApplications       #-}
    {-# LANGUAGE TypeFamilies           #-}
    {-# LANGUAGE UndecidableInstances   #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Generics.Product.Any
    -- Copyright   :  (C) 2020 Csongor Kiss
    -- License     :  BSD3
    -- Maintainer  :  Csongor Kiss <kiss.csongor.kiss@gmail.com>
    -- Stability   :  experimental
    -- Portability :  non-portable
    --
    -- Derive a variety of lenses generically.
    --
    -----------------------------------------------------------------------------

    module Data.Generics.Product.Any
      ( -- *Lenses
        --
        -- $setup
        HasAny (..)
      ) where

    import "this" Data.Generics.Internal.VL.Lens
    import "this" Data.Generics.Product.Fields
    import "this" Data.Generics.Product.Positions
    import "this" Data.Generics.Product.Typed

    -- $setup
    -- == /Running example:/
    --
    -- >>> :set -XTypeApplications
    -- >>> :set -XDataKinds
    -- >>> :set -XDeriveGeneric
    -- >>> import GHC.Generics
    -- >>> :m +Data.Generics.Internal.VL.Lens
    -- >>> :{
    -- data Human = Human
    --   { name    :: String
    --   , age     :: Int
    --   , address :: String
    --   }
    --   deriving (Generic, Show)
    -- human :: Human
    -- human = Human "Tunyasz" 50 "London"
    -- :}

    class HasAny sel s t a b | s sel -> a where
      -- |A lens that focuses on a part of a product as identified by some
      --  selector. Currently supported selectors are field names, positions and
      --  unique types. Compatible with the lens package's 'Control.Lens.Lens'
      --  type.
      --
      --  >>> human ^. the @Int
      --  50
      --
      --  >>> human ^. the @"name"
      --  "Tunyasz"
      --
      --  >>> human ^. the @3
      --  "London"
      the :: Lens s t a b

    instance HasPosition i s t a b => HasAny i s t a b where
      the :: Lens s t a b
    the = forall (i :: Nat) s t a b. HasPosition i s t a b => Lens s t a b
    position @i

    instance HasField field s t a b => HasAny field s t a b where
      the :: Lens s t a b
    the = forall (field :: Symbol) s t a b.
    HasField field s t a b =>
    Lens s t a b
    field @field

    instance (HasType a s, t ~ s, a ~ b) => HasAny a s t a b where
      the :: Lens s t a b
    the = forall a s. HasType a s => Lens s s a a
    typed @a