Roman, Manuel [ccing cvs-ghc for interest]
Consider this:
let xs = build g
in map (\y. ...(foldr k z xs)...) ys
The foldr/build rule doesn't fire, because in principle it might take lots of
work to construct the list 'xs'. By doing it outside the 'let' we share all
the computation of producing xs across every element of ys. But in exchange we
allocate xs, and retain it.
A common case is when xs started life as [p..q]. For example
f x y = length [(a,b) | a <- [1..x], b <- [1..y]]
Then you might reasonably argue that it's cheaper to reconstruct xs for every
y, rather than to share it. By the time we've done a bit of inlining we get
let g = \cn. eftIntFB c n p q
xs = build g
in ...as before...
In short, we'd like to treat (build g) as CONLIKE, *for this particular g*.
Not for all g's. You can see this by just inlining build in your head to get
xs = g (:) []
Now if g was CONLIKE we'd be away!
Somehow we'd like to say
build is CONLIKE if its argument is CONLIKE
Or possibly
when trying to decide if (build g) is CONLIKE
inline 'build' and see if the result is CONLIKE
I wondered whether you've encountered this anywhere in NDP? Or generally
whether it rings any bells.
Simon
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