classP recently deleted from TH.Lib

[Richard Eisenberg]

This one was my fault/decision. The TH.Lib functions tend to mirror exactly the constructors in TH.Syntax. We removed the constructors (for good reason -- predicates and types really are the same now), so I thought it best to remove the TH.Lib function, too. In the cases where code had to be updated, would keeping classP in be enough to prevent the breakage?

We could, of course, just leave the functions there with new implementations, but that feels like it could accumulate legacy functions over time.

Here are some ideas for this, and other similar situations, going forward:
1) Don't remove functions from TH.Lib unless absolutely necessary (which should probably never happen).
2) Remove functions from TH.Lib, but support some other package ("th-compat") which fills in the compatibility gap. This package would not be tied in with GHC. It would use CPP to export functions in order to remain compatible with a range of GHC versions. In this example, the package would export a fresh classP in 7.9+ but just re-export TH's in 7.8-.
3) Do what I've done -- keep TH.Lib parallel with TH.Syntax and leave it to users to sort it out as they see fit.

It is worth noting that a change of this sort in TH.Lib corresponds to a breaking change in TH.Syntax. But, perhaps most people rely on Lib and not on Syntax.

I'm happy to follow what the community thinks is best here -- I don't have any vested opinion.

Thanks -- and sorry for the breakage!
Richard

On May 12, 2014, at 10:35 AM, Johan Tibell <johan.tibell at gmail.com> wrote:

> That would be nice. I had to fix some breakage caused by this in one of Bryan's libraries.
> 
> 
> On Mon, May 12, 2014 at 4:12 PM, Gabor Greif <ggreif at gmail.com> wrote:
> The last two commits from (Apr 9) on
> 
> <https://github.com/ghc/packages-template-haskell/commits/master/Language/Haskell/TH/Lib.hs>
> 
> removed a helper to construct applied type class constraints (`classP`).
> 
> Some packages (notably `llvm-general-pure`) though, depend on it.
> 
> Can we add back something like this:
> 
> {{{
> classP :: Name -> [Q Type] -> Q Pred
> classP cla tys
>   = do
>       tysl <- sequence tys
>       return (foldl' AppT (ConT cla) tysl)
> }}}
> 
> and export it again? Or is there such a helper with a different name already?
> 
> Cheers,
> 
>     Gabor
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