[Haskell-cafe] Help understanding sharing
Albert Y. C. Lai
trebla at vex.net
Mon Apr 14 23:24:36 EDT 2008
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)
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