Re: [boost] [Hana] Formal review

On 6/17/2015 3:55 PM, David Sankel wrote:

On Wed, Jun 10, 2015 at 3:19 AM, Glen Fernandes wrote:

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- Whether you believe the library should be accepted into Boost * Conditions for acceptance

Yes, I think it should be accepted into Boost. The one condition is that the library works with release versions of at least two different mainstream C++ compilers. More on that later...

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- Your name

David J. Sankel

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- Your knowledge of the problem domain.

I've written numerous libraries that make use of Boost.MPL and Boost.Fusion. I also have quite a bit of functional programming expertise.

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You are strongly encouraged to also provide additional information: - What is your evaluation of the library's: * Design

The core technique of combining value and type expressions is solid and makes metaprogramming easier and, as a bonus, improves compilation speeds.

My one question, as I read though the implementation, is "can the core benefits of this library be achived with a simpler 'light' version of this implementation?". While I appreciate the attempt to encode a Haskell-style typeclass hierarchy, I feel like that is not the core competency of hana and should be a separate library and discussion. As it is, this is a 32k header mega library. I'd prefer several small, highly-targeted, highly-composable libraries.

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* Implementation

The code itself looks to be well structured and well documented.

Unfortunately hana only works with one compiler: clang. While I agree that Boost shouldn't need to support Visual C++ 6.0 anymore, I believe this is going too far in the opposite direction. The home page states that boost libraries "are intended to be widely useful, and usable across a broad spectrum of applications". I've always interpreted that statement to be in a practical rather than theoretical sense and I don't think hana meets that criteria. Many other Boost authors have made heroic efforts to meet that criteria and the reputation of Boost is due, in no small part, to those efforts.

I do appreciate the argument that making use of new features encourages compiler implementers to implement then. I maintain, however, that this isn't Boost's job. Boost provides high quality libraries that the every-day Joe C++ developer can benefit from.

That being my position on the issue, my acceptance vote is conditional on hana supporting at least two released versions of mainstream compilers. Given that gcc support seems pretty close, that shouldn't be hard to achieve.

I have not reviewed Hana yet but I feel the need to comment about any arbitrary number of compilers a library must work under. I completely disagree with the notion that any such number would be the cause of accepting or not accepting a library as part of Boost. If a library is found useful and works according to the latest version of the C++ standard that should be more than enough for that library to be accepted into Boost as part of the review process pending individual reviews.