RE: Trees that Grow in the hsSyn AST
1. Which is better to start with: HsSyn or Core? Intuition suggests this sort of thing could be very helpful for making zapping more reliable and ensuring its efficiency, but there may be better reasons to start with HsSyn. Definitely HsSyn. It’s big, riddled with extra info, and has many purposes for different people. Core is small, tight, nailed down. I don’t want to mess with it. 2. If we're making intrusive changes to representations, would now be a sensible era to consider switching to a different variable representation (unbound, bound, abt, etc)? I don’t think so. The issues are quite orthogonal, and no one (to my knowledge) has proposed any vaguely plausible change to variable bindings that would scale to what GHC does. In contrast, this stuff is “just” re-engineering. Simon From: David Feuer [mailto:david@well-typed.com] Sent: 26 May 2017 01:11 To: Simon Peyton Jones <simonpj@microsoft.com>; Alan & Kim Zimmerman <alan.zimm@gmail.com>; ghc-devs@haskell.org Subject: RE: Trees that Grow in the hsSyn AST I haven't looked in detail yet, but there seem to be good ideas. I have two questions: 1. Which is better to start with: HsSyn or Core? Intuition suggests this sort of thing could be very helpful for making zapping more reliable and ensuring its efficiency, but there may be better reasons to start with HsSyn. 2. If we're making intrusive changes to representations, would now be a sensible era to consider switching to a different variable representation (unbound, bound, abt, etc)? David Feuer Well-Typed, LLP -------- Original message -------- From: Simon Peyton Jones via ghc-devs <ghc-devs@haskell.org<mailto:ghc-devs@haskell.org>> Date: 5/25/17 6:48 PM (GMT-05:00) To: Alan & Kim Zimmerman <alan.zimm@gmail.com<mailto:alan.zimm@gmail.com>>, ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> Subject: RE: Trees that Grow in the hsSyn AST Folks Do take a look at this: · We propose to re-engineer HsSyn itself. This will touch a lot of code. · But it’s very neat, and will bring big long-term advantages · And we can do it a bit at a time The wiki page https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow has the details. It’s entirely an internal change, not a change to GHC’s specification, so it’s independent of the GHC proposals process. But I’d value the opinion of other GHC devs. Alan has done a prototype first step, which worked out rather well. Rather than having HsExpr Id (which we all know means “HsExpr after the typechecker” but tha’s a bit inexplicit, we have HsExpr GhcTc meaning “HsExpr after GHC’s Tc pass”. In some ways this is quite superficial, but it unlocks the Trees That Grow machiney. Please ask questions etc. Alan and Shayan can record the answers in the wiki. I’m inclined to go ahead with this, so yell soon if you disagree. Simon From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Alan & Kim Zimmerman Sent: 24 May 2017 22:52 To: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> Subject: Trees that Grow in the hsSyn AST Hi all You may be aware that Shayan Najd presented the paper "Trees that Grow"[1] at HIW last year. Based on the following mandate
As in my previous email to Shayan (attached). Wiki page, describe goals, design, approach. Point to prototype implementation. Seek comments. You can say that I am supportive!
Simon
We have set up a Wiki page at [2] describing a prototype implementation of the first stage of this for the hsSyn AST, which is to change the polymorphic variable from one of RdrName / Name / Id to an index type. This is presented as a fabricator diff at [3]. Please take a look and provide feedback. Regards Alan [1] http://www.jucs.org/jucs_23_1/trees_that_grow/jucs_23_01_0042_0062_najd.pdf<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.jucs.org%2Fjucs_23_1%2Ftrees_that_grow%2Fjucs_23_01_0042_0062_najd.pdf&data=02%7C01%7Csimonpj%40microsoft.com%7C5faccc0d2d534c42c23e08d4a2ef36d8%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C636312595690311243&sdata=fbLJdJqSyXgacCEJwVH880aLsHDgDY46hrc%2FtDXv4VQ%3D&reserved=0<http://www.jucs.org/jucs_23_1/trees_that_grow/jucs_23_01_0042_0062_najd.pdf%3chttps:/na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.jucs.org%2Fjucs_23_1%2Ftrees_that_grow%2Fjucs_23_01_0042_0062_najd.pdf&data=02%7C01%7Csimonpj%40microsoft.com%7C5faccc0d2d534c42c23e08d4a2ef36d8%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C636312595690311243&sdata=fbLJdJqSyXgacCEJwVH880aLsHDgDY46hrc%2FtDXv4VQ%3D&reserved=0>> [2] https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow [3] https://phabricator.haskell.org/D3609
On Friday, May 26, 2017 9:03:15 AM EDT Simon Peyton Jones wrote:
1. Which is better to start with: HsSyn or Core? Intuition suggests this sort of thing could be very helpful for making zapping more reliable and ensuring its efficiency, but there may be better reasons to start with HsSyn.
Definitely HsSyn. It’s big, riddled with extra info, and has many purposes for different people. Core is small, tight, nailed down. I don’t want to mess with it.
Don't get me wrong. I wasn't suggesting that Core should come first; I have absolutely no basis to make any suggestion in that regard. I was just wondering what led to the decision to start with HsSyn. Are you suggesting that Core wouldn't benefit from these ideas? I surely don't see why not. Information about arity and strictness, at least, is introduced in specific compiler phases. I believe that some information needed for join points is only valid/available between certain phases. Making such things explicit in the types seems like it can only help.
2. If we're making intrusive changes to representations, would now be a sensible era to consider switching to a different variable representation (unbound, bound, abt, etc)?
I don’t think so. The issues are quite orthogonal, and no one (to my knowledge) has proposed any vaguely plausible change to variable bindings that would scale to what GHC does. In contrast, this stuff is “just” re-engineering.
All right; I figured it wouldn't hurt to ask. May I ask what sorts of scaling problems you mean? David
| just wondering what led to the decision to start with HsSyn. Are you | suggesting that Core wouldn't benefit from these ideas? I surely don't Core, unlike HsSyn, is heavily optimised and transformed. It's hard to see how the transformations could soundly maintain arbitrary auxiliary information. Also unlike HsSyn, Core is a very small language, so it's no big deal to transform it into something else. | All right; I figured it wouldn't hurt to ask. May I ask what sorts of | scaling problems you mean? Simply maintaining a finite map from binders to another binder, or arbitrary other info (eg strictness) can be challenging. Try it! Simon | -----Original Message----- | From: David Feuer [mailto:david@well-typed.com] | Sent: 30 May 2017 22:06 | To: Simon Peyton Jones <simonpj@microsoft.com> | Cc: Alan & Kim Zimmerman <alan.zimm@gmail.com>; ghc-devs@haskell.org | Subject: Re: Trees that Grow in the hsSyn AST | | On Friday, May 26, 2017 9:03:15 AM EDT Simon Peyton Jones wrote: | > 1. Which is better to start with: HsSyn or Core? Intuition suggests | this sort of thing could be very helpful for making zapping more reliable | and ensuring its efficiency, but there may be better reasons to start | with HsSyn. | > | > Definitely HsSyn. It’s big, riddled with extra info, and has many | purposes for different people. Core is small, tight, nailed down. I | don’t want to mess with it. | | Don't get me wrong. I wasn't suggesting that Core should come first; I | have absolutely no basis to make any suggestion in that regard. I was | just wondering what led to the decision to start with HsSyn. Are you | suggesting that Core wouldn't benefit from these ideas? I surely don't | see why not. Information about arity and strictness, at least, is | introduced in specific compiler phases. I believe that some information | needed for join points is only valid/available between certain phases. | Making such things explicit in the types seems like it can only help. | | > 2. If we're making intrusive changes to representations, would now be a | sensible era to consider switching to a different variable representation | (unbound, bound, abt, etc)? | > | > I don’t think so. The issues are quite orthogonal, and no one (to my | knowledge) has proposed any vaguely plausible change to variable bindings | that would scale to what GHC does. In contrast, this stuff is “just” | re-engineering. | | All right; I figured it wouldn't hurt to ask. May I ask what sorts of | scaling problems you mean? | | David
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Simon Peyton Jones