Accept a special value "null" for LOG_DESTINATION

This parses a new value `"null"` as a `LogDestination`. On non-Windows
systems, it'll be equivalent to `"@/dev/null"`. On Windows systems,
`"@\\.\NUL"` (an explicitly-namespaced form for the `NUL` device[^1]).

Closes #50.

I chose not to investigate the open question about whether or not
`LOG_DESTINATION=NUL` would already Just Work. I decided it didn't
matter, for two reasons:

1. While it's likely "specifying `NUL`" may conceptually work, it's very
   possible you still need to practically use `LOG_DESTINATION=\\.\NUL`
   to do it.

   That would not be very discoverable and the escaping required would
   be quite error-prone, making the `null` sugar valuable.

2. Mixed-OS teams attempting to have null as a default in (e.g.) `.env`
   can't do so today.

   Without this sugar, such teams would have to concretely pick
   `/dev/null` or `NUL`.

Together, this seemed like enough to warrant the small bit of alias
complexity.

[^1]: https://stackoverflow.com/a/58177337

Blammo/Blammo.cabal

@@ -1,6 +1,6 @@
 cabal-version: 1.18
 
--- This file has been generated from package.yaml by hpack version 0.36.0.
+-- This file has been generated from package.yaml by hpack version 0.37.0.
 --
 -- see: https://github.com/sol/hpack
 

Blammo/README.lhs

@@ -144,7 +144,7 @@ setting the format to `json` will automatically enable it (with
 | ----------- | --------------------------- | ----------------------------------------- |
 | Format      | `setLogSettingsFormat`      | `LOG_FORMAT=tty\|json`                    |
 | Level(s)    | `setLogSettingsLevels`      | `LOG_LEVEL=<level>[,<source:level>,...]`  |
-| Destination | `setLogSettingsDestination` | `LOG_DESTINATION=stdout\|stderr\|@<path>` |
+| Destination | `setLogSettingsDestination` | `LOG_DESTINATION=stdout\|stderr\|null\|@<path>` |
 | Color       | `setLogSettingsColor `      | `LOG_COLOR=auto\|always\|never`           |
 | Breakpoint  | `setLogSettingsBreakpoint`  | `LOG_BREAKPOINT=<number>`                 |
 | Concurrency | `setLogSettingsConcurrency` | `LOG_CONCURRENCY=<number>`                |

Blammo/src/Blammo/Logging/LogSettings.hs

@@ -43,6 +43,7 @@ import qualified Blammo.Logging.LogSettings.LogLevels as LogLevels
 import Control.Monad.IO.Class (MonadIO (..))
 import Control.Monad.Logger.Aeson
 import System.IO (Handle, hIsTerminalDevice)
+import qualified System.Info
 
 data LogSettings = LogSettings
   { lsLevels :: LogLevels
@@ -65,12 +66,13 @@ readLogDestination :: String -> Either String LogDestination
 readLogDestination = \case
   "stdout" -> Right LogDestinationStdout
   "stderr" -> Right LogDestinationStderr
+  "null" -> Right $ LogDestinationFile nullDevice
   ('@' : path) -> Right $ LogDestinationFile path
   x ->
     Left $
       "Invalid log destination "
         <> x
-        <> ", must be stdout, stderr, or @{path}"
+        <> ", must be stdout, stderr, null, or @{path}"
 
 data LogFormat
   = LogFormatJSON
@@ -196,3 +198,14 @@ shouldColorAuto LogSettings {..} f = case lsColor of
 shouldColorHandle :: MonadIO m => LogSettings -> Handle -> m Bool
 shouldColorHandle settings h =
   shouldColorAuto settings $ liftIO $ hIsTerminalDevice h
+
+-- | @/dev/null@ or @NUL@ on windows
+--
+-- See <https://stackoverflow.com/a/58177337>.
+nullDevice :: FilePath
+nullDevice
+  | System.Info.os == windowsOS = "\\\\.\\NUL"
+  | otherwise = "/dev/null"
+
+windowsOS :: String
+windowsOS = "mingw32"

Blammo/src/Blammo/Logging/LogSettings/Env.hs

@@ -3,9 +3,9 @@
 -- - @LOG_LEVEL@: a known log level (case insensitive) and optional levels by
 --   source. See "Logging.LogSettings.LogLevels".
 --
--- - @LOG_DESTINATION@: the string @stderr@ or @stdout@ (case sensitive), or
---   @\@{path}@ to log to the file at @path@. Unrecognized values will produce
---   and error.
+-- - @LOG_DESTINATION@: the string @stderr@, @stdout@ or @null@, (case
+--   sensitive), or @\@{path}@ to log to the file at @path@. Unrecognized values
+--   will produce an error.
 --
 -- - @LOG_FORMAT@: the string @tty@ or @json@. Unrecognized values will produce
 --   an error.