ASTLowerer.cpp

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#include <Ark/Compiler/Lowerer/ASTLowerer.hpp>
 
#include <ranges>
#include <utility>
#include <algorithm>
#include <fmt/core.h>
#include <fmt/color.h>
 
#include <Ark/Error/Exceptions.hpp>
#include <Ark/Error/Diagnostics.hpp>
#include <Ark/Utils/Literals.hpp>
#include <Ark/Builtins/Builtins.hpp>
 
namespace Ark::internal
{
    using namespace literals;
 
    ASTLowerer::ASTLowerer(const unsigned debug) :
        m_logger("ASTLowerer", debug)
    {}
 
    void ASTLowerer::process(Node& ast)
    {
        m_logger.traceStart("process");
        m_code_pages.emplace_back();  // create empty page
 
        // gather symbols, values, and start to create code segments
        compileExpression(
            ast,
            /* current_page */ Page { .index = 0, .is_temp = false },
            /* is_result_unused */ false,
            /* is_terminal */ false);
        m_logger.traceEnd();
    }
 
    const std::vector<IR::Block>& ASTLowerer::intermediateRepresentation() const noexcept
    {
        return m_code_pages;
    }
 
    const std::vector<std::string>& ASTLowerer::symbols() const noexcept
    {
        return m_symbols;
    }
 
    const std::vector<ValTableElem>& ASTLowerer::values() const noexcept
    {
        return m_values;
    }
 
    std::optional<Instruction> ASTLowerer::getOperator(const std::string& name) noexcept
    {
        const auto it = std::ranges::find(Language::operators, name);
        if (it != Language::operators.end())
            return static_cast<Instruction>(std::distance(Language::operators.begin(), it) + FIRST_OPERATOR);
        return std::nullopt;
    }
 
    std::optional<uint16_t> ASTLowerer::getBuiltin(const std::string& name) noexcept
    {
        const auto it = std::ranges::find_if(Builtins::builtins,
                                             [&name](const std::pair<std::string, Value>& element) -> bool {
                                                 return name == element.first;
                                             });
        if (it != Builtins::builtins.end())
            return static_cast<uint16_t>(std::distance(Builtins::builtins.begin(), it));
        return std::nullopt;
    }
 
    std::optional<Instruction> ASTLowerer::getListInstruction(const std::string& name) noexcept
    {
        const auto it = std::ranges::find(Language::listInstructions, name);
        if (it != Language::listInstructions.end())
            return static_cast<Instruction>(std::distance(Language::listInstructions.begin(), it) + LIST);
        return std::nullopt;
    }
 
    bool ASTLowerer::nodeProducesOutput(const Node& node)
    {
        if (node.nodeType() == NodeType::List && !node.constList().empty() && node.constList()[0].nodeType() == NodeType::Keyword)
            // a begin node produces a value if the last node in it produces a value
            return (node.constList()[0].keyword() == Keyword::Begin && node.constList().size() > 1 && nodeProducesOutput(node.constList().back())) ||
                // a function always produces a value ; even if it ends with a node not producing one, the VM returns nil
                node.constList()[0].keyword() == Keyword::Fun ||
                // a condition produces a value if all its branches produce a value
                (node.constList()[0].keyword() == Keyword::If &&
                 nodeProducesOutput(node.constList()[2]) &&
                 (node.constList().size() == 3 || nodeProducesOutput(node.constList()[3])));
        // in place list instruction, as well as assert, do not produce values
        if (node.nodeType() == NodeType::List && !node.constList().empty() && node.constList()[0].nodeType() == NodeType::Symbol)
            return std::ranges::find(Language::UpdateRef, node.constList().front().string()) == Language::UpdateRef.end() &&
                node.constList().front().string() != "assert";
        return true;  // any other node, function call, symbol, number...
    }
 
    bool ASTLowerer::isUnaryInst(const Instruction inst) noexcept
    {
        switch (inst)
        {
            case NOT: [[fallthrough]];
            case LEN: [[fallthrough]];
            case EMPTY: [[fallthrough]];
            case TAIL: [[fallthrough]];
            case HEAD: [[fallthrough]];
            case ISNIL: [[fallthrough]];
            case TO_NUM: [[fallthrough]];
            case TO_STR: [[fallthrough]];
            case TYPE:
                return true;
 
            default:
                return false;
        }
    }
 
    bool ASTLowerer::isTernaryInst(const Instruction inst) noexcept
    {
        switch (inst)
        {
            case AT_AT:
                return true;
 
            default:
                return false;
        }
    }
 
    void ASTLowerer::warning(const std::string& message, const Node& node)
    {
        fmt::println("{} {}", fmt::styled("Warning", fmt::fg(fmt::color::dark_orange)), Diagnostics::makeContextWithNode(message, node));
    }
 
    void ASTLowerer::buildAndThrowError(const std::string& message, const Node& node)
    {
        throw CodeError(message, CodeErrorContext(node.filename(), node.position()));
    }
 
    void ASTLowerer::compileExpression(Node& x, const Page p, const bool is_result_unused, const bool is_terminal)
    {
        // register symbols
        if (x.nodeType() == NodeType::Symbol)
            compileSymbol(x, p, is_result_unused);
        else if (x.nodeType() == NodeType::Field)
        {
            // the parser guarantees us that there is at least 2 elements (eg: a.b)
            compileSymbol(x.list()[0], p, is_result_unused);
            for (auto it = x.constList().begin() + 1, end = x.constList().end(); it != end; ++it)
            {
                uint16_t i = addSymbol(*it);
                page(p).emplace_back(GET_FIELD, i);
            }
            page(p).back().setSourceLocation(x.filename(), x.position().start.line);
        }
        // register values
        else if (x.nodeType() == NodeType::String || x.nodeType() == NodeType::Number)
        {
            uint16_t i = addValue(x);
 
            if (!is_result_unused)
                page(p).emplace_back(LOAD_CONST, i);
        }
        // namespace nodes
        else if (x.nodeType() == NodeType::Namespace)
            compileExpression(*x.constArkNamespace().ast, p, is_result_unused, is_terminal);
        else if (x.nodeType() == NodeType::Unused)
        {
            // do nothing, explicitly
        }
        // empty code block should be nil
        else if (x.constList().empty())
        {
            if (!is_result_unused)
            {
                static const std::optional<uint16_t> nil = getBuiltin("nil");
                page(p).emplace_back(BUILTIN, nil.value());
            }
        }
        // list instructions
        else if (const auto head = x.constList()[0]; head.nodeType() == NodeType::Symbol && getListInstruction(head.string()).has_value())
            compileListInstruction(x, p, is_result_unused);
        // registering structures
        else if (x.constList()[0].nodeType() == NodeType::Keyword)
        {
            switch (const Keyword keyword = x.constList()[0].keyword())
            {
                case Keyword::If:
                    compileIf(x, p, is_result_unused, is_terminal);
                    break;
 
                case Keyword::Set:
                    [[fallthrough]];
                case Keyword::Let:
                    [[fallthrough]];
                case Keyword::Mut:
                    compileLetMutSet(keyword, x, p);
                    break;
 
                case Keyword::Fun:
                    compileFunction(x, p, is_result_unused);
                    break;
 
                case Keyword::Begin:
                {
                    for (std::size_t i = 1, size = x.list().size(); i < size; ++i)
                        compileExpression(
                            x.list()[i],
                            p,
                            // All the nodes in a begin (except for the last one) are producing a result that we want to drop.
                            (i != size - 1) || is_result_unused,
                            // If the begin is a terminal node, only its last node is terminal.
                            is_terminal && (i == size - 1));
                    break;
                }
 
                case Keyword::While:
                    compileWhile(x, p);
                    break;
 
                case Keyword::Import:
                    compilePluginImport(x, p);
                    break;
 
                case Keyword::Del:
                    page(p).emplace_back(DEL, addSymbol(x.constList()[1]));
                    page(p).back().setSourceLocation(x.filename(), x.position().start.line);
                    break;
            }
        }
        else if (x.nodeType() == NodeType::List)
        {
            // If we are here, we should have a function name via the m_opened_vars.
            // Push arguments first, then function name, then call it.
            handleCalls(x, p, is_result_unused, is_terminal);
        }
        else
            buildAndThrowError(
                fmt::format(
                    "NodeType `{}' not handled in ASTLowerer::compileExpression. Please fill an issue on GitHub: https://github.com/ArkScript-lang/Ark",
                    typeToString(x)),
                x);
    }
 
    void ASTLowerer::compileSymbol(const Node& x, const Page p, const bool is_result_unused)
    {
        const std::string& name = x.string();
 
        if (const auto it_builtin = getBuiltin(name))
            page(p).emplace_back(Instruction::BUILTIN, it_builtin.value());
        else if (getOperator(name).has_value())
            buildAndThrowError(fmt::format("Found a free standing operator: `{}`", name), x);
        else
        {
            const std::optional<std::size_t> maybe_local_idx = m_locals_locator.lookupLastScopeByName(name);
            if (maybe_local_idx.has_value())
                page(p).emplace_back(LOAD_SYMBOL_BY_INDEX, static_cast<uint16_t>(maybe_local_idx.value()));
            else
                page(p).emplace_back(LOAD_SYMBOL, addSymbol(x));
        }
 
        if (is_result_unused)
        {
            warning("Statement has no effect", x);
            page(p).emplace_back(POP);
        }
    }
 
    void ASTLowerer::compileListInstruction(Node& x, const Page p, const bool is_result_unused)
    {
        const Node head = x.constList()[0];
        std::string name = x.constList()[0].string();
        Instruction inst = getListInstruction(name).value();
 
        // length of at least 1 since we got a symbol name
        const auto argc = x.constList().size() - 1u;
        // error, can not use append/concat/pop (and their in place versions) with a <2 length argument list
        if (argc < 2 && APPEND <= inst && inst <= POP)
            buildAndThrowError(fmt::format("Can not use {} with less than 2 arguments", name), head);
        if (inst <= POP && std::cmp_greater(argc, MaxValue16Bits))
            buildAndThrowError(fmt::format("Too many arguments ({}), exceeds {}", argc, MaxValue16Bits), x);
        if (argc != 3 && inst == SET_AT_INDEX)
            buildAndThrowError(fmt::format("Expected 3 arguments (list, index, value) for {}, got {}", name, argc), head);
        if (argc != 4 && inst == SET_AT_2_INDEX)
            buildAndThrowError(fmt::format("Expected 4 arguments (list, y, x, value) for {}, got {}", name, argc), head);
 
        // compile arguments in reverse order
        for (std::size_t i = x.constList().size() - 1u; i > 0; --i)
        {
            Node& node = x.list()[i];
            if (nodeProducesOutput(node))
                compileExpression(node, p, false, false);
            else
                buildAndThrowError(fmt::format("Invalid node inside call to {}", name), node);
        }
 
        // put inst and number of arguments
        std::size_t inst_argc = 0;
        switch (inst)
        {
            case LIST:
                inst_argc = argc;
                break;
 
            case APPEND:
            case APPEND_IN_PLACE:
            case CONCAT:
            case CONCAT_IN_PLACE:
                inst_argc = argc - 1;
                break;
 
            case POP_LIST:
            case POP_LIST_IN_PLACE:
                inst_argc = 0;
                break;
 
            default:
                break;
        }
        page(p).emplace_back(inst, static_cast<uint16_t>(inst_argc));
        page(p).back().setSourceLocation(head.filename(), head.position().start.line);
 
        if (is_result_unused && name.back() != '!' && inst <= POP_LIST_IN_PLACE)  // in-place functions never push a value
        {
            warning("Ignoring return value of function", x);
            page(p).emplace_back(POP);
        }
    }
 
    void ASTLowerer::compileIf(Node& x, const Page p, const bool is_result_unused, const bool is_terminal)
    {
        if (x.constList().size() == 1)
            buildAndThrowError("Invalid condition: missing 'cond' and 'then' nodes, expected (if cond then)", x);
        if (x.constList().size() == 2)
            buildAndThrowError(fmt::format("Invalid condition: missing 'then' node, expected (if {} then)", x.constList()[1].repr()), x);
 
        // compile condition
        compileExpression(x.list()[1], p, false, false);
        page(p).back().setSourceLocation(x.constList()[1].filename(), x.constList()[1].position().start.line);
 
        // jump only if needed to the "true" branch
        const auto label_then = IR::Entity::Label(m_current_label++);
        page(p).emplace_back(IR::Entity::GotoIf(label_then, true));
 
        // "false" branch code
        if (x.constList().size() == 4)  // we have an else clause
        {
            m_locals_locator.saveScopeLengthForBranch();
            compileExpression(x.list()[3], p, is_result_unused, is_terminal);
            page(p).back().setSourceLocation(x.constList()[3].filename(), x.constList()[3].position().start.line);
            m_locals_locator.dropVarsForBranch();
        }
 
        // when else is finished, jump to end
        const auto label_end = IR::Entity::Label(m_current_label++);
        page(p).emplace_back(IR::Entity::Goto(label_end));
 
        // absolute address to jump to if condition is true
        page(p).emplace_back(label_then);
        // if code
        m_locals_locator.saveScopeLengthForBranch();
        compileExpression(x.list()[2], p, is_result_unused, is_terminal);
        page(p).back().setSourceLocation(x.constList()[2].filename(), x.constList()[2].position().start.line);
        m_locals_locator.dropVarsForBranch();
        // set jump to end pos
        page(p).emplace_back(label_end);
    }
 
    void ASTLowerer::compileFunction(Node& x, const Page p, const bool is_result_unused)
    {
        if (const auto args = x.constList()[1]; args.nodeType() != NodeType::List)
            buildAndThrowError(fmt::format("Expected a well formed argument(s) list, got a {}", typeToString(args)), args);
        if (x.constList().size() != 3)
            buildAndThrowError("Invalid node ; if it was computed by a macro, check that a node is returned", x);
 
        // capture, if needed
        std::size_t capture_inst_count = 0;
        for (const auto& node : x.constList()[1].constList())
        {
            if (node.nodeType() == NodeType::Capture)
            {
                const uint16_t symbol_id = addSymbol(node);
 
                // We have an unqualified name that isn't the captured name
                // This means we need to rename the captured value
                if (const auto& maybe_nqn = node.getUnqualifiedName(); maybe_nqn.has_value() && maybe_nqn.value() != node.string())
                {
                    const uint16_t nqn_id = addSymbol(Node(NodeType::Symbol, maybe_nqn.value()));
 
                    page(p).emplace_back(RENAME_NEXT_CAPTURE, nqn_id);
                    page(p).emplace_back(CAPTURE, symbol_id);
                }
                else
                    page(p).emplace_back(CAPTURE, symbol_id);
 
                ++capture_inst_count;
            }
        }
        const bool is_closure = capture_inst_count > 0;
 
        m_locals_locator.createScope(
            is_closure
                ? LocalsLocator::ScopeType::Closure
                : LocalsLocator::ScopeType::Function);
 
        // create new page for function body
        m_code_pages.emplace_back();
        const auto function_body_page = Page { .index = m_code_pages.size() - 1, .is_temp = false };
        // save page_id into the constants table as PageAddr and load the const
        page(p).emplace_back(is_closure ? MAKE_CLOSURE : LOAD_CONST, addValue(function_body_page.index, x));
 
        // pushing arguments from the stack into variables in the new scope
        for (const auto& node : x.constList()[1].constList())
        {
            if (node.nodeType() == NodeType::Symbol)
            {
                page(function_body_page).emplace_back(STORE, addSymbol(node));
                m_locals_locator.addLocal(node.string());
            }
        }
 
        // Register an opened variable as "#anonymous", which won't match any valid names inside ASTLowerer::handleCalls.
        // This way we can continue to safely apply optimisations on
        // (let name (fun (e) (map lst (fun (e) (name e)))))
        // Otherwise, `name` would have been optimized to a GET_CURRENT_PAGE_ADDRESS, which would have returned the wrong page.
        if (x.isAnonymousFunction())
            m_opened_vars.push("#anonymous");
        // push body of the function
        compileExpression(x.list()[2], function_body_page, false, true);
        if (x.isAnonymousFunction())
            m_opened_vars.pop();
 
        // return last value on the stack
        page(function_body_page).emplace_back(RET);
        m_locals_locator.deleteScope();
 
        // if the computed function is unused, pop it
        if (is_result_unused)
        {
            warning("Unused declared function", x);
            page(p).emplace_back(POP);
        }
    }
 
    void ASTLowerer::compileLetMutSet(const Keyword n, Node& x, const Page p)
    {
        if (const auto sym = x.constList()[1]; sym.nodeType() != NodeType::Symbol)
            buildAndThrowError(fmt::format("Expected a symbol, got a {}", typeToString(sym)), sym);
        if (x.constList().size() != 3)
            buildAndThrowError("Invalid node ; if it was computed by a macro, check that a node is returned", x);
 
        const std::string name = x.constList()[1].string();
        uint16_t i = addSymbol(x.constList()[1]);
 
        if (!m_opened_vars.empty() && m_opened_vars.top() == name)
            buildAndThrowError("Can not define a variable using the same name as the function it is defined inside", x);
 
        const bool is_function = x.constList()[2].isFunction();
        if (is_function)
        {
            m_opened_vars.push(name);
            x.list()[2].setFunctionKind(/* anonymous= */ false);
        }
 
        // put value before symbol id
        // starting at index = 2 because x is a (let|mut|set variable ...) node
        for (std::size_t idx = 2, end = x.constList().size(); idx < end; ++idx)
            compileExpression(x.list()[idx], p, false, false);
 
        if (n == Keyword::Let || n == Keyword::Mut)
        {
            page(p).emplace_back(STORE, i);
            m_locals_locator.addLocal(name);
        }
        else
            page(p).emplace_back(SET_VAL, i);
 
        if (is_function)
            m_opened_vars.pop();
        page(p).back().setSourceLocation(x.filename(), x.position().start.line);
    }
 
    void ASTLowerer::compileWhile(Node& x, const Page p)
    {
        if (x.constList().size() != 3)
            buildAndThrowError("Invalid node ; if it was computed by a macro, check that a node is returned", x);
 
        m_locals_locator.createScope();
        page(p).emplace_back(CREATE_SCOPE);
        page(p).back().setSourceLocation(x.filename(), x.position().start.line);
 
        // save current position to jump there at the end of the loop
        const auto label_loop = IR::Entity::Label(m_current_label++);
        page(p).emplace_back(label_loop);
        // push condition
        compileExpression(x.list()[1], p, false, false);
        // absolute jump to end of block if condition is false
        const auto label_end = IR::Entity::Label(m_current_label++);
        page(p).emplace_back(IR::Entity::GotoIf(label_end, false));
        // push code to page
        compileExpression(x.list()[2], p, true, false);
 
        // reset the scope at the end of the loop so that indices are still valid
        // otherwise, (while true { (let a 5) (print a) (let b 6) (print b) })
        // would print 5, 6, then only 6 as we emit LOAD_SYMBOL_FROM_INDEX 0 and b is the last in the scope
        // loop, jump to the condition
        page(p).emplace_back(IR::Entity::Goto(label_loop, RESET_SCOPE_JUMP));
 
        // absolute address to jump to if condition is false
        page(p).emplace_back(label_end);
 
        page(p).emplace_back(POP_SCOPE);
        m_locals_locator.deleteScope();
    }
 
    void ASTLowerer::compilePluginImport(const Node& x, const Page p)
    {
        std::string path;
        const Node package_node = x.constList()[1];
        for (std::size_t i = 0, end = package_node.constList().size(); i < end; ++i)
        {
            path += package_node.constList()[i].string();
            if (i + 1 != end)
                path += "/";
        }
        path += ".arkm";
 
        // register plugin path in the constants table
        uint16_t id = addValue(Node(NodeType::String, path));
        // add plugin instruction + id of the constant referring to the plugin path
        page(p).emplace_back(PLUGIN, id);
        page(p).back().setSourceLocation(x.filename(), x.position().start.line);
    }
 
    void ASTLowerer::pushFunctionCallArguments(Node& call, const Page p, const bool is_tail_call)
    {
        const auto node = call.constList()[0];
 
        // push the arguments in reverse order because the function loads its arguments in the order they are defined:
        // (fun (a b c) ...) -> load 'a', then 'b', then 'c'
        // We have to push arguments in this order and load them in reverse, because we are using references internally,
        // which can cause problems for recursive functions that swap their arguments around.
        // Eg (let foo (fun (a b c) (if (> a 0) (foo (- a 1) c (+ b c)) 1))) (foo 12 0 1)
        // On the second self-call, b and c would have the same value, since we set c to (+ b c), and we pushed c as the
        // value for argument b, but loaded it as a reference.
        for (Node& value : std::ranges::drop_view(call.list(), 1) | std::views::reverse)
        {
            if (nodeProducesOutput(value))
                compileExpression(value, p, false, false);
            else
            {
                std::string message;
                if (is_tail_call)
                    message = fmt::format("Invalid node inside tail call to `{}'", node.repr());
                else
                    message = fmt::format("Invalid node inside call to `{}'", node.repr());
                buildAndThrowError(message, value);
            }
        }
    }
 
    void ASTLowerer::handleCalls(Node& x, const Page p, bool is_result_unused, const bool is_terminal)
    {
        constexpr std::size_t start_index = 1;
 
        Node& node = x.list()[0];
        const std::optional<Instruction> maybe_operator = node.nodeType() == NodeType::Symbol ? getOperator(node.string()) : std::nullopt;
 
        const std::optional<Instruction> maybe_shortcircuit =
            node.nodeType() == NodeType::Symbol
            ? (node.string() == Language::And
                   ? std::make_optional(Instruction::SHORTCIRCUIT_AND)
                   : (node.string() == Language::Or
                          ? std::make_optional(Instruction::SHORTCIRCUIT_OR)
                          : std::nullopt))
            : std::nullopt;
 
        if (maybe_shortcircuit.has_value())
        {
            // short circuit implementation
            if (x.constList().size() < 3)
                buildAndThrowError(
                    fmt::format(
                        "Expected at least 2 arguments while compiling '{}', got {}",
                        node.string(),
                        x.constList().size() - 1),
                    x);
 
            compileExpression(x.list()[1], p, false, false);
 
            const auto label_shortcircuit = IR::Entity::Label(m_current_label++);
            auto shortcircuit_entity = IR::Entity::Goto(label_shortcircuit, maybe_shortcircuit.value());
            page(p).emplace_back(shortcircuit_entity);
 
            for (std::size_t i = 2, end = x.constList().size(); i < end; ++i)
            {
                compileExpression(x.list()[i], p, false, false);
                if (i + 1 != end)
                    page(p).emplace_back(shortcircuit_entity);
            }
 
            page(p).emplace_back(label_shortcircuit);
        }
        else if (!maybe_operator.has_value())
        {
            if (is_terminal && node.nodeType() == NodeType::Symbol && !m_opened_vars.empty() && m_opened_vars.top() == node.string())
            {
                pushFunctionCallArguments(x, p, /* is_tail_call= */ true);
 
                // jump to the top of the function
                page(p).emplace_back(JUMP, 0_u16);
                page(p).back().setSourceLocation(node.filename(), node.position().start.line);
                return;  // skip the potential Instruction::POP at the end
            }
            else
            {
                if (!nodeProducesOutput(node))
                    buildAndThrowError(fmt::format("Can not call `{}', as it doesn't return a value", node.repr()), node);
 
                m_temp_pages.emplace_back();
                const auto proc_page = Page { .index = m_temp_pages.size() - 1u, .is_temp = true };
 
                // compile the function resolution to a separate page
                if (node.nodeType() == NodeType::Symbol && !m_opened_vars.empty() && m_opened_vars.top() == node.string())
                {
                    // The function is trying to call itself, but this isn't a tail call.
                    // We can skip the LOAD_SYMBOL function_name and directly push the current
                    // function page, which will be quicker than a local variable resolution.
                    // We set its argument to the symbol id of the function we are calling,
                    // so that the VM knows the name of the last called function.
                    page(proc_page).emplace_back(GET_CURRENT_PAGE_ADDR, addSymbol(node));
                }
                else
                {
                    // closure chains have been handled (eg: closure.field.field.function)
                    compileExpression(node, proc_page, false, false);  // storing proc
                }
 
                if (m_temp_pages.back().empty())
                    buildAndThrowError(fmt::format("Can not call {}", x.constList()[0].repr()), x);
 
                const auto label_return = IR::Entity::Label(m_current_label++);
                page(p).emplace_back(IR::Entity::Goto(label_return, PUSH_RETURN_ADDRESS));
 
                pushFunctionCallArguments(x, p, /* is_tail_call= */ false);
                // push proc from temp page
                for (const auto& inst : m_temp_pages.back())
                    page(p).push_back(inst);
                m_temp_pages.pop_back();
 
                // number of arguments
                std::size_t args_count = 0;
                for (auto it = x.constList().begin() + start_index, it_end = x.constList().end(); it != it_end; ++it)
                {
                    if (it->nodeType() != NodeType::Capture)
                        args_count++;
                }
                // call the procedure
                page(p).emplace_back(CALL, args_count);
                page(p).back().setSourceLocation(node.filename(), node.position().start.line);
 
                // patch the PUSH_RETURN_ADDRESS instruction with the return location (IP=CALL instruction IP)
                page(p).emplace_back(label_return);
            }
        }
        else  // operator
        {
            // retrieve operator
            auto op = maybe_operator.value();
 
            if (op == ASSERT)
                is_result_unused = false;
 
            // push arguments on current page
            std::size_t exp_count = 0;
            for (std::size_t index = start_index, size = x.constList().size(); index < size; ++index)
            {
                if (nodeProducesOutput(x.constList()[index]))
                    compileExpression(x.list()[index], p, false, false);
                else
                    buildAndThrowError(fmt::format("Invalid node inside call to operator `{}'", node.repr()), x.constList()[index]);
 
                if ((index + 1 < size && x.constList()[index + 1].nodeType() != NodeType::Capture) || index + 1 == size)
                    exp_count++;
 
                // in order to be able to handle things like (op A B C D...)
                // which should be transformed into A B op C op D op...
                if (exp_count >= 2 && !isTernaryInst(op))
                    page(p).emplace_back(op);
            }
 
            if (isUnaryInst(op))
            {
                if (exp_count != 1)
                    buildAndThrowError(fmt::format("Operator needs one argument, but was called with {}", exp_count), x.constList()[0]);
                page(p).emplace_back(op);
            }
            else if (isTernaryInst(op))
            {
                if (exp_count != 3)
                    buildAndThrowError(fmt::format("Operator needs three arguments, but was called with {}", exp_count), x.constList()[0]);
                page(p).emplace_back(op);
            }
            else if (exp_count <= 1)
                buildAndThrowError(fmt::format("Operator needs two arguments, but was called with {}", exp_count), x.constList()[0]);
 
            page(p).back().setSourceLocation(x.filename(), x.position().start.line);
 
            // need to check we didn't push the (op A B C D...) things for operators not supporting it
            if (exp_count > 2)
            {
                switch (op)
                {
                    // authorized instructions
                    case ADD: [[fallthrough]];
                    case SUB: [[fallthrough]];
                    case MUL: [[fallthrough]];
                    case DIV: [[fallthrough]];
                    case MOD: [[fallthrough]];
                    case AT_AT:
                        break;
 
                    default:
                        buildAndThrowError(
                            fmt::format(
                                "`{}' requires 2 arguments, but got {}.",
                                Language::operators[static_cast<std::size_t>(op - FIRST_OPERATOR)],
                                exp_count),
                            x);
                }
            }
        }
 
        if (is_result_unused)
            page(p).emplace_back(POP);
    }
 
    uint16_t ASTLowerer::addSymbol(const Node& sym)
    {
        // otherwise, add the symbol, and return its id in the table
        auto it = std::ranges::find(m_symbols, sym.string());
        if (it == m_symbols.end())
        {
            m_symbols.push_back(sym.string());
            it = m_symbols.begin() + static_cast<std::vector<std::string>::difference_type>(m_symbols.size() - 1);
        }
 
        const auto distance = std::distance(m_symbols.begin(), it);
        if (std::cmp_less(distance, MaxValue16Bits))
            return static_cast<uint16_t>(distance);
        buildAndThrowError(fmt::format("Too many symbols (exceeds {}), aborting compilation.", MaxValue16Bits), sym);
    }
 
    uint16_t ASTLowerer::addValue(const Node& x)
    {
        const ValTableElem v(x);
        auto it = std::ranges::find(m_values, v);
        if (it == m_values.end())
        {
            m_values.push_back(v);
            it = m_values.begin() + static_cast<std::vector<ValTableElem>::difference_type>(m_values.size() - 1);
        }
 
        const auto distance = std::distance(m_values.begin(), it);
        if (std::cmp_less(distance, MaxValue16Bits))
            return static_cast<uint16_t>(distance);
        buildAndThrowError(fmt::format("Too many values (exceeds {}), aborting compilation.", MaxValue16Bits), x);
    }
 
    uint16_t ASTLowerer::addValue(const std::size_t page_id, const Node& current)
    {
        const ValTableElem v(page_id);
        auto it = std::ranges::find(m_values, v);
        if (it == m_values.end())
        {
            m_values.push_back(v);
            it = m_values.begin() + static_cast<std::vector<ValTableElem>::difference_type>(m_values.size() - 1);
        }
 
        const auto distance = std::distance(m_values.begin(), it);
        if (std::cmp_less(distance, MaxValue16Bits))
            return static_cast<uint16_t>(distance);
        buildAndThrowError(fmt::format("Too many values (exceeds {}), aborting compilation.", MaxValue16Bits), current);
    }
}