Improving containers library

[Milan Straka]

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 <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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