Annotation for unfolding wanted

[Simon Peyton-Jones]

| > {-# INLINE f #-}
| > f x y z = ...  f x' y' z' ...
| >
| > into this:
| >
| > {-# INLINE f #-}
| > f x y z = f' x y z
| >    where f' x y z = ... f' x' y' z' ...
| >
| > That is, shoving (all of) the recursion in a level.  Then inlining f
| > results in a fresh loop, which presumably can be specialized or
| > optimized in various ways.
|
| This transformation is critical for performance of foldr and foldl(').
| The versions in GHC's libraries are manually written in the latter
| style.

It's important if and only if one or more of the parameters is static -- that is, passed on unchanged.  HTis is not the case in the example above. If you have

g x y z = .... (g x y' z') ....x...

then indeed it's sometimes a good plan to transform to

g x y z = g' y z
  where
    g' y z = ....(g' y' z')...x....

because now you can inline g, and thereby specialise for the value of x at the call site.  This is esp good if x is a function.

Simon