Add Helping The Compiler Help Us With Variants 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/).
 
-_619 TILs and counting..._
+_620 TILs and counting..._
 
 ---
 
@@ -444,6 +444,7 @@ _619 TILs and counting..._
 ### ReasonML
 
 - [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)
 - [Multi-Argument Functions As Syntactic Sugar](reason/multi-argument-functions-as-syntactic-sugar.md)
 - [Pattern Match On Exceptions](reason/pattern-match-on-exceptions.md)

reason/helping-the-compiler-help-us-with-variants.md

@@ -0,0 +1,38 @@
+# Helping The Compiler Help Us With Variants
+
+[ReasonML](https://reasonml.github.io/) gives us something called a variant
+which is similar to what other language call enums and union types. By
+defining a variant, we give the compiler some information about exactly what
+values a variable can take on -- its allowed variants.
+
+Here we define the kinds of roles that users in our system can have as well
+as a `user` type for representing individual users:
+
+```reason
+type userType =
+  | Student
+  | Teacher
+  | Admin;
+
+type user = { role: userType, id: int };
+```
+
+And here is how we might use it in some authorization code:
+
+```reason
+let canCreateClasses(u: user) {
+  switch(u.role) {
+  | Student => false
+  | Teacher => true
+  };
+};
+```
+
+We've neglected to include `Admin` in our switch statement. The compiler
+will inform us of this with a warning.
+
+> this pattern-matching is not exhaustive. Here is an example of a value
+> that is not matched: Admin
+
+[source
+code](https://reasonml.github.io/en/try.html?reason=C4TwDgpgBArgzhATgFXNAvAKClAPlAZWBgBMIA7YbPKZCAQwGMALJa-AQRIFsBLcgNyZQkWAkRR0UAN5REAewA2EAFxikqSABoovEmv7AoAXyGZlRxvXIBhRA2AQbi+nARwAFDDXwkAShlqOAB3XmAWLwA6BWUA6XZCYjJKSQA+KAAzekUEBLomVgl0dOBEGAhqU0wqgCk4SMV5AHMPK1t7ekdnV3cPWRjVRNIKYB09NQBGACYAZhM-PwEgA)