+%************************************************************************
+%* *
+%* matchGuard : make a MatchResult from a guarded RHS *
+%* *
+%************************************************************************
+
+\begin{code}
+matchGuard :: [Stmt Id] -- Guard
+ -> DsMatchContext -- Context
+ -> 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)
+
+matchGuard [ResultStmt expr] ctx rhs_ty
+ = do { core_expr <- dsLExpr expr
+ ; return (cantFailMatchResult core_expr) }
+
+ -- ExprStmts must be guards
+ -- Turn an "otherwise" guard is a no-op
+matchGuard (ExprStmt (L _ (HsVar v)) _ : stmts) ctx rhs_ty
+ | v `hasKey` otherwiseIdKey
+ || v `hasKey` getUnique trueDataConId
+ -- trueDataConId doesn't have the same
+ -- unique as trueDataCon
+ = matchGuard stmts ctx rhs_ty
+
+matchGuard (ExprStmt expr _ : stmts) ctx rhs_ty
+ = matchGuard stmts ctx rhs_ty `thenDs` \ match_result ->
+ dsLExpr expr `thenDs` \ pred_expr ->
+ returnDs (mkGuardedMatchResult pred_expr match_result)
+
+matchGuard (LetStmt binds : stmts) ctx rhs_ty
+ = matchGuard stmts ctx rhs_ty `thenDs` \ match_result ->
+ returnDs (adjustMatchResultDs (dsLet 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
+
+matchGuard (BindStmt pat bind_rhs : stmts) ctx rhs_ty
+ = matchGuard stmts ctx rhs_ty `thenDs` \ match_result ->
+ dsLExpr bind_rhs `thenDs` \ core_rhs ->
+ matchSinglePat core_rhs ctx pat rhs_ty match_result
+\end{code}
+
+Should {\em fail} if @e@ returns @D@
+\begin{verbatim}
+f x | p <- e', let C y# = e, f y# = r1
+ | otherwise = r2
+\end{verbatim}