On Thu, Sep 21, 2006 at 12:26:03AM +1000, John Ky wrote:
Given that putStrLn contents did manage to print out the HTTP header before blocking, am I correct in coming to the conlusion that 'contents' is evaluated lazily?
hGetContents breaks the rules of the IO monad - it returns a value (the contents list) that is not entirely pure - evaluation of this value causes execution of IO actions, namely reading from the handle. This way of operation is not a rule, but an exception. BTW, try to implement hGetContents in pure Haskell 98, using only the functions specified there, but excluding (hGetContents, getContents, readFile and interact). As a test try this code on a big or infinite input, like /dev/zero: import IO main = hGetContents stdin >>= print . take 100 hGetContents may cause some confusion, but it also is very handy. It lets you treat file contents as a lazy list, which allows you to use very high-level code to process the data without having to load the whole file into memory. Best regards Tomasz