Refine the GHCI macro into HAVE[_{INTERNAL, EXTERNAL}]_INTERPRETER

As discussed in #16331, the GHCI macro, defined through 'ghci' flags
in ghc.cabal.in, ghc-bin.cabal.in and ghci.cabal.in, is supposed to indicate
whether GHC is built with support for an internal interpreter, that runs in
the same process. It is however overloaded in a few places to mean
"there is an interpreter available", regardless of whether it's an internal
or external interpreter.

For the sake of clarity and with the hope of more easily being able to
build stage 1 GHCs with external interpreter support, this patch splits
the previous GHCI macro into 3 different ones:

- HAVE_INTERNAL_INTERPRETER: GHC is built with an internal interpreter
- HAVE_EXTERNAL_INTERPRETER: GHC is built with support for external interpreters
- HAVE_INTERPRETER: HAVE_INTERNAL_INTERPRETER || HAVE_EXTERNAL_INTERPRETER

2 files changed, 18 insertions, 7 deletions

diff --git a/ghc/Main.hs b/ghc/Main.hs
index f5836f5fe6..ce0aaec6e9 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -25,12 +25,12 @@ import HscMain          ( newHscEnv )
 import DriverPipeline   ( oneShot, compileFile )
 import DriverMkDepend   ( doMkDependHS )
 import DriverBkp   ( doBackpack )
-#if defined(GHCI)
+#if defined(HAVE_INTERNAL_INTERPRETER)
 import GHCi.UI          ( interactiveUI, ghciWelcomeMsg, defaultGhciSettings )
 #endif
 
 -- Frontend plugins
-#if defined(GHCI)
+#if defined(HAVE_INTERPRETER)
 import DynamicLoading   ( loadFrontendPlugin, initializePlugins  )
 import Plugins
 #else
@@ -271,7 +271,7 @@ main' postLoadMode dflags0 args flagWarnings = do
 
 ghciUI :: HscEnv -> DynFlags -> [(FilePath, Maybe Phase)] -> Maybe [String]
        -> Ghc ()
-#if !defined(GHCI)
+#if !defined(HAVE_INTERNAL_INTERPRETER)
 ghciUI _ _ _ _ =
   throwGhcException (CmdLineError "not built for interactive use")
 #else
@@ -521,7 +521,7 @@ isDoEvalMode :: Mode -> Bool
 isDoEvalMode (Right (Right (DoEval _))) = True
 isDoEvalMode _ = False
 
-#if defined(GHCI)
+#if defined(HAVE_INTERNAL_INTERPRETER)
 isInteractiveMode :: PostLoadMode -> Bool
 isInteractiveMode DoInteractive = True
 isInteractiveMode _             = False
@@ -752,7 +752,7 @@ showBanner :: PostLoadMode -> DynFlags -> IO ()
 showBanner _postLoadMode dflags = do
    let verb = verbosity dflags
 
-#if defined(GHCI)
+#if defined(HAVE_INTERNAL_INTERPRETER)
    -- Show the GHCi banner
    when (isInteractiveMode _postLoadMode && verb >= 1) $ putStrLn ghciWelcomeMsg
 #endif
@@ -844,7 +844,7 @@ dumpPackagesSimple dflags = putMsg dflags (pprPackagesSimple dflags)
 -- Frontend plugin support
 
 doFrontend :: ModuleName -> [(String, Maybe Phase)] -> Ghc ()
-#if !defined(GHCI)
+#if !defined(HAVE_INTERPRETER)
 doFrontend modname _ = pluginError [modname]
 #else
 doFrontend modname srcs = do
diff --git a/ghc/ghc-bin.cabal.in b/ghc/ghc-bin.cabal.in
index 0c2cb2db6d..cf5fde03e3 100644
--- a/ghc/ghc-bin.cabal.in
+++ b/ghc/ghc-bin.cabal.in
@@ -24,6 +24,11 @@ Flag ghci
     Default: False
     Manual: True
 
+Flag ext-interp
+    Description: Build external interpreter support
+    Default: True
+    Manual: True
+
 Flag threaded
     Description: Link the ghc executable against the threaded RTS
     Default: True
@@ -63,7 +68,7 @@ Executable ghc
             haskeline      == 0.7.*,
             time           >= 1.8 && < 1.10,
             transformers   == 0.5.*
-        CPP-Options: -DGHCI
+        CPP-Options: -DHAVE_INTERNAL_INTERPRETER
         GHC-Options: -fno-warn-name-shadowing
         Other-Modules:
             GHCi.Leak
@@ -92,6 +97,12 @@ Executable ghc
     if flag(threaded)
       ghc-options: -threaded
 
+    if flag(ext-interp)
+      cpp-options: -DHAVE_EXTERNAL_INTERPRETER
+
+    if flag(ghci) || flag(ext-interp)
+      cpp-options: -DHAVE_INTERPRETER
+
     Other-Extensions:
         CPP
         NondecreasingIndentation