A type checker plugin for row types

Nicolas Frisby nicolas.frisby at gmail.com
Mon Sep 11 19:07:25 UTC 2017

Adam, thanks for:

1) The reference to Iavor's paper --- it is a nice more-detailed
description of the plugin API/semantics, and the Nelson-Oppen parallel is
very illuminating!

2) Asking "Do you mean "touchable" or "unification variable" here and

That prompted me to finally dig deeper into something, and I've
updated/simplified the wiki page accordingly. Basically, I was just using
newFlexiTyVar (since it's pretty much the only option in the "official"
TcPluginsM interface) without understanding if it's the touchability or the
skolem-vs-unification status that was enabling the Given-Given
interactions. I'm happy to report that touchability apparently has nothing
to do with any of my test cases (including the record and variant library,
etc). I'm relieved about that: touchability is a restriction on
unification, and my general goal with my plugin architecture is to leave as
many of the unification details to GHC's type equality solver as possible.

Thanks. -Nick

On Mon, Sep 11, 2017 at 1:34 AM Adam Gundry <adam at well-typed.com> wrote:

> Hi Nick,
> This is great work, and I look forward to seeing the code once it is
> ready. I've had a quick glance over your wiki page, and thought I should
> send you some initial comments, though it deserves deeper attention
> which I will try to find time to give it. :-)
> I don't see a reference to Iavor's paper "Improving Haskell Types with
> SMT" (http://yav.github.io/publications/improving-smt-types.pdf). If
> you've not come across it, it might give a useful alternative
> perspective on how plugins work, especially with regard to derived
> constraints.
> The following is based on my faulty memory, so apologies if it is out of
> date or misleading...
> > When/where exactly do Derived constraints arise?
> Suppose I have a class with an equality superclass
>     class a ~ b => C a b
> and a wanted constraint `C alpha Int`, for some touchable variable
> `alpha`. This leads to a derived constraint `alpha ~ Int` thanks to the
> superclass (derived means we don't actually need evidence for it in
> order to build the core term, but solving it might help fill in some
> touchable variables).  Sorry if this is obvious and not exact enough!
> > When do touchables "naturally" arise in Given constraints?
> Do you mean "touchable" or "unification variable" here (and elsewhere?).
> A skolem is always untouchable, but the converse is not true.
> I think that unification variables can arise in Given constraints, but
> that they will always be untouchable. Suppose we have defined
>     f :: forall a b . ((a ~ b) => a -> b) -> Int
> (never mind that it is ambiguous) and consider type-checking the call `f
> id`. We end up checking `id` against type `a -> b` with given `a ~ b`
> where `a` and `b` are unification variables. They must be untouchable,
> however, otherwise we might unify them, which would be wrong.
> Hope this helps,
> Adam
> On 10/09/17 23:24, Nicolas Frisby wrote:
> > Hi all. I've been spending my free time for the last couple months on a
> > type checker plugin for row types. The free time waxes and wanes;
> > sending an email like this one was my primary goal for the past couple
> > weeks.
> >
> > At the very least, I hoped this project would let me finally get some
> > hands on experience with OutsideIn. And I definitely have. But I've also
> > made more progress than I anticipated, and I think the plugin is
> > starting to have legs!
> >
> > I haven't uploaded the code yet to github -- it's not quite ready to
> > share. But I did do a write up on the dev wiki.
> >
> >
> >
> https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker/RowTypes/Coxswain
> >
> > I would really appreciate and questions, comments, and --- boy, oh boy
> > --- answers.
> >
> > I hope to upload within a week or so, and I'll update that wiki page and
> > reply to this email when I do.
> >
> > Thanks very much. -Nick
> >
> > P.S. -- I've CC'd and BCC'd people who I anticipate would be
> > specifically interested in this (e.g. plugins, row types, etc). Please
> > feel free to forward to others that come to mind; I know some inboxes
> > abjectly can't afford default list traffic.
> >
> > P.P.S. -- One hold up for the upload is: which license? I intend to
> > release under BSD3, mainly to match GHC since one ideal scenario would
> > involve being packaged with/integrated into GHC. But my brief recent
> > research suggests that the Apache license might be more conducive to
> > eventual widespread adoption. If you'd be willing to advise or even just
> > refer me to other write ups, please feel free to email me directly or to
> > start a separate thread on a more appropriate distribution list (CC'ing
> > me, please). Thanks again.
> --
> Adam Gundry, Haskell Consultant
> Well-Typed LLP, http://www.well-typed.com/
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