ghcup config unset

[hasufell]

Following #1057

We have user settings:

data UserSettings = UserSettings
  { uCache             :: Maybe Bool
  , uMetaCache         :: Maybe Integer
  , uMetaMode          :: Maybe MetaMode
  , uNoVerify          :: Maybe Bool
  , uVerbose           :: Maybe Bool
  , uKeepDirs          :: Maybe KeepDirs
  , uDownloader        :: Maybe Downloader
  , uKeyBindings       :: Maybe UserKeyBindings
  , uUrlSource         :: Maybe URLSource
  , uNoNetwork         :: Maybe Bool
  , uGPGSetting        :: Maybe GPGSetting
  , uPlatformOverride  :: Maybe PlatformRequest
  , uMirrors           :: Maybe DownloadMirrors
  , uDefGHCConfOptions :: Maybe [String]
  }

And the actual settings:

data Settings = Settings
  { cache             :: Bool
  , metaCache         :: Integer
  , metaMode          :: MetaMode
  , noVerify          :: Bool
  , keepDirs          :: KeepDirs
  , downloader        :: Downloader
  , verbose           :: Bool
  , urlSource         :: URLSource
  , noNetwork         :: Bool
  , gpgSetting        :: GPGSetting
  , noColor           :: Bool -- this also exists in LoggerConfig
  , platformOverride  :: Maybe PlatformRequest
  , mirrors           :: DownloadMirrors
  , defGHCConfOptions :: [String]
  }

platformOverride here is special, since it's the only Maybe that gets passed through. In UserSettings, all records are Maybe, so we can unset any key.