Patrick Surry wrote:
I've seen other discussions that suggest that lists are always shared while in scope (so the fibs trick works). But is that just a feature of the standard compilers, or is it somewhere mandated in the Hakell spec (I don't see anything obvious in the Haskell Report tho haven't read it cover to cover)?
It is just a feature of most compilers. The Haskell Report does not specify sharing. For most compilers, a sufficient condition for sharing is aliasing, e.g., let y = f x in (y,y,y,y,y) you can be sure that most compilers share one copy of "f x" for those five mentions of "y". As another example, let x = 0:x in x you can be sure that most compilers create a tight cyclic graph for that. In contrast, most compilers may create redundantly new expressions for the following: (f x, f x, f x, f x, f x) -- given the definition: recurse f = f (recurse f) recurse (\x -> 0:x)