xo-reader2 scaffold (fomo+arena version of xo-reader/) [WIP]

include/xo/reader2/DExprSeqState.hpp

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+/** @file DExprSeqState.hpp
+*
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#pragma once
+
+#include "ParserStateMachine.hpp"
+#include "SyntaxStateMachine.hpp"
+#include "syntaxstatetype.hpp"
+#include <xo/facet/obj.hpp>
+
+namespace xo {
+    namespace scm {
+        enum class exprseqtype {
+            /** toplevel interactive sequence.
+             *  allows: rvalue expressions
+             **/
+            toplevel_interactive,
+            /** toplevel non-interactive sequence.
+             *  allows:
+             **/
+            toplevel_batch,
+            /** counts number of valid enums **/
+            N
+        };
+
+        /** @class DExprSeqState
+         *  @brief state machine for parsing a sequence of expression
+         *
+         *  Similar to exprseq_xs in xo-expresion
+         **/
+        class DExprSeqState {
+        public:
+            using AAllocator = xo::mm::AAllocator;
+
+        public:
+            explicit DExprSeqState(exprseqtype ty);
+
+            /** start interactive top-level session **/
+            static void start_interactive(obj<AAllocator> mm,
+                                          ParserStateMachine * p_psm);
+            /** start non-interactive top-level session **/
+            static void start_batch(obj<AAllocator> mm,
+                                    ParserStateMachine * p_psm);
+
+        public:
+            /** @defgroup scm-exprseq-ssm-facet syntaxstatemachine facet methods **/
+            ///@{
+
+            /** identifies the ssm implemented here **/
+            syntaxstatetype ssm_type() const noexcept;
+
+            /** text describing expected/allowed input to this ssm in current state.
+             *  Intended to drive error mesages
+             **/
+            std::string_view get_expect_str() const noexcept;
+
+            /** update state for this syntax on incoming token @p tk,
+             *  overall parser state in @p p_psm
+             **/
+            void on_if_token(const Token & tk, ParserStateMachine * p_psm);
+
+            ///@}
+
+        private:
+            /** sequence type. accept rvalue expressions when
+             *  this is toplevel_interactive.
+             *  Always accept definitions and declarations.
+             **/
+            exprseqtype seqtype_;
+        };
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end DExprSeqState.hpp */