unknown@electronics optics $ cabal repl optics-th
Build profile: -w ghc-8.10.2 -O1
In order, the following will be built (use -v for more details):
 - optics-th-0.3.0.2 (lib) (file src/Optics/TH/Internal/Product.hs changed)
Preprocessing library for optics-th-0.3.0.2..
GHCi, version 8.10.2: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/unknown/.ghci
[1 of 5] Compiling Language.Haskell.TH.Optics.Internal ( src/Language/Haskell/TH/Optics/Internal.hs, interpreted )
[2 of 5] Compiling Optics.TH.Internal.Utils ( src/Optics/TH/Internal/Utils.hs, interpreted )
[3 of 5] Compiling Optics.TH.Internal.Sum ( src/Optics/TH/Internal/Sum.hs, interpreted )
[4 of 5] Compiling Optics.TH.Internal.Product ( src/Optics/TH/Internal/Product.hs, interpreted )
[5 of 5] Compiling Optics.TH        ( src/Optics/TH.hs, interpreted )
Ok, five modules loaded.
λ> :set -XTemplateHaskell
λ> data X = X { xX :: Int };
λ> ; makeFieldLabels ''X;

<interactive>:3:3: error:
    Generating LabelOptic instances requires the following language extensions:

- DataKinds
- FlexibleInstances
- MultiParamTypeClasses
- TypeFamilies
- UndecidableInstances

Please enable the extensions by copy/pasting these lines into the top of your file:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

To enable them in the GHCi session, use the following command:

:set -XDataKinds -XFlexibleInstances -XMultiParamTypeClasses -XTypeFamilies -XUndecidableInstances