Tuple predicates in Template Haskell

[Richard Eisenberg]

Hello Yorick,

Thanks for taking this one on!

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.

Now, on to your question:

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.

Once again, thanks!
Richard

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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