docs: 2.4.0 release announcement draft added

docs/blog/posts/2.4.0-released.md

@@ -0,0 +1,210 @@
+---
+draft: true
+date: 2024-10-27
+authors:
+ - mpusz
+categories:
+ - Releases
+comments: true
+---
+
+# mp-units 2.4.0 released!
+
+**A new product version can be obtained from
+[GitHub](https://github.com/mpusz/mp-units/releases/tag/v2.4.0) and
+[Conan](https://conan.io/center/recipes/mp-units?version=2.4.0).**
+
+This release was unexpected. We planned a significant new feature to happen next, but while
+preparing for it, and also while writing API Reference documentation, we made so many vital fixes
+and improvements that we decided that they deserve a dedicated release first.
+
+This post describes the most significant improvements, while a much longer list of the changes
+introduced by the new version can be found in our [Release Notes](../../release_notes.md#2.4.0).
+
+<!-- more -->
+
+## ISQ quantities cleanup
+
+Initially, we kept quantities defined in "IEC 80000-13: Information science and technology" in
+a standalone `iec80000` namespace, which was renamed to `iec` in the previous release.
+It turned out that this was incorrect. Those quantities are also a part of the ISQ. This is why,
+in this release, we moved all of them to the `isq` namespace (:boom: **breaking change** :boom:).
+
+From now on, `iec` namespace does not provide any quantities and serves purely as a system of units
+definition. It contains binary prefixes (based on the powers of two) and some units introduced
+by IEC (e.g. `var`, `erlang`, `bit`, or ``baud`).
+
+!!! note
+
+    The quantities in `iec` namespace are now deprecated and will be removed in future releases.
+
+Also, it turns out that the latest ISO 80000-3 revision makes a small cleanup to the `phase_speed`
+and `group_speed` quantities. Those were always defined as scalar quantities but also had
+alternative names `phase_velocity` and `group_velocity`. Those were misleading as _velocity_ is
+typically considered a vector quantity. This is why those `XXX_velocity` aliases were
+removed (:boom: **breaking change** :boom:).
+
+
+## Units equality
+
+Previously we assumed that units like `J`, `N m`, and `kg m²/s²` are equal. In some cases,
+this might not be entirely correct. Some quantities require a derived unit instead of a unit with
+a special name. For example:
+
+- `N m` should be used for _moment of force_ (instead of `J`),
+- `V A` should be used for _apparent power_ (instead of `W`).
+
+This is why, starting from this release units like `J`, `N m`, and `kg m²/s²` will not compare equal
+(:boom: **breaking change** :boom:). However, they are deemed equivalent:
+
+```cpp
+static_assert(equivalent(J, N * m));
+static_assert(equivalent(W, V * A));
+```
+
+
+## Portable text output
+
+From the very beginning, the text output of symbols could be formatted in two different ways:
+
+- Unicode,
+- portable using so-called ASCII alternatives.
+
+**mp-units** used the terms "Unicode" or "ASCII" and 'U' or 'A' formatting options for them.
+Even though those terms are widely understood in the C++ community, they are technically
+incorrect.
+
+During the recent SG16 (WG21 Unicode Study Group) meeting, we looked for proper alternatives
+and ended up with the "portable" and "UTF-8" terms (:boom: **breaking change** :boom:).
+
+From now on we will provide the following:
+
+- `text_encoding::utf8`, `symbol_text<N, M>::utf8()`, and `U` formatting option,
+- `text_encoding::portable`, `symbol_text<N, M>::portable()`, and `P` formatting option.
+
+!!! note
+
+    The old identifiers and formatting options are now deprecated and will be removed in future
+    releases.
+
+
+## `char_traits` removed from `fixed_string`
+
+During the same SG16 meeting, the room was strongly against providing `char_traits` for
+`fixed_string`. This is why `char_traits` support was removed in this release
+(:boom: **breaking change** :boom:).
+
+
+## Improved units text output
+
+In the previous release, we introduced common unit abstraction. Initially, all its components
+were printed in parenthesis which contained a list of all the scaled units separated with `=`.
+After some feedback, we decided to change it to a new syntax.
+
+For example, the following:
+
+```cpp
+std::cout << 1 * km + 1 * mi << "\n";
+std::cout << 1 * nmi + 1 * mi << "\n";
+std::cout << 1 * km / h + 1 * m / s << "\n";
+```
+
+will print:
+
+=== "Now"
+
+    ```text
+    40771 EQUIV{[1/25146 mi], [1/15625 km]}
+    108167 EQUIV{[1/50292 mi], [1/57875 nmi]}
+    23 EQUIV{[1/5 km/h], [1/18 m/s]}
+    ```
+
+=== "Before"
+
+    ```text
+    40771 ([1/25146] mi = [1/15625] km)
+    108167 ([1/50292] mi = [1/57875] nmi)
+    23 ([1/5] km/h = [1/18] m/s)
+    ```
+
+As we can see above, the scaled units output changed as well. Now, the entire scaled unit is
+encapsulated within `[...]` brackets to better denote its scope.
+
+Additionally, small magnitudes of scaled units do not use the power of `10`, and also scaled
+units do not have a composition priority over the derived units anymore.
+
+As a result of those changes, the following:
+
+```cpp
+constexpr Unit auto L_per_100km = L / (mag<100> * km);
+std::cout << 6.7 * L_per_100km << "\n";
+```
+
+prints:
+
+=== "Now"
+
+    ```text
+    6.7 L/[100 km]
+    ```
+
+=== "Before"
+
+    ```text
+    6.7 × 10⁻² l/km
+    ```
+
+One more change that we can see above is that litres now use "L" instead of "l" for the symbol.
+The latter one too often is confused with the number `1`.
+
+The next improvement adds proper formatting support for magnitudes. All of the formatting options
+that were working before for the unit symbols now also work for magnitudes.
+
+For example:
+
+```cpp
+using enum text_encoding;
+using enum unit_symbol_solidus;
+using usf = unit_symbol_formatting;
+
+static_assert(unit_symbol(mag<1> / (mag<2> * mag<pi>)*metre) == "[2⁻¹ 𝜋⁻¹ m]");
+static_assert(unit_symbol<usf{.solidus = always}>(mag<1> / (mag<2> * mag<pi>)*metre) == "[1/(2 𝜋) m]");
+static_assert(unit_symbol<usf{.encoding = portable, .solidus = always}>(mag<1> / (mag<2> * mag<pi>)*metre) ==
+              "[1/(2 pi) m]");
+```
+
+As we can see above, the library also learned how to print magnitude symbols. This required
+a change in the `mag_constant` definition. Now, it takes a magnitude symbol and has to be `final`
+like for other similar types in the library (:boom: **breaking change** :boom:):
+
+```cpp
+inline constexpr struct pi final : mag_constant<symbol_text{u8"π", "pi"}, std::numbers::pi_v<long double>> {} pi;
+inline constexpr auto π = pi;
+```
+
+## Unicode identifiers
+
+The example above introduced something interesting: a `π` identifier for a variable. With the
+latest changes to the C++ language, we can officially use Unicode symbols as identifiers in
+the C++ code.
+
+In this release, we've added support for those not only for `π` but also for unit symbols.
+Now we can type the following:
+
+=== "With UTF-8 glyphs"
+
+    ```cpp
+    quantity resistance = 60 * kΩ;
+    quantity capacitance = 100 * µF;
+    ```
+
+=== "Portable"
+
+    ```cpp
+    quantity resistance = 60 * kohm;
+    quantity capacitance = 100 * uF;
+    ```
+
+This might make the source code easier to understand, but typing those identifiers can be tricky.
+Sometimes, the best solution to type it might be a copy-paste approach. If we do not like this
+idea, we can still use old portable identifiers for those as well.