<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Oct 27, 2021, at 9:36 AM, Benjamin Redelings <<a href="mailto:benjamin.redelings@gmail.com" class="">benjamin.redelings@gmail.com</a>> wrote:</div><div class=""><div class=""><blockquote type="cite" cite="mid:010f017c8507c42b-13b49b21-3b8e-4a8d-93b6-56c60f61ed60-000000@us-east-2.amazonses.com" class=""><div class=""><div class=""><br class="">
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<div class="">This won't work.</div>
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</div>
<div class="">class C a where</div>
<div class=""> meth :: a b -> b Int</div>
<div class=""><br class="">
</div>
<div class="">You have to know the kind of local b to learn the kind of
class-variable a. So you have to do it all at once.</div>
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</blockquote><p class="">I was doing it all at once -- but I wasn't sure how to export the
information about 'b' from the procedure. (After you record the
kinds of the typecons like C, I believe the different typecons in
the recursive group become separable.)<br class="">
</p><p class="">I presume (in retrospect) that GHC modifies the declaration to
record the kind of b, and then re-walks the declaration to
substitute kind variables later?<br class=""></p></div></div></blockquote><div>That would be smart, but it's not what GHC does. Instead, GHC first says (a :: kappa), for a fresh unification variable kappa. Then, GHC determines (b :: Type -> Type) and thus unifies kappa := (Type -> Type) -> Type. GHC then *throws away* the information about b. (See the `_ <- ...` at <a href="https://gitlab.haskell.org/ghc/ghc/-/blob/638f65482ca5265c268aa97abfcc14cdc27e46ba/compiler/GHC/Tc/Gen/HsType.hs#L388" class="">https://gitlab.haskell.org/ghc/ghc/-/blob/638f65482ca5265c268aa97abfcc14cdc27e46ba/compiler/GHC/Tc/Gen/HsType.hs#L388</a>) Having now determined conclusively what the kind of `a` is, GHC will later re-check the kind of the type of meth, recording it for good.</div><blockquote type="cite" class=""><div class=""><p class="">
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<blockquote type="cite" cite="mid:010f017c8507c42b-13b49b21-3b8e-4a8d-93b6-56c60f61ed60-000000@us-east-2.amazonses.com" class="">
<div class=""></div></blockquote><p class="">But if that is a difficulty on the right road, I will just go for
it. <br class=""></p></div></blockquote><div>If zonking is a difficulty, there's probably something wrong somewhere. Lengthy, yes, but not difficult. If you're using Data.Data, I imagine you could implement zonking with just the right use of Data.Generics.Schemes.everywhere.</div><div><br class=""></div><div><blockquote type="cite" class=""><div class=""><p class="">Oh, I forgot to add, would it make sense to put some of the information I discovered about implementing kind checking into the wiki somewhere?</p></div></blockquote><div>This is an excellent idea, but I'll counterpropose that you write this in a Note. Indeed, Note [Kind checking for type and class decls] in GHC.Tc.TyCl is probably meant to be the Note you're looking for, but it's too short. Feel free to flesh it out with all of these details. Then, make sure that the functions that implement the details refer to the Note (and vice versa). This will help it to stay up-to-date -- much more so than putting it in the wiki.</div><blockquote type="cite" class=""><div class=""><p class="">It might also help to reference the relevant papers (mostly the PolyKinds paper), and maybe also to mention papers like the THIH paper that don't actually implement kind checking.<br class=""></p></div></blockquote><div>Yes, refer to papers. But please do use their proper names (years and conference names are also helpful). It actually took some inference for me to figure out what the "PolyKinds paper" was in your emails. (The paper that proposes the extension that became PolyKinds is "Giving Haskell a Promotion", TLDI'12, and the paper that describes its extension to work more generally is "System FC with Explicit Kind Equality", ICFP'13. I thought you were referring to one of these -- not to "Kind Inference for Datatypes".)</div><div><br class=""></div><div>Thanks!</div><div>Richard</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><p class="">-BenRI<br class=""></p><div class="moz-cite-prefix">On 10/15/21 1:37 PM, Richard Eisenberg wrote:<br class=""></div><blockquote type="cite" cite="mid:010f017c8507c42b-13b49b21-3b8e-4a8d-93b6-56c60f61ed60-000000@us-east-2.amazonses.com" class=""><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Oct 14, 2021, at 11:59 AM, Benjamin Redelings <<a href="mailto:benjamin.redelings@gmail.com" class="moz-txt-link-freetext" moz-do-not-send="true">benjamin.redelings@gmail.com</a>> wrote:</div><div class=""><div class=""><p class=""><br class="">I asked about this on Haskell-Cafe, and was recommended to ask here instead. Any help is much appreciated!<br class=""></p></div></div></blockquote><div class=""><br class=""></div><div class="">I saw your post over there, but haven't had time to respond.... but this retelling of the story makes it easier to respond, so I'll do so here.</div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><p class="">* The PolyKinds paper was the most helpful thing I've found, but it doesn't cover type classes. I'm also not sure that all implementers can follow algorithm descriptions that are laid out as inference rules, but maybe that could be fixed with a few hints about how to run the rules in reverse. Also, in practice I think an implementer would want to follow GHC in specifying the initial kind of a data type as k1 -> k2 -> ... kn -> *. <br class=""></p></div></div></blockquote><div class=""><br class=""></div><div class="">What is unique about type classes? It seems like you're worried about locally quantified type variables in method types, but (as far as kind inference is concerned) those are very much like existential variables in data constructors. So perhaps take the bit about existential variables from the PolyKinds part of that paper and combine it with the Haskell98 part.</div><div class=""><br class=""></div><div class="">It's true that many implementors may find the notation in that paper to be a barrier, but you just have to read the rules clockwise, starting from the bottom left and ending on the bottom right. :)</div><blockquote type="cite" class=""><div class=""><div class=""><p class=""><br class=""></p><p class="">2. The following question (which I have maybe kind of answered now, but could use more advice on) is an example of what I am hoping such documentation would explain: <br class=""><br class=""></p><blockquote type="cite" class="" style="color: rgb(25, 180, 255);">Q: How do you handle type variables that are present in class methods, but are not type class parameters? If there are multiple types/classes in a single recursive group, the kind of such type variables might not be fully resolved until a later type-or-class is processed. Is there a recommended approach? <br class=""><br class="">I can see two ways to proceed: <br class=""><br class="">i) First determine the kinds of all the data types, classes, and type synonyms. Then perform a second pass over each type or class to determine the kinds of type variables (in class methods) that are not type class parameters. <br class=""></blockquote></div></div></blockquote><div class=""><br class=""></div><div class="">This won't work.</div><div class=""><br class=""></div><div class="">class C a where</div><div class=""> meth :: a b -> b Int</div><div class=""><br class=""></div><div class="">You have to know the kind of local b to learn the kind of class-variable a. So you have to do it all at once.</div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><blockquote type="cite" class="" style="color: rgb(25, 180, 255);"><br class="">ii) Alternatively, record the kind of each type variable as it is encountered -- even though such kinds may contain unification kind variables. After visiting all types-or-classes in the recursive group, replace any kind variables with their definition, or with a * if there is no definition. <br class=""><br class="">I've currently implement approach i), which requires doing kind inference on class methods twice. <br class=""><br class=""></blockquote>Further investigation revealed that GHC takes yet another approach (I think): <br class=""><br class="">iii) Represent kinds with modifiable variables. Substitution can be implemented by modifying kind variables in-place. This is (I think) called "zonking" in the GHC sources. <br class=""></div></div></blockquote><div class=""><br class=""></div><div class="">I don't really see the difference between (ii) and (iii). Maybe (ii) records the kinds in a table somewhere, while (iii) records them "in" the kind variables themselves, but that's not so different, I think.</div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><br class="">This solves a small mystery for me, since I previously thought that zonking was just replacing remaining kind variables with '*'. And indeed this seems to be an example of zonking, but not what zonking is (I think). <br class=""></div></div></blockquote><div class=""><br class=""></div><div class="">We can imagine that, instead of mutation, we build a substitution mapping unification variables to types (or kinds). This would be stored just as a simple mapping or dictionary structure. No mutation. As we learn about a unification variable, we just add to the mapping. If we did this, zonking would be the act of applying the substitution, replacing known unification variables with their values. It just so happens that GHC builds a mapping by using mutable cells in memory, but that's just an implementation detail: zonking is still just applying the substitution.</div><div class=""><br class=""></div><div class="">Zonking does <i class="">not</i><span class=""> replace anything with *. Well, functions that have "zonk" in their name may do this. But it is not really logically part of the zonking operation. If you like, you can pretend that, after zonking a program, we take a separate pass replacing any yet-unfilled kind-level unification variables with *. Sometimes, this is called "zapping" in GHC, I believe.</span></div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><br class="">Zonking looks painful to implement, but approach (i) might require multiple passes over types to update the kind of type variables, which might be worse...<br class=""></div></div></blockquote><div class=""><br class=""></div><div class="">Zonking is a bit laborious to implement, but not painful. Laborious, because it requires a full pass over the AST. Not painful, because all it's trying to do is replace type/kind variables with substitutions: each individual piece of the puzzle is quite simple.</div></div><br class=""><div class="">I hope this is helpful!</div><div class="">Richard</div></blockquote></div></blockquote><div class=""><div class=""><blockquote type="cite" cite="mid:010f017c8507c42b-13b49b21-3b8e-4a8d-93b6-56c60f61ed60-000000@us-east-2.amazonses.com" class=""><div class=""><br class=""></div></blockquote></div></div></div></div></body></html>