Is Safe Haskell intended to allow segfaults?

Edward Kmett ekmett at
Fri Aug 12 13:49:04 UTC 2016

As for a sample list of modules, let's just start with your very first
example, Foreign.Ptr:

In and of itself nothing in Foreign.Ptr is unsafe! It allows a bit of
arithmetic on a type you can't actually use with anything, and provides an
IO action mixed into an otherwise pure module that happens to create a
FunPtr slot from a haskell function. In fact this module is a textbook
example of an otherwise perfectly cromulent Trustworthy module today that
happens to have a single IO action in it.

I can grab Ptr from it, use its Storable instance to make a default
signature for other safe code and still be perfectly safe.

It gives no tools for manipulating the contents of the Ptr. It is no more
dangerous than an Int with a phantom type argument.

You could randomly declare that this module is Unsafe because it combines
badly with APIs that would be safe if you could rely on any Ptr T actually
pointing to a T, and that users could then be granted the power to ferry
them around, but we don't trust a user to be able to do that today.

It's the combinators that read/write to a Ptr are the dangerous bits, not
pure math.


On Wed, Aug 10, 2016 at 10:23 AM, Ryan Newton <rrnewton at> wrote:

> Hi Edward,
> On Tue, Aug 9, 2016 at 11:58 PM, Edward Kmett <ekmett at> wrote:
>> 1.) If you remove IO from being able to be compiled inside Safe code _at
>> all_ most packages I have that bother to expose Safe information will have
>> to stop bothering.
> I definitely wouldn't argue for removing it entirely.  But it's good to
> know that there are instances where IO functions get mixed up in safe
> modules.  I'll try to systematically find all of these on hackage, but in
> the meantime do you have a sample list of modules?
> My modest starting proposal is marking certain Foreign.* modules as Unsafe
> rather than Trustworthy.  We'll find all the modules affected.  But, again,
> are there any modules you know of offhand that are affected?  They should
> fall into two categories:
>    1. Safe modules that must become Trustworthy (if they import Foreign
>    bits, but don't expose the ability to corrupt memory to the clients of
>    their APIs).
>    2. Safe modules that must become Unsafe or be split further into
>    smaller modules.
> Obviously (2) is the biggest source of potential disruption.
> I wouldn't ask anyone to accept a patch on GHC until we'd explored these
> impacts pretty thoroughly.
> I'd have to cut up too many APIs into too many fine-grained pieces.
> Yeah, the module-level business is pretty annoying.  "vector' removed
> ".Safe" modules and no one has gotten around to adding the ".Unsafe".
>> 2.) Assuming instead that you're talking about a stronger-than-Safe
>> additional language extension, say ReallySafe or SafeIO, it all comes down
>> to what the user is allowed to do in IO, doesn't it? What effects are users
>> granted access to? We don't have a very fine-grained system for IO-effect
>> management, and it seems pretty much any choice you pick for what to put in
>> the sandbox will be wrong for some users, so you'd need some sort of pragma
>> for each IO operation saying what bins it falls into and to track that
>> while type checking, etc.
> Well, *maybe* it is a slippery slope that leads to a full effect system.
> But I'd like to see these issues enumerated.  Does memory safety as a goal
> really involve so many different effects?  Do you think there will be 1, 3,
> 10, or 100 things beyond Foreign.Ptr to worry about?
> 3.) On the other hand, someone could _build_ an effect system in Haskell
>> that happens to sit on top of IO, holding effects in an HList, undischarged
>> nullary class constraint, etc.
> Well, sure, I hope we will continue to aim for this as well.  This is
> effectively what we do with our "LVish" Par monad, where we use Safe
> Haskell to ensure users cannot break the effect system in -XSafe code.
> Best,
>  -Ryan
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