Add Using Optional Labeled Function Arguments as a reason 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/).
 
-_727 TILs and counting..._
+_728 TILs and counting..._
 
 ---
 
@@ -542,6 +542,7 @@ _727 TILs and counting..._
 - [String Interpolation With Quoted Strings](reason/string-interpolation-with-quoted-strings.md)
 - [Trying Out ReasonML In CodeSandbox](reason/trying-out-reasonml-in-codesandbox.md)
 - [Two Ways To Find An Item In A List](reason/two-ways-to-find-an-item-in-a-list.md)
+- [Using Optional Labeled Function Arguments](reason/using-optional-labeled-function-arguments.md)
 - [Wrapping A Component For Use In JavaScript](reason/wrapping-a-component-for-use-in-javascript.md)
 
 ### Ruby

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).