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

[Johan Tibell]

I guess for re-exports we could relax the requirement that the symbol comes
from only one module. I don't know if that is better though. If I saw

import Data.Monoid (Monoid)
import Prelude (Monoid)

I'd say that code wasn't the pretties, because it would confuse users, who
would have to figure out that the two were the same. Re-exports are
generally confusing, because they break abstraction by saying that two
things are the same and will continue to be so, which isn't always what you
want.


On Thu, Feb 26, 2015 at 6:06 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> 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
>
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