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diff --git a/src/wat-parser.h b/src/wat-parser.h
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+++ b/src/wat-parser.h
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+/*
+ * Copyright 2022 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 wasm_wat_parser_h
+#define wasm_wat_parser_h
+
+#include <optional>
+#include <string_view>
+
+#include "wasm.h"
+
+namespace wasm::WATParser {
+
+struct Ok {};
+
+struct None {};
+
+struct Err {
+  std::string msg;
+};
+
+template<typename T = Ok> struct Result {
+  std::variant<T, Err> val;
+
+  Result(Result<T>& other) = default;
+  Result(const Err& e) : val(std::in_place_type<Err>, e) {}
+  Result(Err&& e) : val(std::in_place_type<Err>, e) {}
+  template<typename U = T>
+  Result(U&& u) : val(std::in_place_type<T>, std::forward<U>(u)) {}
+
+  Err* getErr() { return std::get_if<Err>(&val); }
+  T& operator*() { return *std::get_if<T>(&val); }
+  T* operator->() { return std::get_if<T>(&val); }
+};
+
+template<typename T = Ok> struct MaybeResult {
+  std::variant<T, None, Err> val;
+
+  MaybeResult() : val(None{}) {}
+  MaybeResult(MaybeResult<T>& other) = default;
+  MaybeResult(const Err& e) : val(std::in_place_type<Err>, e) {}
+  MaybeResult(Err&& e) : val(std::in_place_type<Err>, std::move(e)) {}
+  template<typename U = T>
+  MaybeResult(U&& u) : val(std::in_place_type<T>, std::forward<U>(u)) {}
+
+  // Whether we have an error or a value. Useful for assignment in loops and if
+  // conditions where errors should not get lost.
+  operator bool() const { return !std::holds_alternative<None>(val); }
+
+  Err* getErr() { return std::get_if<Err>(&val); }
+  T& operator*() { return *std::get_if<T>(&val); }
+  T* operator->() { return std::get_if<T>(&val); }
+};
+
+// Parse a single WAT module.
+Result<> parseModule(Module& wasm, std::string_view in);
+
+} // namespace wasm::WATParser
+
+#endif // wasm_wat_parser.h