#7021: Tuple (and other exotic predicates) not yet handled in Template Haskell -------------------------+------------------------------------------------- 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: | -------------------------+------------------------------------------------- Comment (by yoeight): Replying to [comment:9 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.

Do you have any pointer on what irreducible predicates are please ? If it's possible, could you provide a code sample that triggers irreducible predicate TH error ?

* 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. You were right, I got an error when compiling your code. I think we should use it in a new test case in order to increase code coverage. What do you think ?

* 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.

I agree. The thing is, I have `(add-to-list 'write-file-functions 'delete- trailing-whitespace)` in my `.emacs`. I didn't notice until I posted my patches.

Continued thanks for working on this! Thanks for your feedback !

Yorick -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7021#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler