Re: [GHC] #15562: `-XStrict -XNoStrict` is not neutral

24 Aug 2018

      #15562: `-XStrict -XNoStrict` is not neutral
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 I think this extends far beyond `Strict`—in fact, I'd wager that it
 affects every language extension that implies other language extensions.
 As another data point, `TypeFamilies` implies `ExplicitNamespaces`,
 `KindSignatures`, and `MonoLocalBinds`. Here's a GHCi session which shows
 that `-XTypeFamilies -XNoTypeFamilies` is similarly un-neutral:

 {{{
 $ /opt/ghc/8.4.3/bin/ghci
 GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/ryanglscott/.ghci

 λ> :set
 options currently set: none.
 base language is: Haskell2010
 with the following modifiers:
   -XNoDatatypeContexts
   -XNondecreasingIndentation
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fignore-optim-changes
   -fignore-hpc-changes
   -fimplicit-import-qualified
 warning settings:

 λ> :set -XTypeFamilies
 λ> :set
 options currently set: none.
 base language is: Haskell2010
 with the following modifiers:
   -XNoDatatypeContexts
   -XExplicitNamespaces
   -XKindSignatures
   -XMonoLocalBinds
   -XNondecreasingIndentation
   -XTypeFamilies
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fignore-optim-changes
   -fignore-hpc-changes
   -fimplicit-import-qualified
 warning settings:

 λ> :set -XNoTypeFamilies
 λ> :set
 options currently set: none.
 base language is: Haskell2010
 with the following modifiers:
   -XNoDatatypeContexts
   -XExplicitNamespaces
   -XKindSignatures
   -XMonoLocalBinds
   -XNondecreasingIndentation
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fignore-optim-changes
   -fignore-hpc-changes
   -fimplicit-import-qualified
 warning settings:
 }}}

 So ultimately, the question here is: if enabling language extension `A`
 implies `B`, should `NoA` imply `NoB`? I'm not sure that it should. After
 all, someone legitimately might want to combine `NoTypeFamilies` with
 `KindSignatures`, but if `NoTypeFamilies` implied `NoKindSignatures`, then
 this order:

 {{{#!hs
 {-# LANGUAGE NoTypeFamilies #-}
 {-# LANGUAGE KindSignatures #-}
 }}}

 Would enable `KindSignatures`, but this order:

 {{{#!hs
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE NoTypeFamilies #-}
 }}}

 Would disable `KindSignatures`! (This is especially noteworthy since many
 folks like to list language extensions in alphabetical order, so they're
 more likely to hit this scenario.)

 Therefore, my inclination is to say that this is //not// a bug. Of course,
 if this behavior isn't documented, then it should be.

 Do others have differing viewpoints?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15562#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler