Hadrian: introduce userDefaultFlavour, making default flavour overridable

This patch introduces the `userDefaultFlavour` user setting. It should
be the name of the default flavour to use when no --flavour argument is
passed. Before this patch, we would just always default to... the
`default` flavour. With this patch, we default to whatever Flavour whose
name is `userDefaultFlavour`, therefore providing a way for users to
"persist" their choice of flavour, not having to repeat --flavour=[...]
in every hadrian command.

Test Plan:
Set `userDefaultFlavour = "quickest"`, run `hadrian/build.sh`, check
that the quickest flavour is indeed picked.

Reviewers: snowleopard, bgamari

Reviewed By: snowleopard

Subscribers: mpickering, rwbarton, carter

GHC Trac Issues: #15890

Differential Revision: https://phabricator.haskell.org/D5454

1 files changed, 2 insertions, 2 deletions

diff --git a/hadrian/src/Settings.hs b/hadrian/src/Settings.hs
index 3497f43a1e..99a58db71f 100755
--- a/hadrian/src/Settings.hs
+++ b/hadrian/src/Settings.hs
@@ -8,7 +8,7 @@ import CommandLine
 import Expression
 import Flavour
 import Packages
-import UserSettings
+import UserSettings (userFlavours, userPackages, userDefaultFlavour)
 
 import {-# SOURCE #-} Settings.Default
 import Settings.Flavours.Development
@@ -40,7 +40,7 @@ hadrianFlavours =
 
 flavour :: Action Flavour
 flavour = do
-    flavourName <- fromMaybe "default" <$> cmdFlavour
+    flavourName <- fromMaybe userDefaultFlavour <$> cmdFlavour
     let unknownFlavour = error $ "Unknown build flavour: " ++ flavourName
         flavours       = hadrianFlavours ++ userFlavours
     return $ fromMaybe unknownFlavour $ find ((== flavourName) . name) flavours