We need to add role annotations for 7.8

Simon Peyton Jones simonpj at microsoft.com
Mon Mar 24 22:57:03 UTC 2014

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.


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


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.


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