Add 'Burger Discounts' chapter

[feihong]

• Type inference is less effective inside functions that don't call other
  functions. In those cases, you can give the compiler more information:
  • Type annotate the function arguments
  • Use the full name for value
• The Stdlib module is opened by default
• Records are immutable
• Use record copy syntax to make copies of records that have different values
  for some fields
• OCaml doesn't allow you to ignore the return value of functions (unless the
  value is ()), so you can use Stdlib.ignore to explicitly discard return
  values
• Runtime representations of common data types:
  • Variant constructor without argument -> integer
  • Variant constructor with argument -> JS object
  • Record -> JS object
  • None -> undefined
  • Some(value) -> value
• Array facts:
  • Arrays are mutable, just like in JS
  • You can pattern match on arrays of fixed length
  • Array access is unsafe by default
  • What looks like operator usage in array[index] is actually just a call to
    Array.get(array, index)
  • You can create your own Array module to override the behavior of
    Array.get

[jchavarri]

I found some outdated paths and a typo, but otherwise all are minor comments. Amazing work! 💪

[jchavarri]

Wouldn't it be something like this? To make it more generic:

Suggested change

	> Given an order comprising multiple items, find the price of the second most
	> expensive burger. Subtract this number from the pre-tax total of the order.
	> Given an order comprising multiple items, for every two burgers ordered find the
	> price of the less expensive one. Subtract this number from the pre-tax total of the
	> order.

[feihong]

This makes more sense, but I would like to implement it in the next chapter

[jchavarri]

Maybe this could be shared as an alternative approach? Rather than "as an extra precaution", something like "as an alternative approach", and then share the code with exn handling and without the .length check.

[feihong]

Agree, catching the exception is described as alternate approach in 4184366

[jchavarri]

Suggested change

	It looks like `burger[0]` is using an array access operator, but it's actually
	just a function call. That is, `burger[0]` is completely equivalent to
	`Array.get(burger, 0)`.
	In OCaml, array access operators like `burger[0]` are actually just a
	function call. That is, `burger[0]` is completely equivalent to
	`Array.get(burger, 0)`.

[feihong]

Reworded in 20a777b