GHC.Internal.TH.Lift

A Lift instance can have any of its values turned into a Template Haskell expression. This is needed when a value used within a Template Haskell quotation is bound outside the Oxford brackets ([| ... |] or [|| ... ||]) but not at the top level. As an example:

add1 :: Int -> Code Q Int
add1 x = [|| x + 1 ||]

Template Haskell has no way of knowing what value x will take on at splice-time, so it requires the type of x to be an instance of Lift.

A Lift instance must satisfy $(lift x) ≡ x and $$(liftTyped x) ≡ x for all x, where $(...) and $$(...) are Template Haskell splices. It is additionally expected that lift x ≡ unTypeCode (liftTyped x).

Lift instances can be derived automatically by use of the -XDeriveLift GHC language extension:

{-# LANGUAGE DeriveLift #-}
module Foo where

import Language.Haskell.TH.Syntax

data Bar a = Bar1 a (Bar a) | Bar2 String
  deriving Lift

Representation-polymorphic since template-haskell-2.16.0.0.

This is exposed both from the template-haskell-lift and template-haskell packages. Consider importing it from the more stable template-haskell-lift if you don't need the full breadth of the template-haskell interface.