Add Defining Variants With Constructor Arguments as a reasonml til

reason/defining-variants-with-constructor-arguments.md

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+# Defining Variants With Constructor Arguments
+
+In [Helping The Compiler Help Us With
+Variants](https://github.com/jbranchaud/til/blob/master/reason/helping-the-compiler-help-us-with-variants.md),
+I introduced the concept of variants with a basic example of how to define
+and use one. The fun doesn't stop there.
+
+We can take variants a step further by defining them with constructor
+arguments.
+
+```reason
+type listActions =
+  | Length
+  | Nth(int);
+```
+
+The second variant is defined such that it is paired with some extra data --
+a single `int` argument.
+
+Here is how we use that variant in our code:
+
+```reason
+let performListAction = (l: list(int), action: listActions) => {
+  switch(action) {
+  | Length => List.length(l)
+  | Nth(n) => List.nth(l, n)
+  }
+};
+
+performListAction([7,8,9], Nth(1)); /* 8 */
+performListAction([1,2,3], Length); /* 3 */
+```
+
+Our switch statement not only matches on that variant, but it makes the
+`int` argument available as a value we can consume in that step of the
+switch.
+
+[source
+code](https://reasonml.github.io/en/try.html?reason=C4TwDgpgBANglgZ2AQQMbDgewHYKgXgCgooAfKAGQmwHNgALYsqAOQYAo5tgBKAbkKEYEYFEgAnAGaZxAWwqIU6LNgJR2MAFyxFnbjwA0UAIbKc2+EjQYcCHgQB8UAN5MEAdzjBU9dqZvY9q4k5FS0DI6UigB0wuG+MDxM5Gy+gZEKSNHcCUaBTAC+hAUChABSCLGYNOwS0nKZSgHsANoA7AYAHAYAnAC6RqnsAIw8YwIVVTV1MvKK1iqtwwYATAYAzAOU1HT040A)