On Wed, Mar 18, 2009 at 10:37 PM, Chung-chieh Shan <ccs...@post.harvard.edu>wrote:
> You make an important point that sharing is changed only if the variable > (such as x0) is used more than once in the body. Let me note that the > definition of "computation" doesn't have to mention "x0" multiple times > syntactically for x0 to be used more than once. It's enough for "x0" to > appear once under a lambda. Here's a concrete example: > > main :: IO () > main = once >> once > > once :: IO () > once = do > putStrLn "foo" > putStrLn (unsafePerformIO (putStrLn "hello") `seq` "world") > > If I put "() <-" in front of the second-to-last line, then "hello" > appears twice, not once, in the output. You're right. Of course, now you're in the compiler's territory. If you compile the () <- version with optimizations, the unsafePerformIO is lifted out and "hello" is again only printed once. So yes, to ensure sharing under a lambda, it's safest to use an explicit let/where clause. Luke
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