Add Defining Variants With Constructor Arguments as a reasonml til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_620 TILs and counting..._
+_621 TILs and counting..._
 
 ---
 
@@ -443,6 +443,7 @@ _620 TILs and counting..._
 
 ### ReasonML
 
+- [Defining Variants With Constructor Arguments](reason/defining-variants-with-constructor-arguments.md)
 - [Exhaustive Pattern Matching Of List Variants](reason/exhaustive-pattern-matching-of-list-variants.md)
 - [Helping The Compiler Help Us With Variants](reason/helping-the-compiler-help-us-with-variants.md)
 - [Inline Component Styles With Reason React](reason/inline-component-styles-with-reason-react.md)

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

@@ -0,0 +1,39 @@
+# 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)