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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.

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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