Lost in Technopolis

by John Wiegley

Three examples of problems with Lazy I/O

Posted by John Wiegley on May 13, 2013 with labels:

Problem 1: The source of exceptions is obscured

main = getArgs >>= readFile . head >>= print . length

Even though length is a pure function, this is where the I/O will happen (lazily), which means that is where any exceptions relating to I/O will get raised. Pure code should avoid raising exceptions, which this example violates.

Problem 2: Sharing may cause file contents to remain in memory

main = getArgs >>= readFile . head >>= print . (length . words &&& length)

Because of the way that lazy I/O reads in strings, this line of code will cause the entire contents of the file to be loaded into memory by the call to either length or words, and then it will stay in memory to be handled by the other call to length. You would expect it to process the input at the very least one line at a time, to avoid exhausting memory on very large files.

NOTE: It has been pointed out that this is not really a problem with Lazy I/O, but with laziness in general. The only real way, then, in which an iteratee-type library helps here is that it’s more typical to connect sources and sinks directly together, than to read all the data from a source at one time, and then hand it to two sinks that way. So the problem there is not solved either, it’s just less common to the idiom.

Problem 3: File handles are not closed when you might expect

main = getArgs >>= mapM_ (readFile >=> print . length)

If getArgs returns N files, Haskell will open N file handles, rather than one at a time as you might expect, meaning that running this in a very large directory may exhaust system resources.

Conclusion: Use conduit/pipes/io-streams library to avoid surprises

Lazy I/O is great for prototypical simple examples, but for serious code these problems can be hard to track down – and are eliminated by a library such as conduit.


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