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+/**
+ * lisp-compiler: Compiles LISP using LLVM.
+ * Copyright (C) 2022  Clyne Sullivan
+ *
+ * This program is free software: you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later
+ * version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * @file ast.hpp
+ * @brief Abstract Syntax Tree (AST) implementation.
+ */
+
+#ifndef AST_HPP
+#define AST_HPP
+
+#include <string>
+#include <vector>
+#include <variant>
+
+struct CompilerState;
+
+namespace llvm {
+    struct Value;
+}
+
+namespace AST {
+
+using Value = llvm::Value *;
+
+struct Node
+{
+    virtual ~Node() {};
+
+    virtual Value codegen(CompilerState&) = 0;
+    virtual std::string desc() {
+        return "Node";
+    }
+};
+
+struct Identifier : public Node
+{
+    std::string name;
+
+    virtual Value codegen(CompilerState&) final;
+
+    virtual std::string desc() final {
+        return std::string("Identifier: ") + name;
+    }
+};
+
+struct Literal : public Node
+{
+    enum Type {
+        Unknown,
+        Number,
+    } type = Unknown;
+    std::variant<int, double> value;
+
+    virtual Value codegen(CompilerState&) final;
+
+    virtual std::string desc() final {
+        std::string str ("Literal: ");
+
+        switch (type) {
+        default:
+        case Unknown:
+            str += "unknown";
+            break;
+        case Number:
+            str += std::visit([](auto&& v) { return std::to_string(v); }, value);
+            break;
+        }
+
+        return str;
+    }
+};
+
+struct ProcedureCall : public Node
+{
+    Node *callee;
+    std::vector<Node *> operands;
+
+    virtual Value codegen(CompilerState&) final;
+
+    virtual std::string desc() final {
+        std::string str ("ProcedureCall: ");
+        
+        str = str + "<" + (callee ? callee->desc() : "unknown") + ">";
+        str += '(';
+        if (!operands.empty()) {
+            for (const auto& op : operands) {
+                str = str + "<" + (op ? op->desc() : "unknown") + ">";
+                str += ", ";
+            }
+
+            str.pop_back();
+            str.pop_back();
+        }
+        str += ')';
+
+        return str;
+    }
+};
+
+struct LambdaExpression : public Node
+{
+    std::vector<Identifier *> operands;
+    std::vector<Node *> body;
+
+    virtual Value codegen(CompilerState&) final;
+
+    virtual std::string desc() final {
+        return "LambdaExpression";
+    }
+};
+
+struct Conditional : public Node
+{
+    Node *cond;
+    Node *iftrue;
+    Node *iffalse;
+
+    virtual Value codegen(CompilerState&) final;
+
+    virtual std::string desc() final {
+        return "Conditional";
+    }
+};
+
+struct Definition : public Node
+{
+    Identifier *ident;
+    Node *value;
+
+    virtual Value codegen(CompilerState&) final;
+
+    virtual std::string desc() final {
+        return "Definition";
+    }
+};
+
+struct Assignment : public Node
+{
+    Identifier *ident;
+    Node *value;
+
+    virtual Value codegen(CompilerState&) final;
+
+    virtual std::string desc() final {
+        return "Assignment";
+    }
+};
+
+struct DerivedExpression : public Node
+{
+    std::string name;
+
+    virtual Value codegen(CompilerState&) final {
+        return nullptr;
+    }
+
+    virtual std::string desc() final {
+        return "DerivedExpression";
+    }
+};
+
+// MacroUse
+// MacroBlock
+// Includer
+
+}
+
+#endif // AST_HPP
+