Laziness in Haskell

laziness.hs

import Data.IntMap.Lazy as Map

{-
Notice that the smallest is not evaluated under 'go' function, 
otherwise the computation would be ⊥
-}
attachMin :: Ord a => [a] -> [(a, a)]
attachMin l =
  let (l', smallest) = go smallest l
  in l'
    where
      go smallest [x] = ([(x, smallest)], x)
      go smallest (x:xs) =
        let (xs', smallest') = go smallest xs
         in ((x, smallest):xs', min x smallest')

-- Data.IntMap.Strict loops
fib :: Int -> IntMap Int
fib n = m
  where
    m =
      Map.fromList $
        (1, 1) :
        (2, 1) :
          [(i, m ! (i -2) + m ! (i -1)) | i <- [3 .. n]]