TTG hsSyn for Batch and Interactive Parsing

Simon Peyton Jones simonpj at
Wed May 9 08:15:00 UTC 2018


I am absolutely behind this objective:
I propose to move the API Annotations to where they belong, inside the AST.
Indeed I thought that was always part of the TTG plan.

But I don’t understand what this has to do with interactive vs batch parsing.  Why don’t you unconditionally retain API-annotation info?  How would GhcPs be used differently to GhcPsI?

You might want to answer by clarifying on the wiki page, so that it is a persistent record of the design debugged in dialogue by email.


From: Alan & Kim Zimmerman <alan.zimm at>
Sent: 08 May 2018 21:02
To: Simon Peyton Jones <simonpj at>
Cc: ghc-devs <ghc-devs at>
Subject: Re: TTG hsSyn for Batch and Interactive Parsing

I have started a wiki page at

On 8 May 2018 at 10:54, Simon Peyton Jones <simonpj at<mailto:simonpj at>> wrote:
At first blush, “running the parser in two modes” and “changing the Pass” type don’t match up in my mind.  One seems quite local (how to run the parser).  The other seems more pervasive.

Can you say more about your proposed design, perhaps even on a wiki page?


From: ghc-devs <ghc-devs-bounces at<mailto:ghc-devs-bounces at>> On Behalf Of Alan & Kim Zimmerman
Sent: 07 May 2018 16:17
To: ghc-devs <ghc-devs at<mailto:ghc-devs at>>
Subject: TTG hsSyn for Batch and Interactive Parsing

I want to be able to run the GHC parser in one of two modes, batch which functions as now, and interactive which will (eventually) be incremental.
In addition, the hsSyn AST for each will have different TTG[1] annotations, so that it can better support IDE usage.
I think this can be done by changing the types in HsExtension to introduce a 'Process'  type as follows

data Pass = Parsed Process | Renamed | Typechecked
         deriving (Data)

data Process = Batch | Interactive
  deriving (Show, Data)
We then rename the pass synonyms so that batch is the default

type GhcPs   = GhcPass ('Parsed 'Batch)
type GhcPsI  = GhcPass ('Parsed 'Interactive)
I have attached a simple proof of concept file, which emulates parsing and renaming.
Is this an appropriate approach to take?

[1] Trees That Grow

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