GitLab

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