warn-redundant-constraints present as errors

[Alan & Kim Zimmerman]

See https://ghc.haskell.org/trac/ghc/ticket/9973, my original file did not
in fact exhibit the bug.

On Fri, Jan 9, 2015 at 2:18 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

>  Now I get
>
> Foo1.hs:39:8: Not in scope: ‘SYB.everythingStaged’
>
> Foo1.hs:39:29: Not in scope: data constructor ‘SYB.Renamer’
>
>
>
> Do you think you could open a ticket with a reproducible test case?  That
> would be helpful
>
>
> Simon
>
>
>
> *From:* Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com]
> *Sent:* 09 January 2015 11:54
>
> *To:* Simon Peyton Jones
> *Cc:* ghc-devs at haskell.org
> *Subject:* Re: warn-redundant-constraints present as errors
>
>
>
> In the original definingSigsNames requires the constraint, I left that out
> to simplify the example, as the movement of the warning to an error still
> happens.
>
> Original definingSigsNames
>
> ------------------
> -- |Find those type signatures for the specified GHC.Names.
> definingSigsNames :: (SYB.Data t) =>
>             [GHC.Name] -- ^ The specified identifiers.
>             ->t        -- ^ A collection of declarations.
>             ->[GHC.LSig GHC.Name]  -- ^ The result.
> definingSigsNames pns ds = def ds
>   where
>    def decl
>      = SYB.everythingStaged SYB.Renamer (++) [] ([]  `SYB.mkQ` inSig) decl
>      where
>       inSig :: (GHC.LSig GHC.Name) -> [GHC.LSig GHC.Name]
>       inSig (GHC.L l (GHC.TypeSig ns t p))
>        | defines' ns /= [] = [(GHC.L l (GHC.TypeSig (defines' ns) t p))]
>       inSig _ = []
>
>       defines' (p::[GHC.Located GHC.Name])
>         = filter (\(GHC.L _ n) -> n `elem` pns) p
> ----------------------
>
>
>
> On Fri, Jan 9, 2015 at 1:48 PM, Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
>
>  If you remove the constraint from duplicateDecl, then I get
>
>
>
>     Redundant constraint: SYB.Data t
>
>     In the type signature for:
>
>        definingSigsNames :: SYB.Data t =>
>
>                             [GHC.Name] -> t -> [GHC.LSig GHC.Name]
>
>
>
> which is 100% correct: defininingSigssNames doesn’t use its SYB.Data t
> constraint
>
>
>
> Simon
>
>
>
> *From:* Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com]
> *Sent:* 09 January 2015 11:22
> *To:* Simon Peyton Jones
> *Cc:* ghc-devs at haskell.org
> *Subject:* Re: warn-redundant-constraints present as errors
>
>
>
> Thanks.
>
> I've found a case where it warns of a redundant constraint, but if I
> remove the constraint I get an error saying the constraint is required
>
> --------------------------------------------
> import qualified GHC           as GHC
>
> import qualified Data.Generics as SYB
>
> duplicateDecl :: (SYB.Data t) =>   -- **** The constraint being warned
> against *******
>   [GHC.LHsBind GHC.Name]  -- ^ The declaration list
>   ->t                     -- ^ Any signatures are in here
>   ->GHC.Name              -- ^ The identifier whose definition is to be
> duplicated
>   ->GHC.Name              -- ^ The new name (possibly qualified)
>   ->IO [GHC.LHsBind GHC.Name]  -- ^ The result
> duplicateDecl decls sigs n newFunName
>  = do
>      let sspan = undefined
>      newSpan <- case typeSig of
>         [] -> return sspan
>         _  -> do
>           let Just sspanSig = getSrcSpan typeSig
>           toksSig <- getToksForSpan sspanSig
>
>           let [(GHC.L sspanSig' _)] = typeSig
>
>           return sspanSig'
>
>      undefined
>    where
>      typeSig = definingSigsNames [n] sigs
>
> -- |Find those type signatures for the specified GHC.Names.
> definingSigsNames :: (SYB.Data t) =>
>             [GHC.Name] -- ^ The specified identifiers.
>             ->t        -- ^ A collection of declarations.
>             ->[GHC.LSig GHC.Name]  -- ^ The result.
> definingSigsNames pns ds = def ds
>   where def = undefined
>
> getSrcSpan = undefined
> getToksForSpan = undefined
>
> --------------------------------------------
>
>
>
> On Fri, Jan 9, 2015 at 1:08 PM, Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
>
>  I’ve fixed this
>
>
>
> *From:* Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com]
> *Sent:* 08 January 2015 21:46
> *To:* ghc-devs at haskell.org; Simon Peyton Jones
> *Subject:* warn-redundant-constraints present as errors
>
>
>
> This is a great feature, here is some feedback
>
> My syntax highlighter in emacs expects warnings to have the word "warning"
> in them.
>
> So for the two warnings reported below, the first is highlighted as an
> error, and the second as a warning
>
>
> Language/Haskell/Refact/Utils/TypeUtils.hs:3036:17:
>     Redundant constraint: SYB.Data t
>     In the type signature for:
>        duplicateDecl :: SYB.Data t =>
>                         [GHC.LHsBind GHC.Name]
>                         -> t -> GHC.Name -> GHC.Name -> RefactGhc
> [GHC.LHsBind GHC.Name]
>
> Language/Haskell/Refact/Utils/TypeUtils.hs:3045:7: Warning:
>     Defined but not used: ‘toks
>
> This is in a ghci session, and the file loads without problems, so it is
> indeed a warning.
>
> Can we perhaps add the word "Warning" to the output for Redundant
> constraints?
>
> I also had a situation where it asked me to remove a whole lot of
> constraints from different functions, I did them in batches, so did not
> remove them all from the file at once, and at some point I had to add at
> least one of them back, albeit based on an error message.
>
>
>
> Regards
>
>   Alan
>
>
>
>
>
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