Make the eval plugin use the same default language extensions as ghci.

plugins/hls-eval-plugin/src/Ide/Plugin/Eval/Code.hs

@@ -3,7 +3,7 @@
 {-# OPTIONS_GHC -Wwarn -fno-warn-orphans #-}
 
 -- | Expression execution
-module Ide.Plugin.Eval.Code (Statement, testRanges, resultRange, evalExtensions, evalSetup, propSetup, testCheck, asStatements,myExecStmt) where
+module Ide.Plugin.Eval.Code (Statement, testRanges, resultRange, evalSetup, propSetup, testCheck, asStatements,myExecStmt) where
 
 import           Control.Lens                   ((^.))
 import           Data.Algorithm.Diff            (Diff, PolyDiff (..), getDiff)
@@ -13,7 +13,6 @@ import qualified Data.Text                      as T
 import           Development.IDE.Types.Location (Position (..), Range (..))
 import           GHC                            (ExecOptions, ExecResult (..),
                                                  execStmt)
-import           GHC.LanguageExtensions.Type    (Extension (..))
 import           GhcMonad                       (Ghc, liftIO, modifySession)
 import           HscTypes
 import           Ide.Plugin.Eval.Types          (Language (Plain), Loc,
@@ -81,15 +80,6 @@ asStmts (Example e _ _) = NE.toList e
 asStmts (Property t _ _) =
     ["prop11 = " ++ t, "(propEvaluation prop11 :: IO String)"]
 
--- |GHC extensions required for expression evaluation
-evalExtensions :: [Extension]
-evalExtensions =
-    [ OverlappingInstances
-    , UndecidableInstances
-    , FlexibleInstances
-    , IncoherentInstances
-    , TupleSections
-    ]
 
 -- |GHC declarations required for expression evaluation
 evalSetup :: Ghc ()

plugins/hls-eval-plugin/src/Ide/Plugin/Eval/CodeLens.hs

@@ -108,6 +108,7 @@ import           GHC                                  (ClsInst,
                                                        setContext, setLogAction,
                                                        setSessionDynFlags,
                                                        setTargets, typeKind)
+import qualified GHC.LanguageExtensions.Type          as LangExt (Extension (..))
 import           GhcPlugins                           (DynFlags (..),
                                                        defaultLogActionHPutStrDoc,
                                                        elemNameSet, gopt_set,
@@ -118,13 +119,13 @@ import           GhcPlugins                           (DynFlags (..),
                                                        pprInfixName,
                                                        targetPlatform,
                                                        tyThingParent_maybe,
-                                                       xopt_set)
+                                                       xopt_set, xopt_unset)
+
 import           HscTypes                             (InteractiveImport (IIModule),
                                                        ModSummary (ms_mod),
                                                        Target (Target),
                                                        TargetId (TargetFile))
 import           Ide.Plugin.Eval.Code                 (Statement, asStatements,
-                                                       evalExtensions,
                                                        evalSetup, myExecStmt,
                                                        propSetup, resultRange,
                                                        testCheck, testRanges)
@@ -316,8 +317,13 @@ runEvalCmd st EvalParams{..} =
                 -- copy the package state to the interactive DynFlags
                 idflags <- getInteractiveDynFlags
                 df <- getSessionDynFlags
-                setInteractiveDynFlags $
-                    (foldl xopt_set idflags evalExtensions)
+                -- set the identical DynFlags as GHCi
+                -- Source: https://github.com/ghc/ghc/blob/5abf59976c7335df760e5d8609d9488489478173/ghc/GHCi/UI.hs#L473-L483
+                -- This needs to be done manually since the default flags are not visible externally.
+                let df' = flip xopt_set    LangExt.ExtendedDefaultRules
+                        . flip xopt_unset  LangExt.MonomorphismRestriction
+                        $ idflags
+                setInteractiveDynFlags $ df'
 #if MIN_VERSION_ghc(9,0,0)
                         { unitState =
                             unitState

plugins/hls-eval-plugin/test/Main.hs

@@ -91,6 +91,7 @@ tests =
   -- , goldenWithEval "Local Modules can be imported in a test" "TLocalImportInTest" "hs"
   , goldenWithEval "Setting language option TupleSections" "TLanguageOptionsTupleSections" "hs"
   , goldenWithEval ":set accepts ghci flags" "TFlags" "hs"
+  , goldenWithEval "The default language extensions for the eval plugin are the same as those for ghci" "TSameDefaultLanguageExtensionsAsGhci" "hs"
   , goldenWithEval "IO expressions are supported, stdout/stderr output is ignored" "TIO" "hs"
   , goldenWithEval "Property checking" "TProperty" "hs"
   , goldenWithEval "Prelude has no special treatment, it is imported as stated in the module" "TPrelude" "hs"

plugins/hls-eval-plugin/test/testdata/TSameDefaultLanguageExtensionsAsGhci.expected.hs

@@ -0,0 +1,27 @@
+-- The default language extensions for the eval plugin are the same as those for ghci
+
+module TSameDefaultLanguageExtensionsAsGhci where
+
+{-
+Running `:showi language` within ghci currently lists NoDatatypeContexts, ExtendedDefaultRules, NoMonomorphismRestriction and NondecreasingIndentation.
+
+The flags NoDatatypeContexts and NondecreasingIndentation are globally set in Haskell2021, whereas ExtendedDefaultRules and NoMonomorphismRestriction are set manually within ghci.
+(see https://github.com/ghc/ghc/blob/5abf59976c7335df760e5d8609d9488489478173/ghc/GHCi/UI.hs#L473-L483)
+
+It therefore suffices to test for ExtendedDefaultRules and NoMonomorphismRestriction only.
+-}
+
+
+-- ExtendedDefaultRules
+
+-- >>> []
+-- []
+
+-- >>> reverse []
+-- []
+
+-- NoMonomorphismRestriction
+
+-- >>> plus = (+)
+-- >>> :t plus
+-- plus :: forall a. Num a => a -> a -> a

plugins/hls-eval-plugin/test/testdata/TSameDefaultLanguageExtensionsAsGhci.hs

@@ -0,0 +1,24 @@
+-- The default language extensions for the eval plugin are the same as those for ghci
+
+module TSameDefaultLanguageExtensionsAsGhci where
+
+{-
+Running `:showi language` within ghci currently lists NoDatatypeContexts, ExtendedDefaultRules, NoMonomorphismRestriction and NondecreasingIndentation.
+
+The flags NoDatatypeContexts and NondecreasingIndentation are globally set in Haskell2021, whereas ExtendedDefaultRules and NoMonomorphismRestriction are set manually within ghci.
+(see https://github.com/ghc/ghc/blob/5abf59976c7335df760e5d8609d9488489478173/ghc/GHCi/UI.hs#L473-L483)
+
+It therefore suffices to test for ExtendedDefaultRules and NoMonomorphismRestriction only.
+-}
+
+
+-- ExtendedDefaultRules
+
+-- >>> []
+
+-- >>> reverse []
+
+-- NoMonomorphismRestriction
+
+-- >>> plus = (+)
+-- >>> :t plus

plugins/hls-eval-plugin/test/testdata/test.cabal

@@ -50,6 +50,7 @@ library
       TLanguageOptionsTupleSections
       TIO
       TProperty
+      TSameDefaultLanguageExtensionsAsGhci
       TPrelude
       TCPP
       TLHS