Trees that Grow in the hsSyn AST

David Feuer david at
Tue May 30 21:05:41 UTC 2017

On Friday, May 26, 2017 9:03:15 AM EDT Simon Peyton Jones wrote:
> 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.

Don't get me wrong. I wasn't suggesting that Core should come first; I have absolutely no basis to make any suggestion in that regard. I was just wondering what led to the decision to start with HsSyn. Are you suggesting that Core wouldn't benefit from these ideas? I surely don't see why not. Information about arity and strictness, at least, is introduced in specific compiler phases. I believe that some information needed for join points is only valid/available between certain phases. Making such things explicit in the types seems like it can only help.

> 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.

All right; I figured it wouldn't hurt to ask. May I ask what sorts of scaling problems you mean? 


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