unsafePtrCompare, anybody?

Simon Marlow simonmar@microsoft.com
Wed, 19 Sep 2001 11:02:09 +0100

> My intuition says that none of the side-effects in my=20
> implementation are=20
> visible from the abstract level of the module.  However, the use of=20
> unsafePerformIO certainly modifies the behaviour of the=20
> module.  For example,=20
> the following definitions at the beginning of a GHCi session=20
> on the attached=20
> code lead to the subsequent behaviour:
> foo1 <- return (unsafePerformIO (mkAtom "foo"))
> foo2 <- return (unsafePerformIO (mkAtom "foo"))
> bar  <- return (unsafePerformIO (mkAtom "bar"))
> safefoo1 <- mkAtom "foo"
> safefoo2 <- mkAtom "foo"
> safebar  <- mkAtom "bar"
> list <- return [safefoo1, safefoo2, safebar, foo1, foo2, bar]

These uses of unsafePerformIO really *are* unsafe though :-)

Your implementation of mkAtom seems to be similar the the StableName
library, in that it has the property that if

	mkAtom e =3D=3D mkAtom e', then e =3D=3D e'

but the reverse isn't necessarily true (it may or may not be true).
Which is why mkAtom has to be in the IO monad: it has observable

FastString works differently: it guarantees that both

 	mkFastString s =3D=3D mkFastString s'  =3D>   s =3D=3D s'
	s =3D=3D s'  =3D>  mkFastString s =3D=3D mkFastString s'

hold.  So it is safe for mkFastString to be a pure non-I/O function,
because it doesn't have any observable non-deterministic behaviour.