TTG hsSyn for Batch and Interactive Parsing

Simon Peyton Jones simonpj at
Tue May 8 08:54:44 UTC 2018

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> On Behalf Of Alan & Kim Zimmerman
Sent: 07 May 2018 16:17
To: ghc-devs <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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