リストを(非)正格に構成する

strictList.hs

data I a = I {unI :: a}

instance Functor I where
  fmap f (I a) = I (f a)

instance Monad I where
  return    = I
  I x >>= f = f x

foo :: [I a] -> I [a]
foo []     = return []
foo (ix:ixs) = do { x <- ix; xs <- foo ixs; return (x:xs) }

bar :: [I a] -> I [a]
bar = return . map unI

ss :: [I ()]
ss = repeat (I undefined)