primitive library

Simon Peyton Jones simonpj at
Mon Apr 23 20:04:13 UTC 2018

This got longer than I thought it would. I'll work on something that talks more about motivations and features soon.

That would be great, thanks.  It would also mean that more people would use the new stuff, sooner.


From: Andrew Martin <andrew.thaddeus at>
Sent: 23 April 2018 14:20
To: Simon Peyton Jones <simonpj at>
Cc: David Feuer <david at>; ghc-devs <ghc-devs at>; Haskell Libraries <libraries at>
Subject: Re: primitive library

I'll get something together soon that explains my motivations. The short summary is that the types in primitive unpack better that their counterparts in vector (meaning, they unpack into one machine word instead of three). I started using these types in internal data structures in libraries I would write (since I didn't need slicing), and then just started noticing stuff I thought it would be nice to add.

David noticed that a bunch of the typeclass instances were broken. I'd been working on a library quickcheck-classes that I use to test instances at my work place. The primitive library was missing a test suite, so I used it to test all the instances to ensure that the fixes David had written were correct. In the process, I found more broken instances and fixed them. The life lesson here is that property testing is important.

PrimArray is an interesting story. Both winterland1989 and I had independently written libraries that did the same exact thing: implement a typed interface to ByteArray that keeps track of the element type. This makes PrimArray much safer to work with than ByteArray. Eventually, winterland's initial PR to bring this to primitive stalled. It implemented several other features (some of which may still eventually get added), but it's scope was large enough that no maintainer was able to feel comfortable approving it. More recently, I took at stab at doing the same thing, but I only added PrimArray, and after a lot of feedback from David, Carter, and Ryan, it got merged in.

This got longer than I thought it would. I'll work on something that talks more about motivations and features soon.

On Mon, Apr 23, 2018 at 5:48 AM, Simon Peyton Jones <simonpj at<mailto:simonpj at>> wrote:
Andrew, David
I’ve seen a lot of traffic about the primitive library, in which you two are playing a leading role.
Clearly something interesting is going on, but I have not been paying enough attention to work out what.  Maybe lots of unrelated things?  Maybe a handful of closely related things?
Would you consider putting out a summary (to libraries and ghc-devs) to give an overview of the main threads, and any driving motivations.  Why has all this blown up now?
Meanwhile, thank you for being so active.

-Andrew Thaddeus Martin
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