Add function statics and object callable properties (#55)

* Add function statics and object callable properties

* Update function statics and object callables

- Use simpler examples
- Add TypeScript on object callables

* Add TypeScript example for overloading callable

* Updates on callable properties

* Updates on callable properties

- Move to "same syntax"
- Add a few library definitions as reference

* Add back the line # Read-only Types that got deleted by accident

* Not sure how TS supports function statics

* Resolve PR discussion and add function statics

Author: wgao19

README.md

@@ -519,6 +519,8 @@ add("1", "1"); // Ok
 add(1, "1"); // Error
 ```
 
+It is also possible to create function overloads using callable property syntax, see the section [Object callable property](#object-callable-property).
+
 ### TypeScript
 
 TypeScript supports both function and method overloading, in both: type definitions (`.d.ts`) and inline alongside code.
@@ -635,6 +637,122 @@ However, Flow implementation is stricter in this case, as B have a property that
 
 Most of the syntax of Flow and TypeScript is the same. TypeScript is more expressive for certain use-cases (advanced mapped types with keysof, readonly properties), and Flow is more expressive for others (e.g. `$Diff`).
 
+## Object callable property
+
+The basic syntax are the same, except Flow has special syntax for the internal call property slot.
+
+Both can be used to annotate function statics.
+
+### Flow
+
+You can use objects with callable properties as functions: [Try Flow](https://flow.org/try/#0PTAEAEDMBsHsHcBQiAuBPADgU1AMVALygDeiooAFAJQBcoAzigE4CWAdgOaIC+A3IgGNYbRqEh18RaoQB8oAEQALLNDjz+QkSlDLVsOo1adCY6ryA)
+
+```js
+type F = {
+  (): string
+};
+const f: F = () => "hello";
+const hello: string = f();
+```
+
+An overloaded function is a function with multiple call signatures.
+This is supported by Flow. And we list out the different syntaxes here: [Try Flow](https://flow.org/try/#0PTAEAEDMBsHsHcBQiAuBPADgU1AMVALygDeiooAFAJQBcoAzigE4CWAdgOYA0ZoA2nwDGAQ2jQAuuLoU2AVwC2AIyxMqhAHwNm7brwEixkio1adaW0x0QBfZINhtGoSHXxEKADwD8dOUpWgAD4WOmoEmqTkTFgoskxsoB6gXokAdCiwAMranNSgdADkBQDcNohAA)
+
+```js
+type F = {
+  (): string,	
+  [[call]]: (number) => string,	
+  [[call]](string): string
+}
+
+const f: F = (x?: number | string) => {
+  return x ? x.toString() : '';
+}
+```
+
+Use call property to annotate function statics: [Try Flow](https://flow.org/try/#0PTAEAEDMBsHsHcBQiAuBPADgU1AWSwLawCWAXlgCYBiAhgMYqwBOxN0AKpjgLygDeiUKDr0AFlgBc-QaACQAbQB2AVwIAjLEwC6Ules0yAvgBoZ8+SOjQtWgBR6NTAJS7Vj04eR1YigM4pQSHpGFjYpfCIySloGZlYOLlBeRSSAPmkhYkhQWwBCINjQ6AA6ETpxJwyhQOC4tlKxHn5PIRbQLJyCkPiG8qwlLVBc7l5lRQosSGJFSkqBatAmLBRlJhSuupKy8QGjGQ2i3p3FQeSkkdAABlAAflAARlBdUAAqGsL4+1AAWgenGSWKzW7269W2-ROiEMQA)
+
+```js
+type MemoizedFactorialType = {
+  cache: {
+    [number]: number,
+  },
+  [[call]](number): number,
+}
+
+const factorial: MemoizedFactorialType = n => {
+  if (!factorial.cache) {
+    factorial.cache = {}
+  }
+  if (factorial.cache[n] !== undefined) {
+    return factorial.cache[n]
+  }
+  factorial.cache[n] = n === 0 ? 1 : n * factorial(n - 1)
+  return factorial.cache[n]
+}
+```
+
+Reference:
+
+- [Callable Objects](https://flow.org/en/docs/types/functions/#callable-objects-)
+- [immer.js](https://github.com/immerjs/immer/blob/master/src/immer.js.flow) uses it to overload the `produce` (default export) function which has multiple call signatures
+- [Styled Components](https://github.com/flow-typed/flow-typed/blob/master/definitions/npm/styled-components_v4.x.x/flow_v0.75.x-/styled-components_v4.x.x.js#L242) uses it to separate cases of being called on a string and wrapping a component 
+- [Reselect Library Definition](https://github.com/flow-typed/flow-typed/blob/master/definitions/npm/re-reselect_v2.x.x/flow_v0.67.1-/re-reselect_v2.x.x.js) contains massive chunks of overloaded call properties
+
+### TypeScript
+
+You can use objects with callable properties as functions: [TypeScript Playground](https://www.typescriptlang.org/play/#src=type%20F%20%3D%20%7B%0D%0A%20%20()%3A%20string%3B%0D%0A%7D%0D%0Aconst%20foo%3A%20F%20%3D%20()%20%3D%3E%20%22hello%22%3B%0D%0Aconst%20bar%3A%20string%20%3D%20foo()%3B)
+
+```ts
+type F = {
+  (): string;
+}
+const foo: F = () => "hello";
+const bar: string = foo();
+```
+
+An overloaded function is a function with multiple call signatures.
+This is also supported by TypeScript: [TypeScript Playground](https://www.typescriptlang.org/play/#src=type%20F%20%3D%20%7B%0D%0A%20%20()%3A%20string%2C%0D%0A%20%20(number)%3A%20string%2C%0D%0A%20%20(string)%3A%20string%0D%0A%7D%0D%0A%0D%0Aconst%20f%3A%20F%20%3D%20(x%3F%3A%20number%20%7C%20string)%20%3D%3E%20%7B%0D%0A%20%20return%20x%20%3F%20x.toString()%20%3A%20''%3B%0D%0A%7D%0D%0A)
+
+
+```ts
+type F = {
+  (): string,
+  (x: number): string,
+  (x: string): string
+}
+
+const f: F = (x?: number | string) => {
+  return x ? x.toString() : '';
+}
+```
+
+Use call property to annotate function statics: [TypeScript Playground](https://www.typescriptlang.org/play/#src=type%20MemoizedFactorialType%20%3D%20%7B%0D%0A%20%20cache%3F%3A%20%7B%0D%0A%20%20%20%20%5Bn%3A%20number%5D%3A%20number%2C%0D%0A%20%20%7D%2C%0D%0A%20%20(n%3A%20number)%3A%20number%2C%0D%0A%7D%0D%0A%0D%0Aconst%20factorial%3A%20MemoizedFactorialType%20%3D%20n%20%3D%3E%20%7B%0D%0A%20%20if%20(!factorial.cache)%20%7B%0D%0A%20%20%20%20factorial.cache%20%3D%20%7B%7D%0D%0A%20%20%7D%0D%0A%20%20else%20if%20(factorial.cache%5Bn%5D%20!%3D%3D%20undefined)%20%7B%0D%0A%20%20%20%20return%20factorial.cache%5Bn%5D%0D%0A%20%20%7D%0D%0A%20%20factorial.cache%5Bn%5D%20%3D%20n%20%3D%3D%3D%200%20%3F%201%20%3A%20n%20*%20factorial(n%20-%201)%0D%0A%20%20return%20factorial.cache%5Bn%5D%0D%0A%7D)
+
+```ts
+type MemoizedFactorialType = {
+  cache?: {
+    [n: number]: number,
+  },
+  (n: number): number,
+}
+
+const factorial: MemoizedFactorialType = n => {
+  if (!factorial.cache) {
+    factorial.cache = {}
+  }
+  else if (factorial.cache[n] !== undefined) {
+    return factorial.cache[n]
+  }
+  factorial.cache[n] = n === 0 ? 1 : n * factorial(n - 1)
+  return factorial.cache[n]
+}
+```
+
+Reference:
+- [Callable on TypeScript Book](https://github.com/basarat/typescript-book/blob/master/docs/types/callable.md)
+
+
 ## optional parameters
 
 ### Flow and TypeScript