HLint now spots bad unsafePerformIO

Summary: I've just released a new version of HLint that can spot an unsafePerformIO which should have NOINLINE but doesn't.

I've just released HLint v1.9.10, a tool to suggest improvements to Haskell code. I don't usually bother with release announcements of HLint, as each version just fixes a few things and adds a few hints, it's all in the changelog (plus there have now been 102 releases). The latest release attempts to make using unsafePerformIO a little bit safer. A common idiom for top-level mutable state in Haskell is:

myGlobalVar :: IORef Int
myGlobalVar = unsafePerformIO (newIORef 17)

That is, define a top-level CAF (function with no variables) and bind it to unsafePerformIO of some created mutable state. But the definition above is unsafe. GHC might decide myGlobalVar is cheap and inline it into several uses, duplicating the variable so that some functions update different versions. Running this code through the latest version of HLint we see:

Sample.hs:2:1: Error: Missing NOINLINE pragma
Found:
  myGlobalVar = unsafePerformIO (newIORef 17)
Why not:
  {-# NOINLINE myGlobalVar #-}
  myGlobalVar = unsafePerformIO (newIORef 17)

HLint has spotted the problem, and suggested the correct fix.

Trying it for real

Let's take the package slave-thread-0.1.0 and run HLint on it. Slave thread is a new package that helps you ensure you don't end up with ghost threads or exceptions being thrown but missed - a useful package. Running HLint we see:

Sample.hs:19:1: Error: Missing NOINLINE pragma
Found:
  slaves = unsafePerformIO $ Multimap.newIO
Why not:
  {-# NOINLINE slaves #-}
  slaves = unsafePerformIO $ Multimap.newIO

Sample.hs:20:3: Warning: Redundant $
Found:
  unsafePerformIO $ Multimap.newIO
Why not:
  unsafePerformIO Multimap.newIO

Sample.hs:25:1: Error: Eta reduce
Found:
  fork main = forkFinally (return ()) main
Why not:
  fork = forkFinally (return ())

Sample.hs:55:28: Warning: Redundant $
Found:
  PartialHandler.totalizeRethrowingTo_ masterThread $ mempty
Why not:
  PartialHandler.totalizeRethrowingTo_ masterThread mempty

Sample.hs:72:5: Error: Use unless
Found:
  if null then return () else retry
Why not:
  Control.Monad.unless null retry

HLint has managed to spot the missing NOINLINE pragma, and also a handful of tiny cleanups that might make the code a little more readable. Fortunately (and independent of HLint), the NOINLINE pragma was added in slave-thread-0.1.1, so the library no longer suffers from that bug.