Order matters for unused import warnings when reexporting identifiers

Import order seem to change whether unused import warnings trigger when a module re-exports an identifier, with another module importing it and another module exporting the same identifier.

Reproduction:

module Foo ( (<$>) , g  ) where

import Control.Applicative

g :: Int
g = 1

module Main where

import Control.Applicative
import Foo

main :: IO ()
main = print =<< ((+2) <$> return g)

$ ghc -fwarn-unused-imports Main.hs
[1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o )
Linking Main ...

If Main is not using g then this gives a warning that the import of Foo is redundant.

If we switch the order of the imports we do get the warning:

module Main where

import Foo
import Control.Applicative

main :: IO ()
main = print =<< ((+2) <$> return g)

> ghc -fwarn-unused-imports Main.hs
[1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o )

Main.hs:4:1:
    Warning: The import of `Control.Applicative' is redundant
               except perhaps to import instances from `Control.Applicative'
             To import instances alone, use: import Control.Applicative()
Linking Main ...

I expected both versions of Main to produce the same warning. Tested on GHC 7.7.20130824 and 7.4.2.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

Yes, you're right. In general it's a hard problem to find the minimal set of imports that are required. (It's a kind of set-covering problem.) GHC uses a cheap and cheerful heuristic that usually does reasonably well.

The code is very localised, in RnNames.warnUnusedImportDecls. If anyone can do better, go for it!

Simon

added newcomer label

Be aware though that it has become somewhat popular to exploit this bug(?) to avoid writing CPP to manage the changes in Prelude's exports in 7.10.

module Main where

import Control.Applicative -- needed in 7.8, and not a warning in 7.10
                           -- because it comes before the Prelude import,
                           -- though it actually is redundant in 7.10
import Prelude             -- has the effect of canceling the implicit
                           -- import of Prelude

...

removed newcomer label

Trac metadata

Trac field	Value
TypeOfFailure	OtherFailure → IncorrectWarning

added Pnormal label

mentioned in issue #17054

marked this issue as related to #17054

mentioned in issue #21879