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November 2010

264 participants
272 discussions

Equivalent of withForeignPtr for System.Mem.Weak?
by Matthew Steele 01 Nov '10

01 Nov '10

I have an object to which I have added one or more finalizers via addFinalizer from System.Mem.Weak. I would like to have a function that allows me to make use of the object within a block of IO code, and guarantee that the finalizer(s) will not be called during the code block -- sort of like withForeignPtr, but for arbitrary objects. Does such a function exist, and if not, how could I write one? I can imagine writing something like: \begin{code} withObject :: a -> IO b -> IO b withObject obj action = do result <- action touchObject obj -- touchObject :: a -> IO () return result \end{code} However, I don't know how I should implement touchObject and be confident that it won't ever be optimized out. Any suggestions? Cheers, -Matt

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Data records. Default values initialisation. Reading datatype from string.
by Siim Haugas 01 Nov '10

01 Nov '10

Hello, I need some help with data records. Normally I can use a `read' function to convert string to some data type e.g. `(read somestring) :: Type'. However, sometimes I'd like to read a record with some uninitialised fields. I have also set up a default values for my data record, for example: `defrec = {a=0,b=1, ..., z=0}'. Now I would like to override some values, so I have a string "defrec{l=100,r=88}", but it seems that it can't be done with read `(read "defRec{l=100,r=88}") :: defRec'. Any suggestions? Siim

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