Re: Improving containers library

Hi, thanks for enlightening me :) I wished for something like this some time ago... Milan

You should seq or use a bangpattern on the arguments before taking the stable names to ensure that the result is reproducible if the input thunks may later be forced.

...

let y = 1 + 1; x = id y in (ptrEqual x y, x `seq` y `seq` ptrEqual x y) (False,True)

a === b = a `seq` b `seq` unsafePerformIO $ (==) <$> makeStableName a <*> makeStableName b

...

let y = 1 + 1; x = id y in (x === y, x `seq` y `seq` x === y) (True,True)

This may be less of a problem if your tree has bang patterns, but even there it it sometimes easy to trip up and make a mistake with the unforced root of a tree.

-Edward Kmett

On Wed, Mar 3, 2010 at 1:08 PM, Jonathan Cast wrote:

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On Wed, 2010-03-03 at 18:58 +0100, Axel Simon wrote:

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On 03.03.2010, at 17:30, Milan Straka wrote:

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Any easy way of comparing pointers? I mean, if I have something like Tree a = N | B (Tree a) a (Tree a) and have (l::Tree a) and (r::Tree a), can I ask about the "physical equality"?

You can! Despite the names appearing in the following code, the following is safe:

-- | Equality on pointers. ptrEqual :: a -> a -> Bool ptrEqual x y = unsafePerformIO $ do nx <- makeStableName x ny <- makeStableName y return (nx==ny)

Safe but not pure?

jcc

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