Re: Proposal: ValidateMonoLiterals - Initial bikeshed discussion

Hi Merijn, Thanks for persevering with explaining things to me. :-) On 09/02/15 09:47, Merijn Verstraaten wrote:

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On 6 Feb 2015, at 21:31, Adam Gundry wrote: What does "all monomorphic cases" mean? Is the idea what GHC would typecheck an expression involving a literal integer, determine that the occurrence had type Even, then evaluate the TH splice *after* typechecking? Whereas if the occurrence had a non-ground type, it would do something else?

Yes, Typed TH already runs *after* type checking, which is what allows you to do validation based on the resulting type. The main reason why I was only proposing to do this for monomorphic values is, because, how could you possible validate a polymorphic literal? Which validation function would you use?

You could ban polymorphic literals, but that'd involve eliminating most uses of polymorphic Num functions (as I mentioned as another email), which would break so much code it would render the extension unusable in "real" code. I'm open to better ideas on how to tackle this, the main reason I started this discussion is because I don't really like this "polymorphic literals fail at compile time" thing either. I just don't see how to solve it without going all dependent types on the problem.

In the absence of a coherent story for polymorphism, I think the right thing to do is to be able to specify a particular validator, rather than try to have type inference determine a monomorphic type and otherwise get stuck...

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None of this is particularly persuasive, I'm afraid. Is it worthwhile introducing something new just to avoid having to write a quasi quote?

Actually, I would be mildly ok with quasi quoters, BUT there currently is no Typed TH quasi quoter (as mentioned on the wiki page), additionally, such a quoter does not have access to Lift instances for all but a handful of datatypes until we have a more comprehensive way to generate Lift instances. I think both of these points are also highly relevant for this dicussion.

...so is the right solution to introduce Typed TH quasiquoters for expressions? Sorry, I presumed such a thing existed, as Typed TH is rather regrettably underdocumented. Is there any particular difficulty with them, or is it just a Small Matter of Programming? I think the lack of Lift instances is a separate problem; while it looks like 7.10 will be better in this respect and dataToExpQ goes a fair way, I agree that making them easier to generate would be nice. Perhaps a generics-based default method combined with DeriveAnyClass would make "deriving Lift" possible? Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/