Re: Handling source locations in HsSyn via TTG

28 Oct 2019

      ...
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  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 
| Sent: 28 October 2019 10:07
| To: Simon Peyton Jones 
| Cc: ghc-devs@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
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| 
| 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  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
| > ghc-devs@haskell.org
| >
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