More discussion of homomorphic mapped types #473
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Especially the practical uses and comparison to non-homomorphic types.
| @@ -834,6 +834,10 @@ type Partial<T> = { [P in keyof T]?: T[P] } | |||
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| In these examples, the properties list is `keyof T` and the resulting type is some variant of `T[P]`. | |||
| This is a good template for any general use of mapped types. | |||
| That's because this kind of transformation is homomorphic, which means that the mapping applies to every property of `T` and no others. | |||
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might be useful to add a link to wikipedia's definition of Homomorphism: https://en.wikipedia.org/wiki/Homomorphism
| @@ -861,6 +865,16 @@ type Record<K extends string | number, T> = { | |||
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| `Readonly` and `Partial` are homomorphic whereas `Pick` and `Record` are not. | |||
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that is not accurate, Pick is homorophic, since it is declared as type Pick<T, K extends keyof T>, the constraint makes is a homorphic relationship. this is a change in TS 2.1.5, that is not in TS 2.1.4. see microsoft/TypeScript#12826
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To be clear a homomorphic type is one of the form:
type H1<T> = { [P in keyof T] ... };
type H2<T, K extends keyof T> = { [P in K] ... };basically any way for the type system to know that the mapped type is a transfomration on another type.
H3 however, is not a homomorphic mapping:
type H3<T, K extends string> = { [P in K]: Op<T> };There was a problem hiding this comment.
Nice catch. I forgot about that change. Updated.
| Homomorphic mapped types are mapped types that iterate over every property of some type, and only those properties: `{ [P in keyof T]: X }`. | ||
| In the examples above, `Nullable` and `Partial` are homomorphic whereas `Pick` and `Record` are not. | ||
| One clue is that `Pick` and `Record` both take a union of property names in addition to a source type, which they use instead of `keyof T`. | ||
| Note that this unwrapping inference works best on homomorphic mapped types. |
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I'll fix the discussion.
| One clue is that `Pick` and `Record` both take a union of property names in addition to a source type, which they use instead of `keyof T`. | ||
| If the mapped type is not homomorphic you might have to explicitly give a type parameter to your unwrapping function. | ||
| Note that this unwrapping inference only works on homomorphic mapped types. | ||
| If the mapped type is not homomorphic you will have to explicitly give a type parameter to your unwrapping function. |
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give an explicit type parameter.
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Especially the practical uses and comparison to non-homomorphic types.