ASTs with Functor fixpoints

Fix.hs

{-# LANGUAGE DeriveFunctor, StandaloneDeriving #-}

import Data.Functor.Classes (Show1)

data ExprFunctor a = Literal Int | Plus a a | Times a a
  deriving (Show, Functor)

data LineNumber a = LineNumber Int a
  deriving (Show, Functor)

data Fix f = Fix (f (Fix f))

data Comp f g x = Comp (f (g x))
  deriving Functor

type Annotated = Fix (Comp LineNumber ExprFunctor)
type Expr = Fix ExprFunctor

annotated :: Annotated
annotated = Fix (Comp (LineNumber 1 (Plus
                                     (Fix (Comp (LineNumber 2 (Literal 3))))
                                     (Fix (Comp (LineNumber 3 (Literal 5)))))))

mapFix :: (Functor f, Functor g) => (f (Fix f) -> g (Fix f)) -> Fix f -> Fix g
mapFix f (Fix x) = Fix (fmap (mapFix f) (f x))

stripOne :: Comp LineNumber ExprFunctor x -> ExprFunctor x
stripOne (Comp (LineNumber _ e)) = e

strip :: Annotated -> Expr
strip = mapFix stripOne

unannotated :: Expr
unannotated = strip annotated