GHC 8 and Template Haskell

Richard Eisenberg eir at
Thu Apr 14 20:59:16 UTC 2016

I'm happy to be overruled on this, but I vote against this change for GHC 8. Personally, I like to have a policy of "no TH changes after the first RC". This gives ample time for TH clients to update their code. Iavor's suggestion would likely involve a new part of the InstanceD constructor, which would affect anyone constructing or matching on this constructor. If the change involved, say, only adding new functionality without changing anything existing, I would be more willing to include for GHC 8.

And, yes, I agree with Austin.


On Apr 14, 2016, at 2:17 PM, Austin Seipp <austin at> wrote:

> Here's a question, on top of this one: why don't we require
> template-haskell changes for most corresponding syntax changes? We
> tend to play catch up with template-haskell sometimes and it's
> relatively strange. I mean, in some sense, we could have said a while
> back "This needs another revision, please add template haskell
> support" and avoided it all.
> Richard has a better insight into this than I do, I'm sure, but it
> seems - to me, anyway - like template-haskell support is a reasonable
> bar for most surface-level syntax change to cross, before getting
> merged.
> My intuition tells me that, most of the time, a lot of us simply
> forget to make the changes, or ask for them in reviews, and so it
> goes.
> On Thu, Apr 14, 2016 at 1:12 PM, Ben Gamari <ben at> wrote:
>> Iavor Diatchki <iavor.diatchki at> writes:
>>> Hello,
>>> Now that "OVERLAPPING" and "OVERLAPPABLE" are pragmas on the instances, do
>>> we have a way to generate instances with such pragmas using Template
>>> Haskell?   I can't seem to find a way to do this, which is unfortunate.
>>> If I am not missing anything, would there be objections to adding it to the
>>> TH library before the next release---I would volunteer to do the change
>>> ASAP.
>> Indeed this is an unfortunate gap. Given that this shouldn't be a
>> terribly invasive change I would be alright with this if Richard approves.
>> Cheers,
>> - Ben
>> _______________________________________________
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>> Libraries at
> -- 
> Regards,
> Austin Seipp, Haskell Consultant
> Well-Typed LLP,

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