Properly writing typechecker plugins

[Alexis King]

Hi all,

I have recently decided to try writing a GHC typechecker plugin so I can get my hands on some extra operations on type-level strings. My plugin works, but only sort of—I know some things about it are plain wrong, and I have a sneaking suspicion that plenty of other things are not handled properly.

First, some context about what I do and don’t already know: I have a high-level understanding of the basic concepts behind GHC’s solver. I understand what evidence is and what purpose it serves, I mostly understand the different flavors of constraints, and I think I have a decent grasp on some of the operational details of how individual passes work. I’ve spent a little time reading through comments in the GHC source code, along with pieces of the source code itself, but I’m sure my understanding is pretty patchy.

With that out of the way, here are my questions:

First, I’m trying to understand: why are wanted constraints passed to typechecker plugins unflattened? This is my single biggest point of confusion. It certainly seems like the opposite of what I want. Consider that I have a type family

    type family ToUpper (s :: Symbol) :: Symbol where {}

that I wish to solve in my plugin. At first, I just naïvely looked through the bag of wanted constraints and looked for constraints of the shape

    t ~ ToUpper s

but this isn’t good enough, since my plugin regularly receives constraints that look more like

    t ~ SomeOtherTypeFamily (ToUpper s)

so I have to recursively grovel through every type equality constraint looking for an application of a family I care about. Furthermore, once I’ve found one, I’m not sure how to actually let GHC know that I’ve solved it—do I really have to just generate a new given constraint and let GHC’s solver connect the dots?

I have seen the note on the typechecker plugins wiki page <https://gitlab.haskell.org/ghc/ghc/wikis/plugins/type-checker#under-discussion-defining-type-families> about possibly baking support for type families into the plugin interface, which would indeed be nicer than the status quo, but it seems odd to me that they aren’t just passed to plugins flattened, which seems like it would spare a lot of effort. Isn’t the flattened representation really what typechecker plugins would like to see, anyway?

But let’s put families aside for a moment. I’m not just solving type families in my plugin, I’m also solving classes. These classes have no methods, but they do have functional dependencies. For example, I have a class

    class Append (a :: Symbol) (b :: Symbol) (c :: Symbol) | a b -> c, a c -> b, b c -> a

which is like GHC.TypeLits.AppendSymbol, but the fundeps make GHC a bit happier when running it “backwards” (since GHC doesn’t really know about AppendSymbol’s magical injectivity, so it sometimes complains).

In any case, I was hoping that GHC’s solver would handle the improvement afforded by the fundeps for me once I provided evidence for Append constraints, but that doesn’t seem to be the case. Currently, I am therefore manually generating derived constraints based on the functional dependency information, plumbing FunDepOrigin2 through and all. Is there some way to cooperate better with GHC’s solver so I don’t have to duplicate all that logic in my plugin?

I guess one thing I didn’t try is returning given constraints from my solver instead of just solving them and providing evidence. That is, if my plugin received a

    [W] d1 :: Append "foo" "bar" c

constraint, then instead of solving the constraint directly, I could leave it alone and instead return a new constraint

    [G] d2 :: Append "foo" "bar" "baz"

and presumably GHC’s solver would use that constraint to improve and solve d1. But similar to my confusion about type families above, I’m uncertain if that’s the intended method or not, since it seems like it’s sort of circumventing the plugin API.

Finally, on a related note, building evidence for these solver-generated typeclass instances is a bit of a pain. They have no methods, but they do sometimes have superclasses. Currently, I’ve been generating CoreExprs as carefully as I’m able to after reading through the desugaring code: I call dataConWrapId on the result of classDataCon, then use mkTyConApp and mkCoreApps on that directly. It seems to work okay now, but it didn’t always: -dcore-lint thankfully caught my mistakes, but I’ve been wondering if there’s a safer way to build the dictionary that I’ve been missing.

That’s it for now—I’ve just been muddling through until things work. Once I get something that feels closer to right, maybe I’ll put the code somewhere and ask for more low-level comments if anyone would like to take the time to offer them, but for now, I’m still working on the high-level ideas. The wiki pages I’ve found have been very helpful; my appreciation to all who have contributed to them!

Many thanks,
Alexis

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