On Mon, Oct 5, 2015 at 9:02 PM, Erik Hesselink wrote:

On 5 October 2015 at 20:58, Sven Panne wrote:

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2015-10-05 17:09 GMT+02:00 Gershom B :

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[...] As for libraries, it has been pointed out, I believe, that without CPP one can write instances compatible with AMP, and also with AMP +

MRP.

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One can also write code, sans CPP, compatible with pre- and post- AMP. [...]

Nope, at least not if you care about -Wall: If you take e.g. (<$>) which is now part of the Prelude, you can't simply import some compatibility module, because GHC might tell you (rightfully) that that import is redundant, because (<$>) is already visible through the Prelude. So you'll have to use CPP to avoid that import on base >= 4.8, be it from it Data.Functor, Control.Applicative or some compat-* module. And you'll have to use CPP in each and every module using <$> then, unless I miss something obvious. AFAICT all transitioning guides ignore -Wall and friends...

Does the hack mentioned on the GHC trac [1] work for this? It seems a bit fragile but that page says it works and it avoids CPP.

No it doesn't, if you also don't want closed import lists (which you should).