I would like to submit a solution E, which is just a variant of D (or a meshing together of D and B), but may have different pros and cons.
Keep the ping-pong style. But! Let Located take the pass as an argument. Now, Located would be data Located p a = L (XLoc p) a We could define XLoc p to be a source span in GHC passes, and () in Template Haskell. ------------------------------ I’m not arguing in favour of E, at this point: just submitting an alternative. I don’t like A though: I’m assuming that if we are free to add the L constructor or not, it will be forgotten often. This is the sort of mistake which will be hard to catch before releases. It sounds like unnecessary risk. PS: Maybe a friendlier version of Located, in this solution E, could be data Located a p = L (XLoc p) (a p) On Mon, Oct 28, 2019 at 11:46 AM Richard Eisenberg <[email protected]> wrote: > A nice property of Solution D (and I think this is a new observation) is > that we could accommodate both located source and unlocated. For example: > > > data GhcPass (c :: Pass) -- as today > > data Pass = MkPass Phase HasLoc > > data Phase = Parsed | Renamed | Typechecked > > data HasLoc = YesLoc | NoLoc > > > > type instance WrapL (GhcPass (MkPass p YesLoc)) f = Located (f (GhcPass > (MkPass p YesLoc))) > > type instance WrapL (GhcPass (MkPass p NoLoc)) f = f (GhcPass (MkPass p > NoLoc)) > > I don't actually think this is a good idea, as it would mean making a > bunch of functions polymorphic in the HasLoc parameter, which is > syntactically annoying. But the existence of this approach suggests that > the design scales. > > Regardless of what you think of this new idea, I'm in favor of Solution D. > I like my types to stop me from making errors, and I'm willing to put up > with the odd type error asking me to write `unLoc` as I work in order to > avoid errors. > > Richard > > > On Oct 28, 2019, at 10:30 AM, Vladislav Zavialov <[email protected]> > wrote: > > > >> Are you arguing for Solution D? Or are you proposing some new solution > E? I can't tell. > > > > I suspect that I’m arguing for Solution B, but it’s hard to tell because > it’s not described in enough detail in the Wiki. > > > >> Easy > >> > >> type instance WrapL ToolPass t = ... > >> > >> What am I missing? > > > > > > This assumes that `WrapL` is an open type family. In this case, there’s > no problem. The merge request description has the following definition of > WrapL: > > > > type instance WrapL p (f :: * -> *) where > > WrapL (GhcPass p) f = Located (f (GhcPass p)) > > WrapL p f = f p > > type LPat p = WrapL p Pat > > > > That wouldn’t be extensible. However, if WrapL is open, then Solution D > sounds good to me. > > > > - Vlad > > > >> On 28 Oct 2019, at 13:20, Simon Peyton Jones <[email protected]> > wrote: > >> > >> Vlad > >> > >> Are you arguing for Solution D? Or are you proposing some new solution > E? I can't tell. > >> > >> > >> | As to merge request !1970, it isn’t good to special-case GhcPass in a > >> | closed type family, making other tools second-class citizens. Let’s > say I > >> | have `MyToolPass`, how would I write an instance of `WrapL` for it? > >> > >> Easy > >> > >> type instance WrapL ToolPass t = ... > >> > >> What am I missing? > >> > >> Simon > >> > >> | -----Original Message----- > >> | From: Vladislav Zavialov <[email protected]> > >> | Sent: 28 October 2019 10:07 > >> | To: Simon Peyton Jones <[email protected]> > >> | Cc: [email protected] > >> | Subject: Re: Handling source locations in HsSyn via TTG > >> | > >> | I care about this, and I maintain my viewpoint described in > >> | > https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmail.has > >> | kell.org%2Fpipermail%2Fghc-devs%2F2019- > >> | February%2F017080.html&data=02%7C01%7Csimonpj%40microsoft.com > %7C06c859 > >> | > d8bc0f48c73cb208d75b8e895c%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C63 > >> | > 7078540055975603&sdata=FhkqfWGXaNX%2Fz4IiCcvoVCyzVsSAlyz6Y1dxEGUjX9I%3 > >> | D&reserved=0 > >> | > >> | I’m willing to implement this. > >> | > >> | As to merge request !1970, it isn’t good to special-case GhcPass in a > >> | closed type family, making other tools second-class citizens. Let’s > say I > >> | have `MyToolPass`, how would I write an instance of `WrapL` for it? > >> | > >> | - Vlad > >> | > >> | > On 28 Oct 2019, at 12:31, Simon Peyton Jones via ghc-devs <ghc- > >> | [email protected]> wrote: > >> | > > >> | > Friends > >> | > > >> | > As you know > >> | > > >> | > • We are trying to use “Trees That Grow” (TTG) to move HsSyn > towards > >> | a situation in which GHC is merely a client of a generic HsSyn data > type > >> | that can be used by clients other than GHC. > >> | > • One sticking point has been the question of attaching source > >> | locations. We used to have a “ping-pong” style, in which very node is > >> | decorated with a source location, but that’s a bit more awkward in > TTG. > >> | > • This wiki page outlines some choices, while ticket #15495 has a > >> | lot of discussion. > >> | > • HEAD embodies Solution A. But it has the disadvantage that the > >> | type system doesn’t enforce locations to be present at all. That has > >> | undesirable consequences (eg ticket #17330) > >> | > • The proposal is to move to Solution D on that page; you can see > >> | how it plays out in MR !1970. > >> | > • (I think Solutions B and C are non-starters by comparison.) > >> | > If you care, please check out the design and the MR. We can change > >> | later, of course, but doing so changes a lot of code, including client > >> | code, so we’d prefer not to. > >> | > > >> | > Let’s try to converge by the end of the week. > >> | > > >> | > Thanks > >> | > > >> | > Simon > >> | > > >> | > > >> | > > >> | > > >> | > > >> | > _______________________________________________ > >> | > ghc-devs mailing list > >> | > [email protected] > >> | > > >> | > https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.hask > >> | ell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc- > >> | devs&data=02%7C01%7Csimonpj%40microsoft.com > %7C06c859d8bc0f48c73cb208d7 > >> | > 5b8e895c%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637078540055975603&a > >> | > mp;sdata=CRYEYmjuAYYIhoAeTPbi%2FctCiWmkIjL6pKUTBgVBwo8%3D&reserved=0 > >> > > > > _______________________________________________ > > ghc-devs mailing list > > [email protected] > > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > _______________________________________________ > ghc-devs mailing list > [email protected] > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >
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