Latest Template Haskell Breaks Package

[Erik Hesselink]

Hi Dominic,

It looks like just a representation change: a TySynEqn is a data type
containing a [Type] and a Type, and those were the original two
arguments. So it looks like with a little bit of CPP, you could
support both versions. Something like

#if MIN_VERSION_template_haskell(2,9,0)
...
#else
...
#endif

In general, I think each major release of template haskell has quite
some breaking changes, but I don't know of any place where they're
enumerated. The GHC changelog only has a couple of high level bullet
points.

Regards,

Erik

On Tue, Dec 23, 2014 at 2:20 PM, Dominic Steinitz <dominic at steinitz.org> wrote:
> I realise I should have sent this to the libraries list.
>
> Dominic Steinitz
> dominic at steinitz.org
> http://idontgetoutmuch.wordpress.com
>
> Begin forwarded message:
>
> From: Dominic Steinitz <dominic at steinitz.org>
> Subject: Latest Template Haskell Breaks Package
> Date: 23 December 2014 13:14:26 GMT
> To: Haskell-Cafe <haskell-cafe at haskell.org>
>
> Hello Fellow Haskellers,
>
> I have become a maintainer for yarr
> (https://hackage.haskell.org/package/yarr). This no longer compiles with
> ghc-7.8.3 because it specifies base == 4.6. Relaxing this to base >=4.6 &&
> <4.8 tells me I need a newer version of Template Haskell
>
> rejecting: template-haskell-2.7.0.0, 2.6.0.0, 2.5.0.0, 2.4.0.1, 2.4.0.0,
> 2.3.0.1, 2.3.0.0, 2.2.0.0 (conflict: yarr => template-haskell>=2.8 && <2.9)
>
>
> If I now relax the constraint for Template Haskell I get a compiler error as
> there has been a breaking change from Template Haskell 2.9 to 2.10.
>
> Data/Yarr/Utils/FixedVector/VecTuple.hs:45:16:
>     Couldn't match expected type ‘TypeQ -> Q Dec’
>                 with actual type ‘Q Dec’
>     The function ‘tySynInstD’ is applied to three arguments,
>     but its type ‘Name -> TySynEqnQ -> DecQ’ has only two
>
>
> And indeed looking at the changes in
> http://git.haskell.org/packages/template-haskell.git/commitdiff/ccd7891c536b29b8bea96eb92520f46e21390e39
> I can see that the function in question has changed.
>
> -tySynInstD :: Name -> [TypeQ] -> TypeQ -> DecQ
> -tySynInstD tc tys rhs =
> +tySynInstD :: Name -> [TySynEqnQ] -> DecQ
> +tySynInstD tc eqns =
>
>
> Did I miss some announcement of this breaking change and the advice on what
> to do about it?
>
> If I did can someone please point me at the relevant document. If not then I
> feel sad and would be very grateful if someone could help me as I know very
> little about Template Haskell.
>
> Many thanks
>
> Dominic Steinitz
> dominic at steinitz.org
> http://idontgetoutmuch.wordpress.com
>
>
>
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