Explore when to apply static argument transformation

The Static Argument transformation optimises

  f x y = ....f x' y...

into

  f x y = let g x = ....g x'...
          in g x

Instead of passing y along unchanged, we make it into a free variable of a local function definition g.

Unfortunately, it's not always a win. Andre Santos gives a discussion, and quite a few numbers in his thesis.

But sometimes it is a pretty big win. Here's the example that recently motivated me, which Roman Leshchinskiy showed me. You need the attached file Stream.hs, and then try compiling

  import Stream
  foo :: (a -> b) -> [a] -> [c]
  foo f = mapL f

Thus inspired, I think I have a set of criteria that would make the static arg transformation into a guaranteed win:

• there is only one (external) call to the function
• OR its RHS is small enough to inline
• OR it is marked INLINE (?)

So I'd like to try this idea out.

Trac metadata

Trac field	Value
Version	6.4.2
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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