Inliner behaviour with instances is confusing

[bos]

I have a fairly simple typeclass:

class Variate a where
    uniform :: Gen s -> ST s a

With instances like this:

instance Variate Int8 where
    uniform = f where f = uniform1 fromIntegral
                      {-# INLINE f #-}

uniform1 :: (Word32 -> a) -> Gen s -> ST s a
uniform1 f gen = do
  i <- uniformWord32 gen
  return $! f i
{-# INLINE uniform1 #-}

Notice the peculiar form of the instance definition. I had to write the above instead of the more intuitive form:

instance Variate Int8 where
    uniform = uniform1 fromIntegral
    {-# INLINE uniform #-}

With the more obvious form, GHC's inliner didn't fire at all for this, and I was unable to tell why. It was Duncan who suggested the more convoluted instance above. The result is about a 3x performance difference sometimes, so this has a noticeable effect.

I'm not completely sure that this is a bug, but I don't know how to describe why one form works and the other doesn't, so the compiler's behaviour is surprising to me.

[igloo]

Thanks for the report.

[simonpj]

I'll look at this when I've got the new INLINE patch done.

Simon

[simonpj]

Bryan

You don't actually give a reproducible test case, so I can't test this. Nevertheless I believe I have fixed the problem. You should be able to write the straightforward instance, and it should work. Can you try, and let me know?

Simon

[bos]

Thanks, Simon; I'll take a look this weekend and close this bug out if all is now well.

[bos]

As far as I can tell, this is happy in the world of GHC 7.