I've run into a couple of cases when attempting to support multiple GHC versions in my libraries (7.6.3 -> 7.10) where I've repeatedly made mistakes due to being warned about unused imports for various modules that are now part of the Prelude such as Data.Monoid, Data.Foldable, etc. which I subsequently remove either manually or via editor automation sufficiently indistinguishable from magic.

A trick is to import Prelude at the very end, like: import Control.Applicative import Data.Monoid ... import Prelude Since the unused import check is done from top to bottom, and you almost always use *something* from the Prelude, this will suppress the warning. There are some problems with qualified/explicit import lists if I recall correctly though. But it works for me most of the time. Regards, Benno

Hi, the only problem with the "import Prelude" trick is that it does not help when you explicitly list what you import. For example, in one of my files I had: import Control.Applicative ((<$>)) which will still result in a warning. Simply change it to import Control.Applicative import Prelude However, sometimes there are conflicting names due to reexports. For example, forM is both in Data.Traversable and Control.Monad. In such a case, use "hiding" for one of them. Also change import <module> (<f>) to import <module> hiding (<everything except f>) if you want to avoid CPP at all cost. I too went through a few cycles 7.8.4 <-> 7.10.1 trying to make the code warning free on both (I don't use Travis but having minghc on Windows or using HVR's PPA on Ubuntu helps a lot). Michal On Sun, Apr 12, 2015 at 3:45 PM, Brendan Hay wrote:

Thanks for the tips!

On 12 April 2015 at 12:37, Benno Fünfstück wrote:

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I've run into a couple of cases when attempting to support multiple GHC versions in my libraries (7.6.3 -> 7.10) where I've repeatedly made mistakes due to being warned about unused imports for various modules that are now part of the Prelude such as Data.Monoid, Data.Foldable, etc. which I subsequently remove either manually or via editor automation sufficiently indistinguishable from magic.

A trick is to import Prelude at the very end, like:

import Control.Applicative import Data.Monoid ... import Prelude

Since the unused import check is done from top to bottom, and you almost always use *something* from the Prelude, this will suppress the warning.

There are some problems with qualified/explicit import lists if I recall correctly though. But it works for me most of the time.

Regards, Benno

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