Default to using a stack-installed, isolated GHC #2221

ChangeLog.md

@@ -8,6 +8,10 @@ Major changes:
 
 Behavior changes:
 
+* Use a stack-installed, isolated GHC by default, ignoring any system
+  installations (overridable with `--system-ghc`).
+  [#2221](https://github.com/commercialhaskell/stack/issues/2221)
+
 Other enhancements:
 
 Bug fixes:

doc/yaml_configuration.md

@@ -299,13 +299,13 @@ other projects by installing into your shared snapshot database.
 
 ### system-ghc
 
-Enables or disables using the GHC available on the PATH. Useful to disable if
-you want to force stack to use its own installed GHC (via `stack setup`), in
-cases where your system GHC my be incomplete for some reason. Default is `true`.
+Enables or disables using the GHC available on the PATH.
+Useful to disable if you want to save the time, bandwidth or storage space needed to setup an isolated GHC.
+Default is `false`.
 
 ```yaml
-# Turn off system GHC
-system-ghc: false
+# Turn on system GHC
+system-ghc: true
 ```
 
 ### install-ghc

src/Stack/Config.hs

@@ -227,7 +227,7 @@ configFromConfigMonoid configStackRoot configUserConfigPath mresolver mproject C
 
          configGHCVariant0 = getFirst configMonoidGHCVariant
 
-         configSystemGHC = fromFirst (isNothing configGHCVariant0) configMonoidSystemGHC
+         configSystemGHC = fromFirst False configMonoidSystemGHC
          configInstallGHC = fromFirst False configMonoidInstallGHC
          configSkipGHCCheck = fromFirst False configMonoidSkipGHCCheck
          configSkipMsys = fromFirst False configMonoidSkipMsys

src/Stack/Options.hs

@@ -251,7 +251,7 @@ configOptsParser hide0 =
     <*> nixOptsParser True
     <*> firstBoolFlags
             "system-ghc"
-            "using the system installed GHC (on the PATH) if available and a matching version"
+            "using the system installed GHC (on the PATH) if available and a matching version. Disabled by default"
             hide
     <*> firstBoolFlags
             "install-ghc"
@@ -861,7 +861,7 @@ ghcVariantParser hide =
         readGHCVariant
         (long "ghc-variant" <> metavar "VARIANT" <>
          help
-             "Specialized GHC variant, e.g. integersimple (implies --no-system-ghc)" <>
+             "Specialized GHC variant, e.g. integersimple" <>
          hideMods hide
         )
   where

src/Stack/Setup.hs

@@ -403,7 +403,8 @@ ensureCompiler sopts = do
                             (soptsStackYaml sopts)
                             (fromMaybe
                                 ("Try running \"stack setup\" to install the correct GHC into "
-                                <> T.pack (toFilePath (configLocalPrograms config)))
+                                <> T.pack (toFilePath (configLocalPrograms config))
+                                <> " or use \"--system-ghc\" to use a suitable compiler available on your PATH.")
                                 $ soptsResolveMissingGHC sopts)
 
             -- Install msys2 on windows, if necessary

src/Stack/SetupCmd.hs

@@ -46,7 +46,7 @@ setupParser = SetupCmdOpts
                       "The default is to install the version implied by the resolver.")))
     <*> OA.boolFlags False
             "reinstall"
-            "reinstalling GHC, even if available (implies no-system-ghc)"
+            "reinstalling GHC, even if available"
             OA.idm
     <*> OA.boolFlags False
             "upgrade-cabal"