[ghc-steering-committee] Fine-Grained Unused Warnings (#42)

[Simon Marlow]

I mentioned on the github thread that I think this will impact some tooling
(the emacs mode in particular). I do think that moving away from the simple
format of each error/warning having a separate source range in an
easy-to-parse format is not something we should do lightly. That's useful
both for humans and computers, I would argue.

On Fri, 8 Sept 2023 at 03:16, Chris Dornan <chris at chrisdornan.com> wrote:

> Proposal: Fine-Grained Unused Warnings (#42)
> Author: Jakob Brünker
> Rendered proposal:
> https://github.com/JakobBruenker/ghc-proposals/blob/fine-grained-unused/proposals/0000-fine-grained-unused-warnings.rst
> Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/434
> Recommendation: Acceptance
>
> ## Summary
>
> GHC currently warns equally about any superfluous binding that it
> discovers, regardless of whether
> the definition is unreferenced anywhere itself, or is merely mentioned by
> other definitions that are
> similarly superfluous. The proposal proposes to replace these warnings
> with just the warnings about
> definitions that are not referenced anywhere, but list within those
> warnings the entities it
> references that would otherwise not be required. The same information is
> being conveyed under this
> proposal as previously, but in a more structured manner.
>
> The proposal is careful to ensure the -Wunused-* controls
> (-Wunused-top-binds, etc.) are respected,
> so the subwarnings of transitively unused entities are only listed if
> their corresponding -Wunused-*
> control is active.
>
> As Jakob points out in the proposal, getting these warnings into some
> structure is partly motivated
> by IDE usage where the 'additional [transitive] warnings take up a lot of
> valuable screen real
> estate'.
>
> ## Example [From the proposal]
>
> In the following code,
>
> module Foo ()
>
> import Data.List as L
>
> foo = L.intercalate
>
> `intercalate` is transitively unused, since it is only used by `foo`,
> which is unused. The warning
> that's produced will be
>
> Foo.hs:5:1: warning: [-Wunused-top-binds]
> Defined but not used: ‘foo’
> as a consequence the following imports are transitively unused
> [-Wunused-imports]:
> Data.List imported at foo.hs:3:1:
>
> The warning about transitively unused bindings only appears if both the
> warning flag for the unused binding (in this case -Wunused-top-binds) and
> the the one for the transitively unused binding itself (in this case
> -Wunused-imports) are enabled.
>
>
> ## Recommendation
>
> This proposal is not at all disruptive (arguably to the extent of not
> requiring a GHC proposal
> review), is well thought out, and can improve the GHC/IDE UI so I
> recommend that we accept it.
>
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