First off, this kind of question/post is appropriate for putting right into the ticket itself. Posting a comment to the ticket makes it more likely that you'll get a response and saves your thoughts for posterity.

That seems somewhat reasonable, but I think your work could go a little further. It looks like you've introduced TupleP as a new constructor for Pred. This, I believe, would work. But, I think it would be better to have a way of using *any* type as a predicate in TH, as allowed by ConstraintKinds. Perhaps one way to achieve this is to make Pred a synonym of Type, or there could be a TypeP constructor for Pred.

In any case, I would recommend writing a wiki page up with a proposed new TH syntax for predicates and then posting a link to the proposal on the #7021 ticket. Then, it will be easier to debate the merits of any particular approach.

On Jan 3, 2014, at 6:13 PM, Yorick Laupa wrote:

Hi,

I try to make my way through #7021 [1]. Unfortunately, there is nothing in the ticket about what should be expected from the code given as example.

I came with an implementation and I would like feedback from you guys. So, considering this snippet:

--
{-# LANGUAGE ConstraintKinds #-}

type IOable a = (Show a, Read a)

foo :: IOable a => a
foo = undefined
--

This is what I got now when pretty-printing TH.Info after reify "foo" call:

VarI Tuple.foo (ForallT [PlainTV a_1627398594] [TupleP 2 [AppT (ConT GHC.Show.Show) (VarT a_1627398594),AppT (ConT GHC.Read.Read) (VarT a_1627398594)]] (VarT a_1627398594)) Nothing (Fixity 9 InfixL)

Does that sound right to you ?

Thanks for your time

-- Yorick

[1] https://ghc.haskell.org/trac/ghc/ticket/7021
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