On 15 February 2011 11:23, Roman Leshchinskiy
I wouldn't necessarily expect this to guarantee inlining for the same reason that the following code doesn't guarantee that foo gets rewritten to <big>:
foo = bar {-# INLINE bar #-} bar = <big>
It might work with the current implementation (I'm not even sure if it does) but it would always look dodgy to me.
In this case there doesn't seem to be any point inlining anyway, because nothing is known about the context into which you are inlining. Nonetheless, what will happen (I think) is that any users of "foo" will get the definition of "foo" inlined (because that doesn't increase program size) so now they refer to "bar" instead. Now GHC can look at the use site of bar and the definition of bar and decide whether it is a good idea to inline. Basically, I expect the small RHS for the default in my class declaration to be inlined unconditionally, and then GHCs heuristics will determine how and when to inline the "actual" default definition (e.g. default_foo). This differs from the current story in that with the present setup you can write the INLINE and default method directly in the class definition, and then GHC does not need to inline the small RHS of the default to get a chance to apply its inlining heuristics on the "actual" default method. However, given that these small RHSes *should* be inlined eagerly and ubiquitously, there shouldn't be a detectable difference writing default methods directly and the proposed pattern for adding INLINE pragmas to default methods.
Also, what if I write:
class MyClass a where foo :: a -> a foo x = default_foo x
I assume this wouldn't guarantee inlining?
I don't know about any guarantee -- again personally I would only hope the inlining would only occur should GHC decide it is worth it -- but this still looks like it should be OK under the no-size-increase inlining heuristic. I think the simplifier will probably avoid actually inlining unless foo is applied to at least 1 arg to avoid increasing allocation, but any interesting use site will meet that condition. I do not really know what the simplifier does in enough detail to know exactly what will happen here, though. This is just an educated guess as to what will happen, which makes me think that my proposed pattern is OK. Cheers, Max