How do laziness and I/O work together in Haskell?
- by Bill
I'm trying to get a deeper understanding of laziness in Haskell.
I was imagining the following snippet today:
data Image = Image { name :: String, pixels :: String }
image :: String -> IO Image
image path = Image path <$> readFile path
The appeal here is that I could simply create an Image instance and pass it around; if I need the image data it would be read lazily - if not, the time and memory cost of reading the file would be avoided:
main = do
image <- image "file"
putStrLn $ length $ pixels image
But is that how it actually works? How is laziness compatible with IO? Will readFile be called regardless of whether I access pixels image or will the runtime leave that thunk unevaluated if I never refer to it?
If the image is indeed read lazily, then isn't it possible I/O actions could occur out of order? For example, what if immediately after calling image I delete the file? Now the putStrLn call will find nothing when it tries to read.
