Limit completions to top 40

ghcide/src/Development/IDE/Plugin/Completions.hs

@@ -6,6 +6,7 @@ module Development.IDE.Plugin.Completions
     (
       plugin
     , getCompletionsLSP
+    , maxCompletions
     ) where
 
 import Language.Haskell.LSP.Messages
@@ -116,6 +117,12 @@ instance Hashable NonLocalCompletions
 instance NFData   NonLocalCompletions
 instance Binary   NonLocalCompletions
 
+-- | 40 may seem conservative but note that most editors limit how many completions
+--   are displayed in the screen, and most users rarely scroll.
+--   For instance, VSCode only shows 12 completions in its popup, and Emacs has a similar limit.
+maxCompletions :: Int
+maxCompletions = 40
+
 -- | Generate code actions.
 getCompletionsLSP
     :: LSP.LspFuncs Config
@@ -145,11 +152,12 @@ getCompletionsLSP lsp ide
               (Just pfix', _) -> do
                 let clientCaps = clientCapabilities $ shakeExtras ide
                 snippets <- WithSnippets . completionSnippetsOn <$> getClientConfig lsp
-                Completions . List <$> getCompletions ideOpts cci' parsedMod bindMap pfix' clientCaps snippets
+                allCompletions <- getCompletions ideOpts cci' parsedMod bindMap pfix' clientCaps snippets
+                let (topCompletions, rest) = splitAt maxCompletions allCompletions
+                pure $ CompletionList (CompletionListType (null rest) (List topCompletions))
               _ -> return (Completions $ List [])
           _ -> return (Completions $ List [])
       _ -> return (Completions $ List [])
-
 setHandlersCompletion :: PartialHandlers Config
 setHandlersCompletion = PartialHandlers $ \WithMessage{..} x -> return x{
     LSP.completionHandler = withResponse RspCompletion getCompletionsLSP

ghcide/src/Development/IDE/Plugin/Completions/Logic.hs

@@ -565,9 +565,11 @@ getCompletions ideOpts CC { allModNamesAsNS, unqualCompls, qualCompls, importabl
         = filtPragmaCompls (pragmaSuffix fullLine)
         | otherwise
         = let uniqueFiltCompls = nubOrdOn insertText filtCompls
-          in filtModNameCompls ++ map (toggleSnippets caps withSnippets
-                                         . mkCompl ideOpts . stripAutoGenerated) uniqueFiltCompls
-                               ++ filtKeywordCompls
+          in filtModNameCompls
+          ++ filtKeywordCompls
+          ++ map ( toggleSnippets caps withSnippets
+                 . mkCompl ideOpts . stripAutoGenerated
+                 ) uniqueFiltCompls
   return result
 
 

ghcide/test/exe/Main.hs

@@ -64,6 +64,7 @@ import Development.IDE.Plugin.Test (WaitForIdeRuleResult(..), TestRequest(BlockS
 import Control.Monad.Extra (whenJust)
 import qualified Language.Haskell.LSP.Types.Lens as L
 import Control.Lens ((^.))
+import Development.IDE.Plugin.Completions (maxCompletions)
 
 main :: IO ()
 main = do
@@ -3213,7 +3214,17 @@ otherCompletionTests = [
       -- This should be sufficient to detect that we are in a
       -- type context and only show the completion to the type.
       (Position 3 11)
-      [("Integer", CiStruct, "Integer ", True, True, Nothing)]
+      [("Integer", CiStruct, "Integer ", True, True, Nothing)],
+
+    testSessionWait "maxCompletions" $ do
+        doc <- createDoc "A.hs" "haskell" $ T.unlines
+            [ "{-# OPTIONS_GHC -Wunused-binds #-}",
+                "module A () where",
+                "a = Prelude."
+            ]
+        _ <- waitForDiagnostics
+        compls <- getCompletions  doc (Position 3 13)
+        liftIO $ length compls @?= maxCompletions
   ]
 
 highlightTests :: TestTree