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Added in some of the great info about non-homomorphic transformations… #471

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Added in some of the great info about non-homomorphic transformations… #471

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Added in some of the great info about non-homomorphic transformations…
JakeGinnivan Dec 31, 2016
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JakeGinnivan Dec 31, 2016
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23 changes: 23 additions & 0 deletions pages/Advanced Types.md
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Expand Up @@ -793,6 +793,29 @@ type PersonPartial = Partial<Person>;
type ReadonlyPerson = Readonly<Person>;
```

Property mapping is a [homomorphic](https://en.wikipedia.org/wiki/Homomorphism) transformation, so if you wanted to do the
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This is too early for this explanation, and it's too dense for the handbook. It would be better to weave it in with the "Real applications" discussion below.

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Homomorphic mapped types are already described briefly in the "Inference from mapped types" section, so that section needs to be updated as well.

opposite of the `Partial` type and make all optional properties required (a non-homomorphic transformation) you could create
a type `EnsureAll`

```ts
type EnsureAll<T, K extends string> = {
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I like this example but its description of purpose is confusing.

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Did you actually test this EnsureAll type? I get a type error saying you can't index T with K, presumably because K isn't constrained to the keys of T.

[P in K]: T[P];
}
```

And to use it:

```ts
interface Person {
name: string;
age?: number;
}
type PersonWithAge = EnsureAll<Person, keyof Person> // { name: string, age: number }
```

Notice the constraint on K is string; this way the type system can not assert that the result of this
transformation is a homomorphic mapping on T, and thus no modifiers are copied through.
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Need an example of coping vs not-copying.


Let's take a look at the simplest mapped type and its parts:

```ts
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