Allow LambdaCase to be used as a command in proc notation

10 files changed, 112 insertions, 46 deletions

diff --git a/compiler/GHC/Hs/Expr.hs b/compiler/GHC/Hs/Expr.hs
index a03c0aa50d..97eab7d3aa 100644
--- a/compiler/GHC/Hs/Expr.hs
+++ b/compiler/GHC/Hs/Expr.hs
@@ -1330,6 +1330,14 @@ data HsCmd id
 
     -- For details on above see note [Api annotations] in GHC.Parser.Annotation
 
+  | HsCmdLamCase (XCmdLamCase id)
+                 (MatchGroup id (LHsCmd id))    -- bodies are HsCmd's
+    -- ^ - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnLam',
+    --       'ApiAnnotation.AnnCase','ApiAnnotation.AnnOpen' @'{'@,
+    --       'ApiAnnotation.AnnClose' @'}'@
+
+    -- For details on above see note [Api annotations] in GHC.Parser.Annotation
+
   | HsCmdIf     (XCmdIf id)
                 (SyntaxExpr id)         -- cond function
                 (LHsExpr id)            -- predicate
@@ -1371,6 +1379,7 @@ type instance XCmdApp     (GhcPass _) = NoExtField
 type instance XCmdLam     (GhcPass _) = NoExtField
 type instance XCmdPar     (GhcPass _) = NoExtField
 type instance XCmdCase    (GhcPass _) = NoExtField
+type instance XCmdLamCase (GhcPass _) = NoExtField
 type instance XCmdIf      (GhcPass _) = NoExtField
 type instance XCmdLet     (GhcPass _) = NoExtField
 
@@ -1460,6 +1469,9 @@ ppr_cmd (HsCmdCase _ expr matches)
   = sep [ sep [text "case", nest 4 (ppr expr), ptext (sLit "of")],
           nest 2 (pprMatches matches) ]
 
+ppr_cmd (HsCmdLamCase _ matches)
+  = sep [ text "\\case", nest 2 (pprMatches matches) ]
+
 ppr_cmd (HsCmdIf _ _ e ct ce)
   = sep [hsep [text "if", nest 2 (ppr e), ptext (sLit "then")],
          nest 4 (ppr ct),
diff --git a/compiler/GHC/Hs/Extension.hs b/compiler/GHC/Hs/Extension.hs
index 57cd67e65a..0de2ac35a6 100644
--- a/compiler/GHC/Hs/Extension.hs
+++ b/compiler/GHC/Hs/Extension.hs
@@ -599,6 +599,7 @@ type family XCmdApp     x
 type family XCmdLam     x
 type family XCmdPar     x
 type family XCmdCase    x
+type family XCmdLamCase x
 type family XCmdIf      x
 type family XCmdLet     x
 type family XCmdDo      x
diff --git a/compiler/GHC/HsToCore/Arrows.hs b/compiler/GHC/HsToCore/Arrows.hs
index 733ae86d6e..444989a18e 100644
--- a/compiler/GHC/HsToCore/Arrows.hs
+++ b/compiler/GHC/HsToCore/Arrows.hs
@@ -447,45 +447,12 @@ dsCmd ids local_vars stack_ty res_ty (HsCmdApp _ cmd arg) env_ids = do
             free_vars `unionDVarSet`
               (exprFreeIdsDSet core_arg `uniqDSetIntersectUniqSet` local_vars))
 
--- D; ys |-a cmd : stk t'
--- -----------------------------------------------
--- D; xs |-a \ p1 ... pk -> cmd : (t1,...(tk,stk)...) t'
---
---              ---> premap (\ ((xs), (p1, ... (pk,stk)...)) -> ((ys),stk)) cmd
-
 dsCmd ids local_vars stack_ty res_ty
         (HsCmdLam _ (MG { mg_alts
           = (L _ [L _ (Match { m_pats  = pats
                              , m_grhss = GRHSs _ [L _ (GRHS _ [] body)] _ })]) }))
-        env_ids = do
-    let pat_vars = mkVarSet (collectPatsBinders pats)
-    let
-        local_vars' = pat_vars `unionVarSet` local_vars
-        (pat_tys, stack_ty') = splitTypeAt (length pats) stack_ty
-    (core_body, free_vars, env_ids')
-       <- dsfixCmd ids local_vars' stack_ty' res_ty body
-    param_ids <- mapM newSysLocalDsNoLP pat_tys
-    stack_id' <- newSysLocalDs stack_ty'
-
-    -- the expression is built from the inside out, so the actions
-    -- are presented in reverse order
-
-    let
-        -- build a new environment, plus what's left of the stack
-        core_expr = buildEnvStack env_ids' stack_id'
-        in_ty = envStackType env_ids stack_ty
-        in_ty' = envStackType env_ids' stack_ty'
-
-    fail_expr <- mkFailExpr LambdaExpr in_ty'
-    -- match the patterns against the parameters
-    match_code <- matchSimplys (map Var param_ids) LambdaExpr pats core_expr
-                    fail_expr
-    -- match the parameters against the top of the old stack
-    (stack_id, param_code) <- matchVarStack param_ids stack_id' match_code
-    -- match the old environment and stack against the input
-    select_code <- matchEnvStack env_ids stack_id param_code
-    return (do_premap ids in_ty in_ty' res_ty select_code core_body,
-            free_vars `uniqDSetMinusUniqSet` pat_vars)
+        env_ids
+  = dsCmdLam ids local_vars stack_ty res_ty pats body env_ids
 
 dsCmd ids local_vars stack_ty res_ty (HsCmdPar _ cmd) env_ids
   = dsLCmd ids local_vars stack_ty res_ty cmd env_ids
@@ -626,6 +593,12 @@ dsCmd ids local_vars stack_ty res_ty
     return (do_premap ids in_ty sum_ty res_ty core_matches core_choices,
             exprFreeIdsDSet core_body `uniqDSetIntersectUniqSet` local_vars)
 
+dsCmd ids local_vars stack_ty res_ty
+      (HsCmdLamCase _ mg@MG { mg_ext = MatchGroupTc [arg_ty] _ }) env_ids = do
+  arg_id <- newSysLocalDs arg_ty
+  let case_cmd  = noLoc $ HsCmdCase noExtField (nlHsVar arg_id) mg
+  dsCmdLam ids local_vars stack_ty res_ty [nlVarPat arg_id] case_cmd env_ids
+
 -- D; ys |-a cmd : stk --> t
 -- ----------------------------------
 -- D; xs |-a let binds in cmd : stk --> t
@@ -693,7 +666,7 @@ dsCmd ids local_vars stack_ty res_ty (XCmd (HsWrap wrap cmd)) env_ids = do
     core_wrap <- dsHsWrapper wrap
     return (core_wrap core_cmd, env_ids')
 
-dsCmd _ _ _ _ _ c = pprPanic "dsCmd" (ppr c)
+dsCmd _ _ _ _ c _ = pprPanic "dsCmd" (ppr c)
 
 -- D; ys |-a c : stk --> t      (ys <= xs)
 -- ---------------------
@@ -753,6 +726,52 @@ trimInput build_arrow
         (core_cmd, free_vars) <- build_arrow env_ids
         return (core_cmd, free_vars, dVarSetElems free_vars))
 
+-- Desugaring for both HsCmdLam and HsCmdLamCase.
+--
+-- D; ys |-a cmd : stk t'
+-- -----------------------------------------------
+-- D; xs |-a \ p1 ... pk -> cmd : (t1,...(tk,stk)...) t'
+--
+--              ---> premap (\ ((xs), (p1, ... (pk,stk)...)) -> ((ys),stk)) cmd
+dsCmdLam :: DsCmdEnv            -- arrow combinators
+         -> IdSet               -- set of local vars available to this command
+         -> Type                -- type of the stack (right-nested tuple)
+         -> Type                -- return type of the command
+         -> [LPat GhcTc]        -- argument patterns to desugar
+         -> LHsCmd GhcTc        -- body to desugar
+         -> [Id]                -- list of vars in the input to this command
+                                -- This is typically fed back,
+                                -- so don't pull on it too early
+         -> DsM (CoreExpr,      -- desugared expression
+                 DIdSet)        -- subset of local vars that occur free
+dsCmdLam ids local_vars stack_ty res_ty pats body env_ids = do
+    let pat_vars = mkVarSet (collectPatsBinders pats)
+    let local_vars' = pat_vars `unionVarSet` local_vars
+        (pat_tys, stack_ty') = splitTypeAt (length pats) stack_ty
+    (core_body, free_vars, env_ids')
+       <- dsfixCmd ids local_vars' stack_ty' res_ty body
+    param_ids <- mapM newSysLocalDsNoLP pat_tys
+    stack_id' <- newSysLocalDs stack_ty'
+
+    -- the expression is built from the inside out, so the actions
+    -- are presented in reverse order
+
+    let -- build a new environment, plus what's left of the stack
+        core_expr = buildEnvStack env_ids' stack_id'
+        in_ty = envStackType env_ids stack_ty
+        in_ty' = envStackType env_ids' stack_ty'
+
+    fail_expr <- mkFailExpr LambdaExpr in_ty'
+    -- match the patterns against the parameters
+    match_code <- matchSimplys (map Var param_ids) LambdaExpr pats core_expr
+                    fail_expr
+    -- match the parameters against the top of the old stack
+    (stack_id, param_code) <- matchVarStack param_ids stack_id' match_code
+    -- match the old environment and stack against the input
+    select_code <- matchEnvStack env_ids stack_id param_code
+    return (do_premap ids in_ty in_ty' res_ty select_code core_body,
+            free_vars `uniqDSetMinusUniqSet` pat_vars)
+
 {-
 Translation of command judgements of the form
 
diff --git a/compiler/GHC/HsToCore/Coverage.hs b/compiler/GHC/HsToCore/Coverage.hs
index 8130565837..d8b83bb25e 100644
--- a/compiler/GHC/HsToCore/Coverage.hs
+++ b/compiler/GHC/HsToCore/Coverage.hs
@@ -861,6 +861,8 @@ addTickHsCmd (HsCmdCase x e mgs) =
         liftM2 (HsCmdCase x)
                 (addTickLHsExpr e)
                 (addTickCmdMatchGroup mgs)
+addTickHsCmd (HsCmdLamCase x mgs) =
+        liftM (HsCmdLamCase x) (addTickCmdMatchGroup mgs)
 addTickHsCmd (HsCmdIf x cnd e1 c2 c3) =
         liftM3 (HsCmdIf x cnd)
                 (addBinTickLHsExpr (BinBox CondBinBox) e1)
diff --git a/compiler/GHC/Iface/Ext/Ast.hs b/compiler/GHC/Iface/Ext/Ast.hs
index ffd7d26415..ddb29ce63d 100644
--- a/compiler/GHC/Iface/Ext/Ast.hs
+++ b/compiler/GHC/Iface/Ext/Ast.hs
@@ -1240,6 +1240,9 @@ instance ( a ~ GhcPass p
         [ toHie expr
         , toHie alts
         ]
+      HsCmdLamCase _ alts ->
+        [ toHie alts
+        ]
       HsCmdIf _ _ a b c ->
         [ toHie a
         , toHie b
diff --git a/compiler/GHC/Parser.y b/compiler/GHC/Parser.y
index 34d46fd4db..bafed741be 100644
--- a/compiler/GHC/Parser.y
+++ b/compiler/GHC/Parser.y
@@ -2765,11 +2765,10 @@ aexp    :: { ECP }
                                                (mj AnnLet $1:mj AnnIn $3
                                                  :(fst $ unLoc $2)) }
         | '\\' 'lcase' altslist
-            {% runPV $3 >>= \ $3 ->
-               fmap ecpFromExp $
-               ams (sLL $1 $> $ HsLamCase noExtField
+            {  ECP $ $3 >>= \ $3 ->
+               amms (mkHsLamCasePV (comb2 $1 $>)
                                    (mkMatchGroup FromSource (snd $ unLoc $3)))
-                   (mj AnnLam $1:mj AnnCase $2:(fst $ unLoc $3)) }
+                    (mj AnnLam $1:mj AnnCase $2:(fst $ unLoc $3)) }
         | 'if' exp optSemi 'then' exp optSemi 'else' exp
                          {% runECP_P $2 >>= \ $2 ->
                             return $ ECP $
diff --git a/compiler/GHC/Parser/PostProcess.hs b/compiler/GHC/Parser/PostProcess.hs
index fdc3085e3d..94137f07ba 100644
--- a/compiler/GHC/Parser/PostProcess.hs
+++ b/compiler/GHC/Parser/PostProcess.hs
@@ -1760,6 +1760,8 @@ class b ~ (Body b) GhcPs => DisambECP b where
   mkHsOpAppPV :: SrcSpan -> Located b -> Located (InfixOp b) -> Located b -> PV (Located b)
   -- | Disambiguate "case ... of ..."
   mkHsCasePV :: SrcSpan -> LHsExpr GhcPs -> MatchGroup GhcPs (Located b) -> PV (Located b)
+  -- | Disambiguate @\\case ...@ (lambda case)
+  mkHsLamCasePV :: SrcSpan -> MatchGroup GhcPs (Located b) -> PV (Located b)
   -- | Function argument representation
   type FunArg b
   -- | Bring superclass constraints on FunArg into scope.
@@ -1874,6 +1876,7 @@ instance DisambECP (HsCmd GhcPs) where
     let cmdArg c = L (getLoc c) $ HsCmdTop noExtField c
     return $ L l $ HsCmdArrForm noExtField op Infix Nothing [cmdArg c1, cmdArg c2]
   mkHsCasePV l c mg = return $ L l (HsCmdCase noExtField c mg)
+  mkHsLamCasePV l mg = return $ L l (HsCmdLamCase noExtField mg)
   type FunArg (HsCmd GhcPs) = HsExpr GhcPs
   superFunArg m = m
   mkHsAppPV l c e = do
@@ -1930,6 +1933,7 @@ instance DisambECP (HsExpr GhcPs) where
   mkHsOpAppPV l e1 op e2 = do
     return $ L l $ OpApp noExtField e1 op e2
   mkHsCasePV l e mg = return $ L l (HsCase noExtField e mg)
+  mkHsLamCasePV l mg = return $ L l (HsLamCase noExtField mg)
   type FunArg (HsExpr GhcPs) = HsExpr GhcPs
   superFunArg m = m
   mkHsAppPV l e1 e2 = do
@@ -2014,6 +2018,7 @@ instance DisambECP (PatBuilder GhcPs) where
   superInfixOp m = m
   mkHsOpAppPV l p1 op p2 = return $ L l $ PatBuilderOpApp p1 op p2
   mkHsCasePV l _ _ = addFatalError l $ text "(case ... of ...)-syntax in pattern"
+  mkHsLamCasePV l _ = addFatalError l $ text "(\\case ...)-syntax in pattern"
   type FunArg (PatBuilder GhcPs) = PatBuilder GhcPs
   superFunArg m = m
   mkHsAppPV l p1 p2 = return $ L l (PatBuilderApp p1 p2)
diff --git a/compiler/GHC/Rename/Expr.hs b/compiler/GHC/Rename/Expr.hs
index 773b194db8..6ec473134d 100644
--- a/compiler/GHC/Rename/Expr.hs
+++ b/compiler/GHC/Rename/Expr.hs
@@ -495,6 +495,10 @@ rnCmd (HsCmdCase x expr matches)
        ; (new_matches, ms_fvs) <- rnMatchGroup CaseAlt rnLCmd matches
        ; return (HsCmdCase x new_expr new_matches, e_fvs `plusFV` ms_fvs) }
 
+rnCmd (HsCmdLamCase x matches)
+  = do { (new_matches, ms_fvs) <- rnMatchGroup CaseAlt rnLCmd matches
+       ; return (HsCmdLamCase x new_matches, ms_fvs) }
+
 rnCmd (HsCmdIf x _ p b1 b2)
   = do { (p', fvP) <- rnLExpr p
        ; (b1', fvB1) <- rnLCmd b1
@@ -540,6 +544,8 @@ methodNamesCmd (HsCmdLam _ match)        = methodNamesMatch match
 
 methodNamesCmd (HsCmdCase _ _ matches)
   = methodNamesMatch matches `addOneFV` choiceAName
+methodNamesCmd (HsCmdLamCase _ matches)
+  = methodNamesMatch matches `addOneFV` choiceAName
 
 --methodNamesCmd _ = emptyFVs
    -- Other forms can't occur in commands, but it's not convenient
diff --git a/compiler/GHC/Tc/Gen/Arrow.hs b/compiler/GHC/Tc/Gen/Arrow.hs
index 5d26927ed4..6ac42a76d0 100644
--- a/compiler/GHC/Tc/Gen/Arrow.hs
+++ b/compiler/GHC/Tc/Gen/Arrow.hs
@@ -151,13 +151,14 @@ tc_cmd env (HsCmdLet x (L l binds) (L body_loc body)) res_ty
 tc_cmd env in_cmd@(HsCmdCase x scrut matches) (stk, res_ty)
   = addErrCtxt (cmdCtxt in_cmd) $ do
       (scrut', scrut_ty) <- tcInferRho scrut
-      matches' <- tcMatchesCase match_ctxt scrut_ty matches (mkCheckExpType res_ty)
+      matches' <- tcCmdMatches env scrut_ty matches (stk, res_ty)
       return (HsCmdCase x scrut' matches')
-  where
-    match_ctxt = MC { mc_what = CaseAlt,
-                      mc_body = mc_body }
-    mc_body body res_ty' = do { res_ty' <- expTypeToType res_ty'
-                              ; tcCmd env body (stk, res_ty') }
+
+tc_cmd env in_cmd@(HsCmdLamCase x matches) (stk, res_ty)
+  = addErrCtxt (cmdCtxt in_cmd) $ do
+      (co, [scrut_ty], stk') <- matchExpectedCmdArgs 1 stk
+      matches' <- tcCmdMatches env scrut_ty matches (stk', res_ty)
+      return (mkHsCmdWrap (mkWpCastN co) (HsCmdLamCase x matches'))
 
 tc_cmd env (HsCmdIf x NoSyntaxExprRn pred b1 b2) res_ty    -- Ordinary 'if'
   = do  { pred' <- tcLExpr pred (mkCheckExpType boolTy)
@@ -330,6 +331,20 @@ tc_cmd _ cmd _
   = failWithTc (vcat [text "The expression", nest 2 (ppr cmd),
                       text "was found where an arrow command was expected"])
 
+-- | Typechecking for case command alternatives. Used for both
+-- 'HsCmdCase' and 'HsCmdLamCase'.
+tcCmdMatches :: CmdEnv
+             -> TcType                           -- ^ type of the scrutinee
+             -> MatchGroup GhcRn (LHsCmd GhcRn)  -- ^ case alternatives
+             -> CmdType
+             -> TcM (MatchGroup GhcTcId (LHsCmd GhcTcId))
+tcCmdMatches env scrut_ty matches (stk, res_ty)
+  = tcMatchesCase match_ctxt scrut_ty matches (mkCheckExpType res_ty)
+  where
+    match_ctxt = MC { mc_what = CaseAlt,
+                      mc_body = mc_body }
+    mc_body body res_ty' = do { res_ty' <- expTypeToType res_ty'
+                              ; tcCmd env body (stk, res_ty') }
 
 matchExpectedCmdArgs :: Arity -> TcType -> TcM (TcCoercionN, [TcType], TcType)
 matchExpectedCmdArgs 0 ty
diff --git a/compiler/GHC/Tc/Utils/Zonk.hs b/compiler/GHC/Tc/Utils/Zonk.hs
index 8fbbba22b1..4372a39e9d 100644
--- a/compiler/GHC/Tc/Utils/Zonk.hs
+++ b/compiler/GHC/Tc/Utils/Zonk.hs
@@ -995,6 +995,10 @@ zonkCmd env (HsCmdCase x expr ms)
        new_ms <- zonkMatchGroup env zonkLCmd ms
        return (HsCmdCase x new_expr new_ms)
 
+zonkCmd env (HsCmdLamCase x ms)
+  = do new_ms <- zonkMatchGroup env zonkLCmd ms
+       return (HsCmdLamCase x new_ms)
+
 zonkCmd env (HsCmdIf x eCond ePred cThen cElse)
   = do { (env1, new_eCond) <- zonkSyntaxExpr env eCond
        ; new_ePred <- zonkLExpr env1 ePred