[Haskell-cafe] Is there a way to query the availability of an extension? Or could we?

[David Feuer]

That name sounds fine. Whatever we do on this front won't become
terribly practically relevant for several years, even if it makes it
into the next Report, but it would be nice to have a better story in
four or five years.

On Tue, May 17, 2016 at 5:22 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> Hello David,
>
> This would be simple to add, since GHC 8.0 already generates
> macros for package versions.  The relevant code is
> compiler/main/DriverPipeline.hs search for "MIN_VERSION".
>
> I would bikeshed the macro name a little; how about something
> like LANGUAGE_ScopedTypeVariables?
>
> Cheers,
> Edward
>
> Excerpts from David Feuer's message of 2016-05-17 14:19:45 -0700:
>> We have __GLASGOW_HASKELL__ to tell us what GHC version we're running
>> (if we're running GHC), and Cabal sets up MIN_VERSION_blah macros and
>> (when applicable) the __HADDOCK_VERSION__ macro. But what if we're
>> running some other compiler? It seems rather painful to have to write
>> code that enables or disables various extensions based on what version
>> N of compiler C happens to support. Not to mention that this is even a
>> pain when just dealing with GHC, since it involves digging through
>> release notes or waiting to see how Travis throws up.
>>
>> Is there some better way? If not, could we add one?
>>
>> __Have_ScopedTypeVariables__
>>
>> could tell us if the ScopedTypeVariables extension is available. Then
>> instead of saying "We need ScopedTypeVariables" when we can
>> (painfully) do without, we can just use it precisely when we have it.