Re: warn-redundant-constraints present as errors

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 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@gmail.com] *Sent:* 09 January 2015 11:54

*To:* Simon Peyton Jones *Cc:* ghc-devs@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 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@gmail.com] *Sent:* 09 January 2015 11:22 *To:* Simon Peyton Jones *Cc:* ghc-devs@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 wrote:

I’ve fixed this

*From:* Alan & Kim Zimmerman [mailto:alan.zimm@gmail.com] *Sent:* 08 January 2015 21:46 *To:* ghc-devs@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