Add brackets if needed for hint "Avoid lambda"

src/GHC/Util/HsExpr.hs

@@ -141,7 +141,7 @@ niceDotApp a b = dotApp a b
 
 -- Generate a lambda expression but prettier if possible.
 niceLambda :: [String] -> LHsExpr GhcPs -> LHsExpr GhcPs
-niceLambda ss e = fst (niceLambdaR ss e)-- We don't support refactorings yet.
+niceLambda ss e = fst (niceLambdaR Nothing ss e)-- We don't support refactorings yet.
 
 allowRightSection :: String -> Bool
 allowRightSection x = x `notElem` ["-","#"]
@@ -150,99 +150,111 @@ allowLeftSection x = x /= "#"
 
 -- Implementation. Try to produce special forms (e.g. sections,
 -- compositions) where we can.
-niceLambdaR :: [String]
-            -> LHsExpr GhcPs
+niceLambdaR :: Maybe (LHsExpr GhcPs) -- parent expression
+            -> [String]
+            -> LHsExpr GhcPs -- the expression being processed
             -> (LHsExpr GhcPs, R.SrcSpan -> [Refactoring R.SrcSpan])
--- Rewrite @\ -> e@ as @e@
--- These are encountered as recursive calls.
-niceLambdaR xs (SimpleLambda [] x) = niceLambdaR xs x
-
--- Rewrite @\xs -> (e)@ as @\xs -> e@.
-niceLambdaR xs (L _ (HsPar _ x)) = niceLambdaR xs x
-
--- @\vs v -> ($) e v@ ==> @\vs -> e@
--- @\vs v -> e $ v@ ==> @\vs -> e@
-niceLambdaR (unsnoc -> Just (vs, v)) (view -> App2 f e (view -> Var_ v'))
-  | isDol f
-  , v == v'
-  , vars e `disjoint` [v]
-  = niceLambdaR vs e
-
--- @\v -> thing + v@ ==> @\v -> (thing +)@  (heuristic: @v@ must be a single
--- lexeme, or it all gets too complex)
-niceLambdaR [v] (L _ (OpApp _ e f (view -> Var_ v')))
-  | isLexeme e
-  , v == v'
-  , vars e `disjoint` [v]
-  , L _ (HsVar _ (L _ fname)) <- f
-  , isSymOcc $ rdrNameOcc fname
-  = let res = nlHsPar $ noLocA $ SectionL noExtField e f
-     in (res, \s -> [Replace Expr s [] (unsafePrettyPrint res)])
-
--- @\vs v -> f x v@ ==> @\vs -> f x@
-niceLambdaR (unsnoc -> Just (vs, v)) (L _ (HsApp _ f (view -> Var_ v')))
-  | v == v'
-  , vars f `disjoint` [v]
-  = niceLambdaR vs f
-
--- @\vs v -> (v `f`)@ ==> @\vs -> f@
-niceLambdaR (unsnoc -> Just (vs, v)) (L _ (SectionL _ (view -> Var_ v') f))
-  | v == v' = niceLambdaR vs f
-
--- Strip one variable pattern from the end of a lambdas match, and place it in our list of factoring variables.
-niceLambdaR xs (SimpleLambda ((view -> PVar_ v):vs) x)
-  | v `notElem` xs = niceLambdaR (xs++[v]) $ lambda vs x
-
--- Rewrite @\x -> x + a@ as @(+ a)@ (heuristic: @a@ must be a single
--- lexeme, or it all gets too complex).
-niceLambdaR [x] (view -> App2 op@(L _ (HsVar _ (L _ tag))) l r)
-  | isLexeme r, view l == Var_ x, x `notElem` vars r, allowRightSection (occNameStr tag) =
-      let e = rebracket1 $ addParen (noLocA $ SectionR noExtField op r)
-      in (e, \s -> [Replace Expr s [] (unsafePrettyPrint e)])
--- Rewrite (1) @\x -> f (b x)@ as @f . b@, (2) @\x -> f $ b x@ as @f . b@.
-niceLambdaR [x] y
-  | Just (z, subts) <- factor y, x `notElem` vars z = (z, \s -> [mkRefact subts s])
+niceLambdaR parent = go
   where
-    -- Factor the expression with respect to x.
-    factor :: LHsExpr GhcPs -> Maybe (LHsExpr GhcPs, [LHsExpr GhcPs])
-    factor (L _ (HsApp _ ini lst)) | view lst == Var_ x = Just (ini, [ini])
-    factor (L _ (HsApp _ ini lst)) | Just (z, ss) <- factor lst
-      = let r = niceDotApp ini z
-        in if astEq r z then Just (r, ss) else Just (r, ini : ss)
-    factor (L _ (OpApp _ y op (factor -> Just (z, ss))))| isDol op
-      = let r = niceDotApp y z
-        in if astEq r z then Just (r, ss) else Just (r, y : ss)
-    factor (L _ (HsPar _ y@(L _ HsApp{}))) = factor y
-    factor _ = Nothing
-    mkRefact :: [LHsExpr GhcPs] -> R.SrcSpan -> Refactoring R.SrcSpan
-    mkRefact subts s =
-      let tempSubts = zipWith (\a b -> (a, toSSA b)) substVars subts
-          template = dotApps (map (strToVar . fst) tempSubts)
-      in Replace Expr s tempSubts (unsafePrettyPrint template)
--- Rewrite @\x y -> x + y@ as @(+)@.
-niceLambdaR [x,y] (L _ (OpApp _ (view -> Var_ x1) op@(L _ HsVar {}) (view -> Var_ y1)))
-    | x == x1, y == y1, vars op `disjoint` [x, y] = (op, \s -> [Replace Expr s [] (unsafePrettyPrint op)])
--- Rewrite @\x y -> f y x@ as @flip f@.
-niceLambdaR [x, y] (view -> App2 op (view -> Var_ y1) (view -> Var_ x1))
-  | x == x1, y == y1, vars op `disjoint` [x, y] =
-      ( gen op
-      , \s -> [Replace Expr s [("x", toSSA op)] (unsafePrettyPrint $ gen (strToVar "x"))]
-      )
-  where
-    gen :: LHsExpr GhcPs -> LHsExpr GhcPs
-    gen = noLocA . HsApp noExtField (strToVar "flip")
-        . if isAtom op then id else addParen
-
--- We're done factoring, but have no variables left, so we shouldn't make a lambda.
--- @\ -> e@ ==> @e@
-niceLambdaR [] e = (e, \s -> [Replace Expr s [("a", toSSA e)] "a"])
--- Base case. Just a good old fashioned lambda.
-niceLambdaR ss e =
-  let grhs = noLocA $ GRHS noAnn [] e :: LGRHS GhcPs (LHsExpr GhcPs)
-      grhss = GRHSs {grhssExt = emptyComments, grhssGRHSs=[grhs], grhssLocalBinds=EmptyLocalBinds noExtField}
-      match = noLocA $ Match {m_ext=noExtField, m_ctxt=LamAlt LamSingle, m_pats=noLocA $ map strToPat ss, m_grhss=grhss} :: LMatch GhcPs (LHsExpr GhcPs)
-      matchGroup = MG {mg_ext=Generated OtherExpansion SkipPmc, mg_alts=noLocA [match]}
-  in (noLocA $ HsLam noAnn LamSingle matchGroup, const [])
+    -- Rewrite @\ -> e@ as @e@
+    -- These are encountered as recursive calls.
+    go xs (SimpleLambda [] x) = go xs x
+
+    -- Rewrite @\xs -> (e)@ as @\xs -> e@.
+    go xs (L _ (HsPar _ x)) = go xs x
+
+    -- @\vs v -> ($) e v@ ==> @\vs -> e@
+    -- @\vs v -> e $ v@ ==> @\vs -> e@
+    go (unsnoc -> Just (vs, v)) (view -> App2 f e (view -> Var_ v'))
+      | isDol f
+      , v == v'
+      , vars e `disjoint` [v]
+      = go vs e
+
+    -- @\v -> thing + v@ ==> @\v -> (thing +)@  (heuristic: @v@ must be a single
+    -- lexeme, or it all gets too complex)
+    go [v] (L _ (OpApp _ e f (view -> Var_ v')))
+      | isLexeme e
+      , v == v'
+      , vars e `disjoint` [v]
+      , L _ (HsVar _ (L _ fname)) <- f
+      , isSymOcc $ rdrNameOcc fname
+      = let res = nlHsPar $ noLocA $ SectionL noExtField e f
+        in (res, \s -> [Replace Expr s [] (unsafePrettyPrint res)])
+
+    -- @\vs v -> f x v@ ==> @\vs -> f x@
+    go (unsnoc -> Just (vs, v)) (L _ (HsApp _ f (view -> Var_ v')))
+      | v == v'
+      , vars f `disjoint` [v]
+      = go vs f
+
+    -- @\vs v -> (v `f`)@ ==> @\vs -> f@
+    go (unsnoc -> Just (vs, v)) (L _ (SectionL _ (view -> Var_ v') f))
+      | v == v' = go vs f
+
+    -- Strip one variable pattern from the end of a lambdas match, and place it in our list of factoring variables.
+    go xs (SimpleLambda ((view -> PVar_ v):vs) x)
+      | v `notElem` xs = go (xs++[v]) $ lambda vs x
+
+    -- Rewrite @\x -> x + a@ as @(+ a)@ (heuristic: @a@ must be a single
+    -- lexeme, or it all gets too complex).
+    go [x] (view -> App2 op@(L _ (HsVar _ (L _ tag))) l r)
+      | isLexeme r, view l == Var_ x, x `notElem` vars r, allowRightSection (occNameStr tag) =
+          let e = rebracket1 $ addParen (noLocA $ SectionR noExtField op r)
+          in (e, \s -> [Replace Expr s [] (unsafePrettyPrint e)])
+    -- Rewrite (1) @\x -> f (b x)@ as @f . b@, (2) @\x -> f $ b x@ as @f . b@.
+    go [x] y
+      | Just (z, subts) <- factor y, x `notElem` vars z = (z, \s -> [mkRefact subts s])
+      where
+        -- Factor the expression with respect to x.
+        factor :: LHsExpr GhcPs -> Maybe (LHsExpr GhcPs, [LHsExpr GhcPs])
+        factor (L _ (HsApp _ ini lst)) | view lst == Var_ x = Just (ini, [ini])
+        factor (L _ (HsApp _ ini lst)) | Just (z, ss) <- factor lst
+          = let r = niceDotApp ini z
+            in if astEq r z then Just (r, ss) else Just (r, ini : ss)
+        factor (L _ (OpApp _ y op (factor -> Just (z, ss))))| isDol op
+          = let r = niceDotApp y z
+            in if astEq r z then Just (r, ss) else Just (r, y : ss)
+        factor (L _ (HsPar _ y@(L _ HsApp{}))) = factor y
+        factor _ = Nothing
+        mkRefact :: [LHsExpr GhcPs] -> R.SrcSpan -> Refactoring R.SrcSpan
+        mkRefact subts s =
+          let tempSubts = zipWith (\a b -> (a, toSSA b)) substVars subts
+              template = dotApps (map (strToVar . fst) tempSubts)
+          in Replace Expr s tempSubts (unsafePrettyPrint template)
+    -- Rewrite @\x y -> x + y@ as @(+)@.
+    go [x,y] (L _ (OpApp _ (view -> Var_ x1) op@(L _ HsVar {}) (view -> Var_ y1)))
+        | x == x1, y == y1, vars op `disjoint` [x, y] = (op, \s -> [Replace Expr s [] (unsafePrettyPrint op)])
+    -- Rewrite @\x y -> f y x@ as @flip f@.
+    go [x, y] (view -> App2 op (view -> Var_ y1) (view -> Var_ x1))
+      | x == x1, y == y1, vars op `disjoint` [x, y] =
+          ( gen op
+          , \s -> [Replace Expr s [("x", toSSA op)] (unsafePrettyPrint $ gen (strToVar "x"))]
+          )
+      where
+        gen :: LHsExpr GhcPs -> LHsExpr GhcPs
+        gen = noLocA . HsApp noExtField (strToVar "flip")
+            . if isAtom op then id else addParen
+
+    -- We're done factoring, but have no variables left, so we shouldn't make a lambda.
+    -- @\ -> e@ ==> @e@
+    go [] e =
+      let -- Add brackets if needed, primarily for handling BlockArguments.
+          -- e.g., parent = `f \x -> g 3 x`; e = `g 3`.
+          -- Brackets should be placed around `e` to produce `f (g 3)` instead of `f g 3`.
+          addBrackets = case parent of
+            Just p -> isApp p && not (isVar e)
+            Nothing -> False
+          e' = if addBrackets then mkHsPar e else e
+          tpl = if addBrackets then "(a)" else "a"
+      in (e', \s -> [Replace Expr s [("a", toSSA e)] tpl])
+    -- Base case. Just a good old fashioned lambda.
+    go ss e =
+      let grhs = noLocA $ GRHS noAnn [] e :: LGRHS GhcPs (LHsExpr GhcPs)
+          grhss = GRHSs {grhssExt = emptyComments, grhssGRHSs=[grhs], grhssLocalBinds=EmptyLocalBinds noExtField}
+          match = noLocA $ Match {m_ext=noExtField, m_ctxt=LamAlt LamSingle, m_pats=noLocA $ map strToPat ss, m_grhss=grhss} :: LMatch GhcPs (LHsExpr GhcPs)
+          matchGroup = MG {mg_ext=Generated OtherExpansion SkipPmc, mg_alts=noLocA [match]}
+      in (noLocA $ HsLam noAnn LamSingle matchGroup, const [])
 
 
 -- 'case' and 'if' expressions have branches, nothing else does (this

src/Hint/Lambda.hs

@@ -39,6 +39,10 @@ f = foo (\y -> g x . h $ y) -- g x . h
 f = foo (\y -> g x . h $ y) -- @Message Avoid lambda
 f = foo ((*) x) -- (x *)
 f = (*) x
+f = g \x -> h 3 x -- (h 3)
+f = g (\x -> h 3 x) -- h 3
+f = g \x -> (`h` 3) x -- (`h` 3)
+f = g \x -> h x -- h
 f = foo (flip op x) -- (`op` x)
 f = foo (flip op x) -- @Message Use section
 f = foo (flip x y) -- (`x` y)
@@ -217,7 +221,7 @@ lambdaExp _ o@(L _ (HsPar _ (view -> App2 (view -> Var_ "flip") origf@(view -> R
 
 lambdaExp p o@(L _ (HsLam _ LamSingle _))
     | not $ any isOpApp p
-    , (res, refact) <- niceLambdaR [] o
+    , (res, refact) <- niceLambdaR p [] o
     , not $ isLambda res
     , not $ any isQuasiQuoteExpr $ universe res
     , not $ "runST" `Set.member` Set.map occNameString (freeVars o)