Haskell IO and closing files

Question

When I open a file for reading in Haskell, I\'ve found that I can\'t use the contents of the file after closing it. For example, this program will print the contents of a file:<

Answer 1

As previously noted, hGetContents is lazy. readFile is strict, and closes the file when it's done:

main = do contents <- readFile "foo"
          putStr contents

yields the following in Hugs

> main
blahblahblah

where foo is

blahblahblah

Interestingly, seq will only guarantee that some portion of the input is read, not all of it:

main = do inFile <- openFile "foo" ReadMode
          contents <- hGetContents $! inFile
          contents `seq` hClose inFile
          putStr contents

yields

> main
b

A good resource is: Making Haskell programs faster and smaller: hGetContents, hClose, readFile