Demo: Lazy evaluation of the writer monad

README.md

The output from ghci demonstrates that wa >>= f produces the result even though wa contains diverging computations.

% ghci lazy-writer.hs
GHCi, version 9.4.4: https://www.haskell.org/ghc/  :? for help
[1 of 2] Compiling Main             ( lazy-writer.hs, interpreted )
Ok, one module loaded.
ghci> go
False, 42
ghci> 

lazy-writer.hs

data Wr w a = Wr w a

a ⅋⅋ b = b && a

instance Functor (Wr Bool) where
  fmap f (Wr w a) = (Wr w (f a))

instance Applicative (Wr Bool) where
  pure a = Wr True a
  (Wr v f) <*> (Wr w a) = Wr (v ⅋⅋ w) (f a)

instance Monad (Wr Bool) where
  (Wr v a) >>= f = let (Wr w b) = f a in Wr (v ⅋⅋ w) b

instance Show a => Show (Wr Bool a) where
  show (Wr w a) = (show w) <> ", " <> (show a)

zzz :: forall a. a
zzz = zzz

wa :: Wr Bool Int
wa = Wr zzz zzz

f :: Int -> Wr Bool Int
f n = Wr False 42

go = wa >>= f