On Tue, Oct 6, 2015 at 4:44 AM, Ben Gamari wrote:

Sven Panne writes:

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

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On October 5, 2015 at 10:59:35 AM, Bryan O'Sullivan (bos@serpentine.com ) wrote: [...] 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. 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...

This is a fair point that comes up fairly often. The fact that CPP is required to silence redundant import warnings is quite unfortunate. Others languages have better stories in this area. One example is Rust, which has a quite flexible `#[allow(...)]` pragma which can be used to acknowledge and silence a wide variety of warnings and lints [1].

I can think of a few ways (some better than others) how we might introduce a similar idea for import redundancy checks in Haskell,

1. Attach a `{-# ALLOW redundant_import #-}` pragma to a definition,

-- in Control.Applicative {-# ALLOW redundant_import (<$>) #-} (<$>) :: (a -> b) -> f a -> f b (<$>) = fmap

asking the compiler to pretend that any import of the symbol did not exist when looking for redundant imports. This would allow library authors to appropriately mark definitions when they are moved, saving downstream users from having to make any change whatsoever.

2. Or alternatively we could make this a idea a bit more precise,

-- in Control.Applicative {-# ALLOW redundant_import Prelude.(<$>) #-} (<$>) :: (a -> b) -> f a -> f b (<$>) = fmap

Which would ignore imports of `Control.Applicative.(<$>)` only if `Prelude.(<$>)` were also in scope.

One obvious solution I haven't seen mentioned is the ability to add nonexistent identifier to a hiding clause (these identifiers might presumably exist in some other version of the import): import Prelude hiding ((<$>)) I can see the argument for marking such imports with a pragma, though it gets a bit ugly. -Jan-Willem Maessen