-matchGuard :: [TypecheckedStmt] -- Guard
- -> MatchResult -- What to do if the guard succeeds
- -> DsM MatchResult
-
-matchGuard [] body_result = returnDs body_result
-
- -- Turn an "otherwise" guard is a no-op
-matchGuard (GuardStmt (HsVar v) _ : stmts) body_result
- | uniqueOf v == otherwiseIdKey
- = matchGuard stmts body_result
-
-matchGuard (GuardStmt expr _ : stmts) body_result
- = matchGuard stmts body_result `thenDs` \ (MatchResult _ ty body_fn cxt) ->
- dsExpr expr `thenDs` \ core_expr ->
- let
- expr_fn = \ fail -> mkCoreIfThenElse core_expr (body_fn fail) fail
- in
- returnDs (MatchResult CanFail ty expr_fn cxt)
-
-matchGuard (LetStmt binds : stmts) body_result
- = matchGuard stmts body_result `thenDs` \ match_result ->
- dsBinds False{-don't auto scc-} binds `thenDs` \ core_binds ->
- returnDs (mkCoLetsMatchResult core_binds match_result)
-
-matchGuard (BindStmt pat rhs _ : stmts) body_result
- = matchGuard stmts body_result `thenDs` \ match_result ->
- dsExpr rhs `thenDs` \ core_rhs ->
- newSysLocalDs (coreExprType core_rhs) `thenDs` \ scrut_var ->
- match [scrut_var] [EqnInfo [pat] match_result] [] `thenDs` \ match_result' ->
- returnDs (mkCoLetsMatchResult [NonRec scrut_var core_rhs] match_result')
+matchGuards :: [Stmt Id] -- Guard
+ -> HsMatchContext Name -- Context
+ -> LHsExpr Id -- RHS
+ -> Type -- Type of RHS of guard
+ -> DsM MatchResult
+
+-- See comments with HsExpr.Stmt re what an ExprStmt means
+-- Here we must be in a guard context (not do-expression, nor list-comp)
+
+matchGuards [] ctx rhs rhs_ty
+ = do { core_rhs <- dsLExpr rhs
+ ; return (cantFailMatchResult core_rhs) }
+
+ -- ExprStmts must be guards
+ -- Turn an "otherwise" guard is a no-op. This ensures that
+ -- you don't get a "non-exhaustive eqns" message when the guards
+ -- finish in "otherwise".
+ -- NB: The success of this clause depends on the typechecker not
+ -- wrapping the 'otherwise' in empty HsTyApp or HsCoerce constructors
+ -- If it does, you'll get bogus overlap warnings
+matchGuards (ExprStmt (L _ (HsVar v)) _ _ : stmts) ctx rhs rhs_ty
+ | v `hasKey` otherwiseIdKey
+ || v `hasKey` getUnique trueDataConId
+ -- trueDataConId doesn't have the same unique as trueDataCon
+ = matchGuards stmts ctx rhs rhs_ty
+
+matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty
+ = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
+ dsLExpr expr `thenDs` \ pred_expr ->
+ returnDs (mkGuardedMatchResult pred_expr match_result)
+
+matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty
+ = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
+ returnDs (adjustMatchResultDs (dsLocalBinds binds) match_result)
+ -- NB the dsLet occurs inside the match_result
+ -- Reason: dsLet takes the body expression as its argument
+ -- so we can't desugar the bindings without the
+ -- body expression in hand
+
+matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty
+ = matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
+ dsLExpr bind_rhs `thenDs` \ core_rhs ->
+ matchSinglePat core_rhs ctx pat rhs_ty match_result