ircd:Ⓜ️ Add an m::query() template suite for DRY'ing common m::get() usage pattern.

include/ircd/m/m.h

@@ -46,6 +46,7 @@ namespace ircd::m
 #include "self.h"
 #include "id.h"
 #include "event/event.h"
+#include "query.h"
 #include "dbs/dbs.h"
 #include "vm.h"
 #include "invite_3pid.h"

include/ircd/m/query.h

@@ -0,0 +1,93 @@
+// Matrix Construct
+//
+// Copyright (C) Matrix Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2019 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+#pragma once
+#define HAVE_IRCD_M_QUERY_H
+
+namespace ircd::m
+{
+	template<class R> R query(std::nothrow_t, const event::idx &, const string_view &key, R&& def, const std::function<R (const string_view &)> &);
+	template<class F> auto query(std::nothrow_t, const event::idx &, const string_view &key, F&&);
+	template<class F> auto query(const event::idx &, const string_view &key, F&&);
+}
+
+/// Like m::get(), but the closure returns a value which will then be returned
+/// by these functions. This elides the pattern of returning a result through
+/// the lambda capture when one simply wants to condition on the fetched value
+/// rather than make further use of it.
+///
+template<class F>
+auto
+ircd::m::query(const event::idx &event_idx,
+               const string_view &key,
+               F&& closure)
+{
+	using R = typename std::invoke_result<F, const string_view &>::type;
+
+	R ret;
+	m::get(event_idx, key, [&ret, &closure]
+	(const string_view &value)
+	{
+		ret = closure(value);
+	});
+
+	return ret;
+}
+
+/// See other overload documentation first. This overload implements
+/// non-throwing behavior when the event_idx/key(column) is not found by
+/// calling the closure with a default-constructed string so the closure can
+/// return a default value.
+///
+template<class F>
+auto
+ircd::m::query(std::nothrow_t,
+               const event::idx &event_idx,
+               const string_view &key,
+               F&& closure)
+{
+	using R = typename std::invoke_result<F, const string_view &>::type;
+
+	R ret;
+	const auto assign
+	{
+		[&ret, &closure](const string_view &value)
+		{
+			ret = closure(value);
+		}
+	};
+
+	if(unlikely(!m::get(std::nothrow, event_idx, key, assign)))
+		assign(string_view{});
+
+	return ret;
+}
+
+/// See other overload documentation first. This overload implements
+/// non-throwing behavior when the event_idx/key(column) is not found by
+/// returning a default value supplied by the caller.
+///
+template<class R>
+R
+ircd::m::query(std::nothrow_t,
+               const event::idx &event_idx,
+               const string_view &key,
+               R&& default_,
+               const std::function<R (const string_view &)> &closure)
+{
+	R ret{std::forward<R>(default_)};
+	m::get(std::nothrow, event_idx, key, [&ret, &closure]
+	(const string_view &value)
+	{
+		ret = closure(value);
+	});
+
+	return ret;
+}