Trees that Grow in the hsSyn AST

Simon Peyton Jones simonpj at
Thu May 25 22:48:47 UTC 2017


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 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.


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Alan & Kim Zimmerman
Sent: 24 May 2017 22:52
To: ghc-devs at
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.

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