[ANNOUNCE] GHC 8.6.1-alpha2 available

Andrés Sicard-Ramírez asr at eafit.edu.co
Sat Jul 21 13:25:04 UTC 2018

Thank you for a very clear explanation and the solution.


On 21 July 2018 at 08:03, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
> Presumably, you're compiling this program outside of a git repository,
> and so the Template Haskell splice $(gitHash) evaluates to "UNKNOWN"
> at compile time. Here is an even more minimal example, which does not
> depend on gitrev:
>     f :: ()
>     f = case "UNKNOWN" of
>           "UNKNOWN" -> ()
>           _         -> ()
> This, when compiled with GHC 8.6.1 or later, gives the same warning:
>     Test.hs:4:7: warning: [-Woverlapping-patterns]
>         Pattern match is redundant
>         In a case alternative: _ -> ...
>       |
>     4 |       _         -> ()
>       |       ^^^^^^^^^^^^^^^
> This, I would argue, is a correct error message. The string literal
> "UNKNOWN" is, well, statically known to be "UNKNOWN", so GHC knows
> that that catch-all case cannot be reached. (See [1] for the commit
> which introduced this check.)
> Granted, this makes things slightly inconvenient for your purposes,
> since in your actual program, $(gitHash) might evaluate to *different*
> things at compile time depending on the directory you compile it in.
> I'd recommend rewriting the commitInfo function to pattern guards:
>     commitInfo :: Maybe String
>     commitInfo
>       | hash == "UNKNOWN" = Nothing
>       | otherwise         = Just hash
>       where
>         hash = $(gitHash)
> Since that does not trigger -Woverlapping-patterns. (Granted, it's
> possible that GHC could become much smarter in the future and be able
> to detect that that `otherwise` case is unreachable when $(gitHash)
> evaluates to "UNKNOWN" at compile time. But GHC certainly isn't that
> smart today!)
> Ryan S.
> -----
> [1] http://git.haskell.org/ghc.git/commit/1f88f541aad1e36d01f22f9e71dfbc247e6558e2
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