Re: We need to add role annotations for 7.8

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 As someone completely oblivious on this development in Haskell until now, but researching myself in the same direction (on the distinction of computational, irrelevant, and parametric arguments in Agda), I am allowing myself some comments on the process, and the syntax discussion given in the paper http://www.cis.upenn.edu/~eir/papers/2014/coercible/coercible-ext.pdf 1. The alternative of using a pragma as syntax is dismissed with the argument: "We felt that, in several years, we would regret this decision. Backwards-compatibility would no longer be an issue, and we would be stuck with a pragma syntax for a core language feature." I strongly disagree. One can introduce a pragma syntax first, for smooth backward compatibility, and after the feature has passed the test of time, still introduce a proper, "first-class" syntax. If role annotations become standard, introduction of a proper syntax instead of ugly pragmas will be received with applause, unlike now, where you burden ugly CPP-ifs onto the library developers. You could do for now with a pragma-like declaration {-# TYPEROLE identifier role ... role #-} or {-# TYPE_ROLE identifier role ... role #-} instead of the declaration type role identifier role ... role 2. The chosen role names mean nothing to me. It feels a bit like the term "delegate" used in C# instead of just speaking of a higher-order function. Looking at the semantics, I find the following translation nominal = computational That is, the choice of type expression has a computational effect (like a different implementation of Ord). representational = parametric The choice of type expression is parametric, i.e., does not lead to other choices, but is purely propagated through. Further, "representational" is a bit long for a keyword. 'nominal' invokes the exact opposite association for me than it means. In nominal calculi (Pitts et al), everything is *parametric* in the choice of names. For introducing a non-backwards compatible language feature, this process feels rushed, imho. You might wanna pull the break before the release. Cheers, Andreas On 25.03.2014 04:26, Richard Eisenberg wrote:

I have a few responses to various themes in this thread, but nothing terribly unexpected:

- The introduction of roles is the end of the story that began with the discovery of bug #1496. The alternative would be to do away with GND. `coerce` is just a convenient application of roles, not the reason they were introduced.

- The concrete syntax for role ascriptions was debated in public, in bug #8185. There is further discussion of the design choice in the appendix of the extended version of our recent draft paper on the subject: www.cis.upenn.edu/~eir/papers/2014/coercible/coercible-ext.pdf http://www.cis.upenn.edu/~eir/papers/2014/coercible/coercible-ext.pdf

I'm afraid it's too late now to make changes. I don't love what we

ended up with, but I believe it's the best syntax that was proposed.

- I agree with Simon that `coerce` is quite a bit safer than `unsafeCoerce`. Under the assumption that all libraries are written correctly (that is, with proper role annotations), `coerce` is in fact fully safe.

- I surely recognize why and how this causes a Major Pain for Mark, and for other library maintainers. I wish there were an easier solution. However, I will perhaps repeat others in saying that a library that doesn't add role annotations is no more wrong in 7.8 than it was since GND was introduced. The only difference with 7.8 is that, now, there is a way to be right.

Richard

On Mar 24, 2014, at 9:32 PM, Edward A Kmett wrote:

...

Fair enough. I did try to convey that in the following sentence about how it at least enforces representational equality, but I can see how my statement might be taken as understating the importance of that property.

Sent from my iPhone

On Mar 24, 2014, at 6:57 PM, Simon Peyton Jones mailto:simonpj@microsoft.com> wrote:

...

In that light, `coerce` then can be viewed as a more friendly but still evil version of unsafeCoerce

Coerce embodies one rather compelling improvement: *it is type-sound.* unsafeCoerce can cause arbitrary seg-faults etc. ‘coerce’ cannot. Call me an old-fashioned “well-typed programs don’t go wrong” man, but I think that’s a big plus. Much more than “an occasional situation improvement”.

Granted, “type-sound” doesn’t guarantee “correct”, but then it never did.

The role machinery doesn’t exactly hoist us on a dilemma – it merely exposes the dilemma that was there all the time.

Simon

*From:*Edward Kmett [mailto:ekmett@gmail.com] *Sent:* 24 March 2014 19:11 *To:* Mark Lentczner *Cc:* Simon Peyton Jones; libraries@haskell.org mailto:libraries@haskell.org Libraries; ghc-devs@haskell.org mailto:ghc-devs@haskell.org *Subject:* Re: We need to add role annotations for 7.8

Mark,

We're currently planning to retain the existing behavior of GeneralizedNewtypeDeriving with regards to Safe Haskell. That is, Safe Haskell and GND still won't mix in 7.8 due to these same security concerns.

I think a key observation with regards to GeneralizedNewtypeDeriving is with representational roles as default the new roles machinery with the representational default lets you write nothing you couldn't write before. No new security vulnerabilities are introduced. They were there all along!

We're also disabling the Safe flag on Data.Coerce. In that light, `coerce` then can be viewed as a more friendly but still evil version of unsafeCoerce. It lets you write nothing you couldn't write before with `unsafeCoerce`. I view it as merely an occasional situational improvement over the existing unsafeCoerce in that it at least enforces representational equality.

Making the default role annotation nominal comes at a very very real cost. Namely, *all* of generalized newtype deriving anywhere breaks, and everyone forever will have to put annotations in to fix it.

The 'backwards' representational default puts the burden on a small minority of library authors.

I'm not a huge fan of the representational machinery, in that it hoists us upon this dilemma, but given the choice between everyone paying in perpetuity and a small minority of skilled library authors adding a handful of annotations that for the most part have already been added, and which expose them to no more risk than they'd had before if they forget, I'm definitely in favor of the current solution.

-Edward

On Mon, Mar 24, 2014 at 11:26 AM, Mark Lentczner mailto:mark.lentczner@gmail.com> wrote:

Thanks for the pointers, Simon. I appologize for coming to this quite so late... I didn't realize the global impact of this feature.

From a "meaning" perspective, I'm agnostic on the default.

From a "engineering" perspective, I want a default that "does a good enough, reasonably safe thing" if programmers ignore the feature.

The later is subtle as there are different vantage points for different developers. In the Platform, we have many libraries that we are encouraging both end-programmers, and other library authors to make use of and depend on extensively. This means those libraries have to work for both programmers that are ignoring the feature, and those that use it. In that later case, there is the even more subtle distinction of those that use the feature for their own code, and those that use it in libraries they make available.

The later case is issue: It seems a real mess if a library author who wanted to use the new feature, had to circumvent a HP library because it didn't annotate. Similar thought experiment: What would be the downside if containers didn't annotate? Would that just make the feature unusable because everything uses containers?

To put it more directly: with the satus-quo default of representations, what is the down side if a library, a widely used library, doesn't bother to annotate? What would be the loss if containers didn't annotate? (I know it did, this is the thought experiment... because I've got 30+ libraries in HP that are in this boat.)

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