Add Using Optional Labeled Function Arguments as a reason til

reason/using-optional-labeled-function-arguments.md

@@ -0,0 +1,32 @@
+# Using Optional Labeled Function Arguments
+
+If you are constructing a function that takes some arguments, but one of
+those arguments has a reasonable default value, then you can use an optional
+labeled argument. Labeled arguments are those arguments prefixed with a `~`.
+If you give the argument a default value, then it becomes optional.
+
+```reason
+let thing = (~a=1, b: int, c: int) => {
+  a + b + c;
+};
+```
+
+In this case `~a` is a labeled argument. It is also optional and will
+default to `1` if not specified. The other two arguments, `b` and `c`, are
+positional arguments and thus required in order for the function to
+evaluate.
+
+Here are two ways of using this function either by specifying `~a` or
+excluding it so that it defaults to `1`.
+
+```reason
+thing(~a=2, 1, 1)
+|> string_of_int
+|> print_endline /* 4 */
+
+thing(1, 1)
+|> string_of_int
+|> print_endline /* 3 */
+```
+
+See more details [here](https://reasonml.github.io/docs/en/function).