#13064: Incorrect redudant imports warning -------------------------------------+------------------------------------- Reporter: phadej | Owner: (none) Type: bug | Status: new Priority: low | Milestone: 8.8.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: #15393 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * cc: core-libraries-committee@… (added) * milestone: => 8.8.1 Comment: I admit I'm a bit nervous about merging this. The fact that the new scheme results in so many warnings even in GHC's core libraries is deeply concerning. I suspect that this new scheme may break the guidance that the Core Libraries Committee (or, at very least, Ed Kmett) has long offered for dealing with additions to `Prelude`: adding a (seemingly redundant) import of `Prelude`. For instance, consider the case of the Semigroup/Monoid proposal (SMP), where `(<>)` was added to `Prelude`. Imagine that before SMP a user had a module with, {{{#!hs import Data.Semigroup (Semigroup, (<>)) squash :: Semigroup a => a -> a -> a squash = (<>) }}} However, post-SMP the import of `Data.Semigroup` is redundant, leading to a warning which would cause failures with `-Wall`. Of course, the user could drop the `import`, but only at the expense of compatibility with earlier GHC releases. One way around this is to guard the import with CPP, {{{#!hs #if MIN_VERSION_base(4,11,0) import Data.Semigroup (Semigroup, (<>)) #endif squash :: Semigroup a => a -> a -> a squash = (<>) }}} However, requiring this sort of refactoring stands in violation of the CLC's [[https://prime.haskell.org/wiki/Libraries/3-Release-Policy|Three Release Policy]] which states that no library change will cause `-Wall` failures that are avoidable only with CPP. This is why the CLC has instead recommended this solution instead: {{{#!hs import Data.Semigroup (Semigroup, (<>)) import Prelude squash :: Semigroup a => a -> a -> a squash = (<>) }}} Under the current redundancy check this throws no warnings (assuming something is used from `Prelude`). However, under the new scheme GHC deems the `Data.Semigroup` to be redundant. Moreover, under the new semantics I don't see any way to recover the previous level of compatibility short of CPP guards. Note that I'm not saying that either behavior is more correct; rather, I'm merely saying that a truly massive amount of code may be relying on the status quo and we should be very careful before making changes here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13064#comment:22 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler