[Haskell-cafe] Significant changes in -Wall for GHC 7.10

[Omari Norman]

I was basing this solely off the documentation:

https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/options-sanity.html

so this may just be a documentation bug.  Someone emailed me off-list and
said he reported it:

https://ghc.haskell.org/trac/ghc/ticket/10386

Thanks.  --Omari

On Tue, May 5, 2015 at 12:25 PM, Edward Z. Yang <ezyang at mit.edu> wrote:

> Hello Omari,
>
> Just to double check, could you send us a program which fails
> to produce warnings on 7.10, which does produce warnings on 7.8,
> and the command line arguments to run it?
> I tried the following test:
>
> [ezyang at hs01 ghc-bisect]$ cat A.hs
> module A where
> -- f :: Bool -> Bool
> -- f x = x && (\x -> x) x
> g :: IO ()
> g = do  (readLn :: IO Int)
>         return ()
> [ezyang at hs01 ghc-bisect]$ ghc -c A.hs -Wall -fforce-recomp
>
> A.hs:5:9: Warning:
>     A do-notation statement discarded a result of type ‘Int’
>     Suppress this warning by saying ‘_ <- (readLn :: IO Int)’
>     or by using the flag -fno-warn-unused-do-bind
> [ezyang at hs01 ghc-bisect]$ ghc --version
> The Glorious Glasgow Haskell Compilation System, version 7.10.1
>
> This is 7.10 that is distributed by Arch Linux.  I haven't
> tried any other distributions, including the pre-built binaries.
>
> Cheers,
> Edward
>
> Excerpts from Omari Norman's message of 2015-05-05 07:54:59 -0700:
> > I noticed some significant changes to the flags included with -Wall in
> GHC
> > 7.10.  Many flags that were previously included in -Wall are now not
> > included.  These include:
> >
> > -fwarn-type-defaults
> > -fwarn-name-shadowing
> > -fwarn-missing-signatures
> > -fwarn-hi-shadowing
> > -fwarn-orphans
> > -fwarn-unused-do-bind
> > -fwarn-trustworthy-safe
> >
> > I can't find any mention in the release notes of this change in behavior.
> >
> > I was wondering what the rationale is for these changes?  Any link to
> > relevant discussion would be appreciated if it exists.
> >
> > Also, what opinion do people have?  I previously kept my code -Wall
> clean,
> > but sometimes that would be a pain.  For instance I would munge local
> names
> > so they wouldn't trigger -fwarn-name-shadowing, and I would add
> signatures
> > to integers so they wouldn't trigger -fwarn-type-defaults.  In fact, I
> only
> > noticed this change in behavior in 7.10 because I was considering dumping
> > -Wall, and when I looked at the 7.10 manual I saw that it does not enable
> > -fwarn-name-shadowing.  (I still use 7.8 but Google pulled the 7.10
> > manual.)  Does this change indicate that people were finding -Wall too
> > onerous and, thus, not using it?
> >
> > I do remember reading complaints about -fwarn-unused-do-bind being added
> to
> > -Wall, but I previously felt dirty about name shadowing though I wondered
> > if munging names to avoid shadowing was actually worse.
>
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