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diff --git a/src/parser/input.h b/src/parser/input.h
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+/*
+ * Copyright 2023 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef parser_input_h
+#define parser_input_h
+
+#include "lexer.h"
+#include "support/result.h"
+#include "wasm.h"
+
+namespace wasm::WATParser {
+
+using namespace std::string_view_literals;
+
+// Wraps a lexer and provides utilities for consuming tokens.
+struct ParseInput {
+  Lexer lexer;
+
+  explicit ParseInput(std::string_view in) : lexer(in) {}
+
+  ParseInput(std::string_view in, size_t index) : lexer(in) {
+    lexer.setIndex(index);
+  }
+
+  ParseInput(const ParseInput& other, size_t index) : lexer(other.lexer) {
+    lexer.setIndex(index);
+  }
+
+  bool empty() { return lexer.empty(); }
+
+  std::optional<Token> peek();
+  bool takeLParen();
+  bool takeRParen();
+  bool takeUntilParen();
+  std::optional<Name> takeID();
+  std::optional<std::string_view> takeKeyword();
+  bool takeKeyword(std::string_view expected);
+  std::optional<uint64_t> takeOffset();
+  std::optional<uint32_t> takeAlign();
+  std::optional<uint64_t> takeU64();
+  std::optional<int64_t> takeS64();
+  std::optional<int64_t> takeI64();
+  std::optional<uint32_t> takeU32();
+  std::optional<int32_t> takeS32();
+  std::optional<int32_t> takeI32();
+  std::optional<uint8_t> takeU8();
+  std::optional<double> takeF64();
+  std::optional<float> takeF32();
+  std::optional<std::string_view> takeString();
+  std::optional<Name> takeName();
+  bool takeSExprStart(std::string_view expected);
+
+  Index getPos();
+  [[nodiscard]] Err err(Index pos, std::string reason);
+  [[nodiscard]] Err err(std::string reason) { return err(getPos(), reason); }
+};
+
+#include "input-impl.h"
+
+} // namespace wasm::WATParser
+
+#endif // parser_input_h