The suggested redundant import solution doesn't work for explicit import lists

[Simon Peyton Jones]

OK. I've created a ticket
   https://ghc.haskell.org/trac/ghc/ticket/10117

In fact there is a pretty good specification here
https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/UnusedImports

So back to you: based on the existing, can you think of a variant that would work better?

Simon

|  -----Original Message-----
|  From: Herbert Valerio Riedel [mailto:hvriedel at gmail.com]
|  Sent: 24 February 2015 12:00
|  To: Simon Peyton Jones
|  Cc: Herbert Valerio Riedel; Johan Tibell; Edward Kmett; Austin Seipp
|  Subject: Re: The suggested redundant import solution doesn't work for
|  explicit import lists
|  
|  Simon,
|  
|  The problem:
|  
|   trying to avoid redundant-import warnings due symbols having moved to
|  'Prelude' due to AMP/FTP w/o CPP usage
|  
|  e.g.
|  
|    import Control.Applicative (Applicative)
|  
|    foo :: Applicative f => ...
|    foo = ...
|  
|  now warns that 'Applicative' is redundant, as it's exported from
|  'Prelude' now. 'Monoid' is another such example that's now re-exported
|  from 'Prelude' (but wasn't before GHC 7.10).
|  
|  
|  One thing that happens to work is the trick shown at
|  https://ghc.haskell.org/trac/ghc/wiki/Migration/7.10#GHCsaysTheimporto
|  f...isredundant
|  i.e.
|  
|    module Foo (Int, Word, Monoid(..)) where
|  
|    import Data.Monoid
|    import Data.Word
|    import Prelude
|  
|  as in that case, 'Prelude' is explicitly imported last, and so GHC
|  doesn't warn about Data.{Word,Monoid} being redundant (due to the way
|  the redundancy-check is implemented in GHC)
|  
|  However, if the example above is rewritten as
|  
|    module Foo (Int, Word, Monoid(..)) where
|  
|    import Data.Monoid (Monoid(..))
|    import Data.Word (Word)
|    import Prelude
|  
|  GHC would warn, as it handles import-listed symbols differently than
|  wildcard imports...
|  
|  and the question is, whether we can easily get the latter example to
|  become warning free as well w/o risking breakages elsewhere...
|  
|  ...is it clearer now what I'm suggesting?
|  
|  Cheers,
|    hvr
|  
|  On 2015-02-24 at 12:50:35 +0100, Simon Peyton Jones wrote:
|  > I'm sorry, but can someone re-articulate the question?   I'm deep
|  underwater, so reluctant to reverse-engineer the question from the
|  thread.
|  > I think you are asking for some feature.  But I'm not sure what the
|  spec is.
|  >
|  > Simon
|  >
|  > | -----Original Message-----
|  > | From: Herbert Valerio Riedel [mailto:hvriedel at gmail.com]
|  > | Sent: 24 February 2015 11:29
|  > | To: Johan Tibell
|  > | Cc: Edward Kmett; Simon Peyton Jones; Austin Seipp
|  > | Subject: Re: The suggested redundant import solution doesn't work
|  > | for explicit import lists
|  > |
|  > | Johan,
|  > |
|  > | You're right, I'm afraid the Prelude-trick doesn't work well with
|  > | import-lists... not sure though if it's worth the risk to tweak
|  GHC
|  > | 7.10.1's redundant-warning detection to make it work here too...
|  > |
|  > | @SPJ, any comments?
|  > |
|  > | for more context:
|  > |
|  > |  -
|  > |
|  https://ghc.haskell.org/trac/ghc/wiki/Migration/7.10#GHCsaysTheimporto
|  f..
|  > | .isredundant
|  > |  -
|  > |
|  http://www.reddit.com/r/haskell/comments/2wx64g/ghc_710_will_use_pla
|  > | n_ftp
|  > | /covdas0
|  > |
|  > | Cheers,
|  > |   hvr
|  > |
|  > | On 2015-02-24 at 11:47:30 +0100, Johan Tibell wrote:
|  > | > The suggested fix for unused imports resulting from the move of
|  e.g.
|  > | > Data.Monoid doesn't work when using explicit import lists:
|  > | >
|  > | > $ cat Test.hs
|  > | > module Test where
|  > | >
|  > | > import Data.Monoid (Monoid)
|  > | > import Prelude
|  > | >
|  > | > f :: Monoid a => a
|  > | > f = undefined
|  > | > $ ghc-7.10.0.20150123 -Wall /tmp/Test.hs
|  > | > [1 of 1] Compiling Test             ( /tmp/Test.hs, /tmp/Test.o
|  )
|  > | >
|  > | > /tmp/Test.hs:3:1: Warning:
|  > | >     The import of ‘Data.Monoid’ is redundant
|  > | >       except perhaps to import instances from ‘Data.Monoid’
|  > | >     To import instances alone, use: import Data.Monoid()
|  > | >
|  > | > It does work if you don't have an import list (bad idea) or use
|  > | qualified
|  > | > imports (good idea).
|  > | >
|  > | > -- Johan
|  > |
|  > | --
|  > | "Elegance is not optional" -- Richard O'Keefe
|  
|  --
|  "Elegance is not optional" -- Richard O'Keefe