On 2012-11-03T10:05+0100, Peter Divianszky wrote:
Suppose we have a record update
r { x = f (r x)}
Hi Peter, I think you mean this: r { x = f (x r) }
1. Does the generated code for the record update build an identical record when f returns it's argument unchanged instead of sharing the old one? (I guess yes.)
In GHC: In your example a new record is built, but all its entries (x in this case) are shared. Here's how I found out: λ> let f = id λ> data R = R { x :: Int } λ> let r = R 0 λ> let u = r { x = f (x r) } λ> :view r λ> :view u :view is from ghc-vis[1], the resulting visualization is attached. In this simple example, when you compile it with -O1 the compiler figures out that u will always be the same as r and shares them. This does not work when it's dynamic whether u will be identical to r. Hope to help, Dennis [1] http://hackage.haskell.org/package/ghc-vis
