Handling source locations in HsSyn via TTG

Wed Oct 30 13:05:57 UTC 2019

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 <rae at richarde.dev> 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 <vladislav at serokell.io>
> 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 <simonpj at microsoft.com>
> 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 <vladislav at serokell.io>
> >> | Sent: 28 October 2019 10:07
> >> | To: Simon Peyton Jones <simonpj at microsoft.com>
> >> | Cc: ghc-devs at haskell.org
> >> | 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-
> >> | devs at haskell.org> 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
> >> | >
> >> | >
> >> | >
> >> | >
> >> | >
> >> | > _______________________________________________
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> >> | > ghc-devs at haskell.org
> >> | >
> >> |
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> >
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