Current description of Core?

Simon Peyton Jones simonpj at
Wed Oct 22 09:59:09 UTC 2014

Interesting.  There is a pretty high bar for changes to Core itself.  Currently arrow notation desugars into Core with no changes.  If you want to change Core, then arrow “notation” is actually much more than syntactic sugar.   Go for it – but it would be a much more foundational change than previously, and hence would require more motivation.


From: Sophie Taylor [mailto:sophie at]
Sent: 22 October 2014 10:53
To: Simon Peyton Jones
Cc: ghc-devs at
Subject: Re: Current description of Core?

Ah, thanks HEAPS. I've been banging my head against a wall for the last few days trying to see exactly what is going on :) I'm trying to find a way to minimise/eliminate the changes required to Core for the arrow notation rewrite  - specifically, introducing kappa abstraction and application - semantically different to lambda abstraction/application but close enough that I can probably get away with either adding a simple flag to the Abstraction/Application constructors or doing it higher up in the HsExpr land, but the latter method leaves a sour taste in my mouth.

On 22 October 2014 19:35, Simon Peyton Jones <simonpj at<mailto:simonpj at>> wrote:
Is the current description of Core still System FC_2 (described in

We never implemented that particular version (too complicated!).

This is the full current story (thanks to Richard for keeping it up to date), in the GHC source tree


From: ghc-devs [mailto:ghc-devs-bounces at<mailto:ghc-devs-bounces at>] On Behalf Of Sophie Taylor
Sent: 22 October 2014 10:26
To: ghc-devs at<mailto:ghc-devs at>
Subject: Current description of Core?


Is the current description of Core still System FC_2 (described in

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