class-plugin: Add code action for implementing instance method when default implementation doesn't work

plugins/hls-class-plugin/src/Ide/Plugin/Class/CodeAction.hs

@@ -5,7 +5,6 @@
 
 module Ide.Plugin.Class.CodeAction where
 
-import           Control.Applicative                  (liftA2)
 import           Control.Lens                         hiding (List, use)
 import           Control.Monad.Extra
 import           Control.Monad.IO.Class               (liftIO)
@@ -89,7 +88,7 @@ codeAction recorder state plId (CodeActionParams _ _ docId _ context) = pluginRe
         List diags = context ^. J.diagnostics
 
         ghcDiags = filter (\d -> d ^. J.source == Just "typecheck") diags
-        methodDiags = filter (\d -> isClassMethodWarning (d ^. J.message)) ghcDiags
+        methodDiags = filter (\d -> isClassMethodWarning (d ^. J.message) || isInstanceMissingError (d ^. J.message)) ghcDiags
 
         mkActions
             :: NormalizedFilePath
@@ -111,11 +110,17 @@ codeAction recorder state plId (CodeActionParams _ _ docId _ context) = pluginRe
                 $ use GetInstanceBindTypeSigs docPath
             implemented <- findImplementedMethods ast instancePosition
             logWith recorder Info (LogImplementedMethods cls implemented)
+            let groups =
+                    -- If the mininal def contains nothing, the plugin offers to
+                    -- add stub implementations of all methods with default
+                    -- implementations
+                    case classMinimalDef cls of
+                        And [] -> classOpItemsGroups range sigs cls
+                        Or []  -> classOpItemsGroups range sigs cls
+                        minDef -> minDefToMethodGroups range sigs minDef
             pure
                 $ concatMap mkAction
-                $ fmap (filter (\(bind, _) -> bind `notElem` implemented))
-                $ minDefToMethodGroups range sigs
-                $ classMinimalDef cls
+                $ fmap (filter (\(bind, _) -> bind `notElem` implemented)) groups
             where
                 range = diag ^. J.range
 
@@ -207,6 +212,11 @@ isClassNodeIdentifier ident = (isNothing . identType) ident && Use `Set.member`
 isClassMethodWarning :: T.Text -> Bool
 isClassMethodWarning = T.isPrefixOf "• No explicit implementation for"
 
+isInstanceMissingError :: T.Text -> Bool
+isInstanceMissingError err =
+    "• No instance for" `T.isPrefixOf` err
+    && "In the instance declaration for" `T.isInfixOf` err
+
 isInstanceValBind :: ContextInfo -> Bool
 isInstanceValBind (ValBind InstanceBind _ _) = True
 isInstanceValBind _                          = False
@@ -215,11 +225,19 @@ isInstanceValBind _                          = False
 minDefToMethodGroups :: Range -> [InstanceBindTypeSig] -> BooleanFormula Name -> [[(T.Text, T.Text)]]
 minDefToMethodGroups range sigs = go
     where
-        go (Var mn)   = [[ (T.pack . occNameString . occName $ mn, bindRendered sig)
-                        | sig <- sigs
-                        , inRange range (getSrcSpan $ bindName sig)
-                        , printOutputable mn == T.drop (T.length bindingPrefix) (printOutputable (bindName sig))
-                        ]]
+        go (Var mn)   = mkGroups range sigs mn
         go (Or ms)    = concatMap (go . unLoc) ms
         go (And ms)   = foldr (liftA2 (<>)) [[]] (fmap (go . unLoc) ms)
         go (Parens m) = go (unLoc m)
+
+mkGroups :: Range -> [InstanceBindTypeSig] -> Name -> [[(T.Text, T.Text)]]
+mkGroups range sigs name =
+    [[ (T.pack . occNameString . occName $ name, bindRendered sig)
+     | sig <- sigs
+     , inRange range (getSrcSpan $ bindName sig)
+     , printOutputable name == T.drop (T.length bindingPrefix) (printOutputable (bindName sig))
+     ]]
+
+classOpItemsGroups :: Range -> [InstanceBindTypeSig] -> Class -> [[(T.Text, T.Text)]]
+classOpItemsGroups range sigs cls =
+    concatMap (mkGroups range sigs . getName . fst) $ classOpItems cls

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

@@ -12,13 +12,10 @@ module Main
 import           Control.Lens                 (Prism', prism', (^.), (^..),
                                                (^?))
 import           Control.Monad                (void)
-import           Data.Aeson                   (toJSON, (.=))
 import           Data.Functor.Contravariant   (contramap)
 import           Data.Maybe
 import           Development.IDE.Types.Logger
 import qualified Ide.Plugin.Class             as Class
-import           Ide.Plugin.Config            (PluginConfig (plcConfig))
-import qualified Ide.Plugin.Config            as Plugin
 import qualified Language.LSP.Types.Lens      as J
 import           System.FilePath
 import           Test.Hls
@@ -78,6 +75,8 @@ codeActionTests recorder = testGroup
       executeCodeAction eqWithSig
   , goldenWithClass recorder "Only insert pragma once" "InsertPragmaOnce" "" $ \(_:multi:_) -> do
       executeCodeAction multi
+  , goldenWithClass recorder "Creates a placeholder for a default method implementation" "DefaultImplementation" "" $ \(_:fAction:_) -> do
+      executeCodeAction fAction
   ]
 
 codeLensTests :: Recorder (WithPriority Class.Log) -> TestTree

plugins/hls-class-plugin/test/testdata/DefaultImplementation.expected.hs

@@ -0,0 +1,16 @@
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE InstanceSigs #-}
+module DefaultImplementation where
+
+class Test1 a where
+
+class Test a where
+  f :: a -> a
+  default f :: Test1 a => a -> a
+  f = id
+
+data X = X | Y
+
+instance Test X where
+  f :: X -> X
+  f = _

plugins/hls-class-plugin/test/testdata/DefaultImplementation.hs

@@ -0,0 +1,13 @@
+{-# LANGUAGE DefaultSignatures #-}
+module DefaultImplementation where
+
+class Test1 a where
+
+class Test a where
+  f :: a -> a
+  default f :: Test1 a => a -> a
+  f = id
+
+data X = X | Y
+
+instance Test X where