Re: [Haskell-cafe] the Haskell "with" "pattern"?

On Wed, 2008-08-06 at 18:21 +0200, Johan Tibell wrote:

On Wed, Aug 6, 2008 at 5:44 PM, Jules Bean wrote:

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Johan Tibell wrote:

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You probably knew this already but there's nothing in the "with idiom" that prevents the resource to escape.

And, it doesn't always matter. Some withs are more unsafe that others :)

if the 'with' constructs a nice ordinary heap allocated haskell structure from the external resource then it may not matter one jot if it escapes: it may be out-of-date, perhaps, but still a useful data value.

I'm sure you're right but at the moment I can't really think of any such structure. Perhaps you could give an example? Why would you need the "with" in the first place for such a structure?

http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurren... withMVar :: MVar a -> (a -> IO b) -> IO b (useful for synchronization, among other things). modifyMVar :: MVar a -> (a -> IO (a, b)) -> IO b although that isn't quite within the boundaries of what `bracket' can do. jcc