Improving containers library

[Edward Kmett]

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 <jonathanccast at fastmail.fm>wrote:

> On Wed, 2010-03-03 at 18:58 +0100, Axel Simon wrote:
> > On 03.03.2010, at 17:30, Milan Straka wrote:
> >
> > > 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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