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 ; Alan & Kim Zimmerman ; 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 mailto:ghc-devs@haskell.org> Date: 5/25/17 6:48 PM (GMT-05:00) To: Alan & Kim Zimmerman mailto:alan.zimm@gmail.com>, ghc-devs@haskell.orgmailto: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.orgmailto: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=0http://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