[Template-haskell] GADTs

[Simon Peyton-Jones]

| Also, TH tries to present the completely sugared language to the user;
| otherwise it would probably work on something more like core.

This really is a tension, and one I don't really know how to resolve.  

GHC does represent fully-sugared syntax, including even the placement of
parens.

I don't think TH need go to these lengths.  TH generates code that we
might read, but which is mainly intended to be compiled.  It's *also*
intended to be processed by other TH code, so the smaller the TH data
type, the better.  

I conclude that TH should avoid gratuitous syntactic sugar.  Anything
that can be converted to a simpler equivalent form, should be.  Hence I
rather think that infix operators in TH are a mistake.  What do they
really buy us?  Similarly an if-expression, and arithmetic sequence, and
list expressions.   On the other hand, a list comprehension is much more
complicated to desugar, so probably deserves to be there.

It'd be good to discuss this and perhaps agree some changes.

Concerning data types, I think it'd be fine to present data types in a
single, canonical representation as a data type, probably something like
the GADT style.  That'd be a breaking change, mind you.


Because there are design choices here, I'm dubious about getting any of
this into 6.6.  Time is very short, and we don't want to make a change
that we re-change later. 

Simon