The problems with lazy I/O

Let's use the hGetLine function alongside the hGetContents function:

main = do
  h <- openFile "jabberwocky.txt" ReadMode
  firstLine <- hGetLine h     -- returns a string
  contents  <- hGetContents h -- returns a "promise"

  hClose h           -– close file
  print $ words firstLine
  print $ words contents

We close the file before consuming the firstLine string and the contents stream:

  print $ words firstLine
    ["'Twas","brillig,","and","the","slithy","toves"]
  print $ words contents
    []

The contents is a live stream that gets turned off when the file is closed. The firstLine is an eager string and survives the closing of the file.

The preceding example points to some serious problems with the lazy I/O:

• The order of the side effects is tied to the order of the lazy evaluation. Because the order of lazy evaluation is not explicit, the order of effects...