add duration module proposal

proposals/duration_module.md

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+# Duration Module
+
+Brian Grenier, February 21st, 2025
+
+## Motivation
+
+Representing time durations in programming can be a general hassle at best,
+and a source of bugs at worst. Different functions interpret the same plain
+integer value in different ways, and developers must be careful that
+they calculate the intended time interval properly. Consider the following
+example, where you wish to create a recurring function call that is
+executed every 15 minutes.
+
+```mojo
+alias TIMEOUT_IN_MILLISECONDS = 15 * (60 * 1000)
+struct Job:
+    fn set_timeout(t: Int):
+        """
+        Args:
+            t: The timeout duration in milliseconds
+        """
+        ...
+
+def main():
+    var j = Job()
+    j.set_timeout(TIMEOUT_IN_MILLISECONDS)
+```
+
+Now most programmers have done such calculations frequently enough that we do
+not consider this to be such a terrible burden, but we _can_ do better,
+removing the risk entirely. One piece of the C++ standard library that
+was actually done quite well in my opinion is
+[`std::chrono::duration`](https://en.cppreference.com/w/cpp/chrono/duration).
+By encoding the meaning of the numerical value in the type system, we
+can write much safer,
+and easy to understand APIs.
+
+## Proposal
+
+I propose we follow a similar pattern to `std::chrono::duration`, and
+define a `Duration` struct that takes a `Ratio` parameter to denote its
+relation to the base value of one second.
+
+```mojo
+@value
+@register_passable("trivial")
+struct Ratio[N: UInt, D: UInt = 1]:
+    alias Milli = Ratio[1, 1000]
+    pass
+
+alias Seconds = Duration[Ratio[1](), 's']
+alias Milliseconds = Duration[Milli, 'ms']
+alias Minutes = Duration[Ratio[60](), 'm']
+
+@value
+@register_passable("trivial")
+struct Duration[R: Ratio, postfix: StringLiteral='']:
+    var _value: Int
+
+    fn cast[R: Ratio](self) -> Duration[R]:
+        return Duration[R](0)
+
+    fn write_to[W: Writer](self, mut writer: W):
+        writer.write(self.count(), postfix)
+
+    ...
+```
+
+With these pieces we can now convert the above example into something much
+more ergonomic.
+
+```mojo
+struct Job:
+    fn set_timeout(t: Milliseconds):
+        """
+        Args:
+            t: The timeout duration in milliseconds
+        """
+        ...
+
+def main():
+    var j = Job()
+    j.set_timeout(Minutes(15).cast[Ratio.Milli]())
+
+    sleep(Seconds(10))
+```
+
+## Limitations
+
+I've currently identified two limitations when implementing something
+like this, but I believe they can both be easily remedied when the appropriate
+language features are available.
+
+### Inter-ratio arithmetic
+
+In C++ you can do arithmetic on durations of differing ratios
+because the compiler can determine the higher precision timeframe, which will
+be used as the return type of the operation, and the other value will be cast
+down.
+
+```c++
+#include <chrono>
+
+int main()
+{
+
+    using namespace std::chrono;
+
+    seconds s(10);
+    milliseconds m(100);
+
+    std::cout << m + s; // prints 10100ms
+}
+```
+
+Here `s` will be implicitly cast to `milliseconds`. This is facilitated by
+[`std::common_type`](
+    https://en.cppreference.com/w/cpp/chrono/duration/common_type),
+which as far as I am aware, Mojo currently cannot express. So for now we will
+have to stick to doing arithmetic with matching types.
+
+### Generic representation type
+
+`std::chrono::duration` also makes the representation type of the value
+generic, but since Mojo doesn't currently have something like a `Numeric`
+trait, we would either have to stick to using `Int`, or be generic over
+`Scalar`