"Neil Mitchell"
I should have been more precise with my question. Given the code:
fred = 2 + 2
bob = fred + fred
In a Haskell implementation fred would be evaluated once to 4, then used twice. The 2+2 would only happen once (ignore defaulting and overloaded numerics for now).
Not necessarily. The Haskell 98 standard very carefully avoids mandating any particular evaluation strategy beyond that it should be non-strict. So bob is always going to be 8, but just how it gets there is up to the implementation. If you'd had fred = [1..] and bob = do something with fred a lot of other stuff something else with fred it's much less obvious that keeping the first-calculated value for fred around the whole time is the right thing to do.
Do all Haskell compilers support the sharing.
I don't know all Haskell compilers, but I'm pretty sure there have been implementations that don't, or don't always because of the above problem. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk