Clarify subtyping for shapes

[knider]

Numerous typos found:

Types: Shapes

• Consider a shape type S2 whose field set is a superset of that in shape type S1. As such, S2 is a subtype of S1. If S2 is a superset of S1, how can it be a subtype of S1?

Expressions And Operators: Await, Source Code Fundamentals: Namespaces, Attributes: Predefined Attributes, Types: Classes

• $$ in the links

Asynchronous Operations: Guidelines

• memmoized has too many m's
• memoize example should have <<__Memoize>>

I found more but they seem to have been corrected in the past few weeks and many of those pages have disappeared so props to getting all those fixed.

---

Please don't change anything below this point.

---

• Build ID: 2019-03-15T20:34:09+0000:361688e6c740dea5615800d530efcf4090ad0b4c
• Page requested: /hack/types/shapes
• Page requested at: Wed, 27 Mar 2019 21:26:25 +0000
• Controller: GuidePageController

[Wilfred]

Thanks for the report!

If S2 is a superset of S1, how can it be a subtype of S1?

You can pass a shape('x' => int, 'y' => int) to a function that's only expecting a shape('x' => int). You can think of the shape with more fields as more specific, which is what subtyping gives.

Similarly I could have a class Animal with a subtype Dog. That subclass Dog might have additional properties (like $this->tail). So Dog's properties are a superset of Animal.

We could certainly look at clarifying this section though.

$$ in the links

Duplicate of #594.