Writerっぽくリスト作れて、遅延評価とかfoldr/buildとか効くヤツ

builder.hs

module Main where

import Control.Monad.Free.Church
import GHC.Exts (build)
import Control.Monad (forever)

type Builder b = F ((,) b)

putB :: b -> Builder b ()
putB x = liftF (x, ())

bToList :: Builder b a -> [b]
bToList x = build $ \cons nil -> runF x (const nil) (uncurry cons)

main :: IO ()
main = print $ take 10 $ bToList $ forever (putB "a")