Tuple predicates in Template Haskell

[Yorick Laupa]

Thanks Richard !

I followed your advice and posted a new comment to the ticket

Yorick


2014/1/6 Richard Eisenberg <eir at cis.upenn.edu>

> 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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