[GHC] #7021: Tuple (and other exotic predicates) not yet handled in Template Haskell

[GHC]

#7021: Tuple (and other exotic predicates) not yet handled in Template Haskell
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        Reporter:        |            Owner:
  goldfire               |           Status:  new
            Type:        |        Milestone:  7.8.1
  feature request        |          Version:  7.5
        Priority:        |         Keywords:  ConstraintKinds
  normal                 |  TemplateHaskell
       Component:        |     Architecture:  Unknown/Multiple
  Template Haskell       |       Difficulty:  Unknown
      Resolution:        |       Blocked By:
Operating System:        |  Related Tickets:
  Unknown/Multiple       |
 Type of failure:        |
  None/Unknown           |
       Test Case:        |
        Blocking:        |
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Comment (by goldfire):

 This looks nice -- I can easily believe it works. Here is some feedback I
 have on the code:

 * I would want to see this handle `IrredPred` predicates, as well as
 tuples. That way, synonyms built with !ConstraintKinds would be fully
 supported by TH. I don't think this should be too much work beyond what
 you've already done.

 * I still think a change to !DsMeta is warranted. For example, I would
 imagine code like `[t| (Show a, (Read a, Num a)) => a -> a |]` would fail
 to compile, even though TH would now be able to represent such a
 predicate.

 * The functions `classP` and `equalP` in the `Lib` module are meant to
 echo the constructors of `Pred`. I would say that, now that `Pred` is a
 synonym, `classP` and `equalP` should be removed. (It took me a while to
 understand why all those functions are there in `Lib` -- the answer is
 that !DsMeta is ''much'' easier to write with those functions there.)
 Leaving `classP` and `equalP` doesn't cause anyone any real harm, but it
 would just be a small wart on the design that could easily be eliminated.

 * Along similar lines, you will probably find that you need an `equalityT`
 function in `Lib` when updating !DsMeta.

 * It looks like you left the old `Pred` commented-out in `Syntax`.

 * While it is probably too late now (and I don't think worth going back to
 fix), it's best to do whitespace changes in separate commits from
 substantive changes. Perhaps before editing a new file, removing the
 trailing whitespace, make a commit, and then go back and to the real
 changes. You can always then rebase at the end of your session to lump
 together all the whitespace commits.

 Continued thanks for working on this!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7021#comment:9>
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