See https://ghc.haskell.org/trac/ghc/wiki/Migration/7.10.

I have just solved one of these by doing

{-# LANGUAGE CPP #-}

#if __GLASGOW_HASKELL__ < 709
import Data.Monoid hiding ((<>))
#endif

Alan

On Sun, Apr 12, 2015 at 9:55 AM, Brendan Hay <brendan.g.hay@gmail.com> wrote:

Hi,

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.

This then results in successful compilation on 7.10 and failure on earlier

versions of GHC due to missing imports (ie. Data.Monoid (mappend, mempty)),

which prior to my current workflow of manually building on multiple versions of

GHC before releasing a new version, manifested once or twice only after

uploading to Hackage.

Now this is all user/workflow error on my part, but I wondered if others have

some kind of trick up their sleeve for avoiding these kind of issues? I could

attempt to tailor the compiler's warning flags appropriately, but it bodes ill for

spotting genuinely unused imports.

Cheers,

Brendan

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