Add Helping The Compiler Help Us With Variants as a reasonml til

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

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+# 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)