4 files changed, 147 insertions, 2 deletions
diff --git a/asl/log/BUILD.bazel b/asl/log/BUILD.bazel
index 9291a8a..51f41df 100644
--- a/asl/log/BUILD.bazel
+++ b/asl/log/BUILD.bazel
@@ -1,10 +1,10 @@
 cc_library(
     name = "log",
     srcs = [
-        "log.hpp",
+        "log.cpp",
     ],
     hdrs = [
-        "log.cpp",
+        "log.hpp",
     ],
     deps = [
         "//asl",
diff --git a/asl/log/log.cpp b/asl/log/log.cpp
index e69de29..cd5d287 100644
--- a/asl/log/log.cpp
+++ b/asl/log/log.cpp
@@ -0,0 +1,52 @@
+#include "asl/log/log.hpp"
+#include "asl/print.hpp"
+#include "asl/string_builder.hpp"
+
+// @Todo Don't use internal get_stdout_writer, make console module
+
+static asl::log::DefaultLogger g_default_logger{asl::print_internals::get_stdout_writer()};
+static asl::log::Logger* g_head = &g_default_logger;
+
+static constexpr asl::string_view kLevelName[] = {
+    "  DEBUG  ",
+    "  INFO   ",
+    " WARNING ",
+    "  ERROR  ",
+};
+
+void asl::log::register_logger(box<Logger> logger_box)
+{
+    auto* logger = leak(ASL_MOVE(logger_box));
+    logger->m_next = exchange(g_head, logger);
+}
+
+void asl::log::DefaultLogger::log(const message& msg)
+{
+    asl::format(m_writer, "[{}] {}:{}: {}\n",
+        kLevelName[msg.level], // NOLINT
+        msg.location.file,
+        msg.location.line,
+        msg.message);
+}
+
+void asl::log::log_inner(
+    level l,
+    string_view fmt, span<const format_internals::type_erased_arg> args,
+    const source_location& sl)
+{
+    // @Todo Use temporary allocator
+    StringWriter msg_writer{};
+    asl::format_internals::format(&msg_writer, fmt, args);
+
+    message m{
+        .level = l,
+        .message = msg_writer.as_string_view(),
+        .location = sl,
+    };
+
+    for (auto* it = g_head; it != nullptr; it = it->next_logger())
+    {
+        it->log(m);
+    }
+}
+
diff --git a/asl/log/log.hpp b/asl/log/log.hpp
index e69de29..a6b8f03 100644
--- a/asl/log/log.hpp
+++ b/asl/log/log.hpp
@@ -0,0 +1,83 @@
+#pragma once
+
+#include <asl/format.hpp>
+#include <asl/utility.hpp>
+#include <asl/box.hpp>
+
+namespace asl::log
+{
+
+enum level : uint8_t
+{
+    kDebug = 0,
+    kInfo,
+    kWarning,
+    kError,
+};
+
+struct message
+{
+    level level;
+    string_view message;
+    source_location location;
+};
+
+// @Todo Write and use an intrusive doubly-linked list
+class Logger
+{
+    Logger* m_next{};
+
+public:
+    Logger() = default;
+    ASL_DEFAULT_COPY_MOVE(Logger);
+    virtual ~Logger() = default;
+
+    virtual void log(const message&) = 0;
+
+    friend void register_logger(box<Logger>);
+
+    constexpr Logger* next_logger() const { return m_next; }
+};
+
+// @Todo Make a deref_as trait & deref utility
+// @Todo Accept writer as box, pointer, reference, or value
+class DefaultLogger : public Logger
+{
+    Writer* m_writer;
+
+public:
+    explicit constexpr DefaultLogger(Writer* writer) : m_writer{writer} {}
+
+    void log(const message&) override;
+};
+
+void register_logger(box<Logger>);
+
+// @Todo Add a way to remove loggers (including all)
+
+template<typename T, typename... Args>
+requires constructible_from<T, Args&&...> && convertible_from<Logger*, T*>
+void register_logger(Args&&... args)
+{
+    register_logger(make_box<T>(ASL_FWD(args)...));
+}
+
+void log_inner(level l, string_view fmt, span<const format_internals::type_erased_arg> args, const source_location& sl);
+
+template<formattable... Args>
+void log(level l, const source_location& sl, string_view fmt, const Args&... args)
+{
+    format_internals::type_erased_arg type_erased_args[] = {
+        format_internals::type_erased_arg(args)...
+    };
+    log_inner(l, fmt, type_erased_args, sl);
+}
+
+} // namespace asl::log
+
+// @Todo Compile-time configuration of logging
+
+#define ASL_LOG_DEBUG(...) ::asl::log::log(::asl::log::kDebug, ::asl::source_location{}, __VA_ARGS__)
+#define ASL_LOG_INFO(...) ::asl::log::log(::asl::log::kInfo, ::asl::source_location{}, __VA_ARGS__)
+#define ASL_LOG_WARNING(...) ::asl::log::log(::asl::log::kWarning, ::asl::source_location{}, __VA_ARGS__)
+#define ASL_LOG_ERROR(...) ::asl::log::log(::asl::log::kError, ::asl::source_location{}, __VA_ARGS__)
diff --git a/asl/log/log_tests.cpp b/asl/log/log_tests.cpp
index e69de29..01e8fd7 100644
--- a/asl/log/log_tests.cpp
+++ b/asl/log/log_tests.cpp
@@ -0,0 +1,10 @@
+#include <asl/log/log.hpp>
+#include <asl/testing/testing.hpp>
+
+ASL_TEST(log)
+{
+    asl::log::log(asl::log::kInfo, asl::source_location{}, "Hello, {}!", "world"_sv);
+
+    ASL_LOG_ERROR("Oh no! {}", 42);
+}
+