e9ca833448
Summary:
This is useful for metaprogramming. For instance, suppose we have an
object like this:
```js
const stringifiers = {
ISSUE: (stringifyIssue: (Node<IssueNodePayload>) => string),
COMMENT: (stringifyComment: (Node<CommentPayload>) => string),
...
}
```
How do we type this? We might try
```js
{[type: NodeType]: (Node<NodePayload>) => string}
```
but this is not correct, because `Node<IssueNodePayload>` is a subtype of
`Node<NodePayload>`, and `(_) => K` is contravariant, not covariant. (In
other words, a function from `Node<IssueNodePayload>` is not as general
as a function from `Node<NodePayload>`.) We need to express a dependency
between the object key and the value. We instead write:
```js
type TypedNodeToStringifier = <T: $Values<NodeTypes>>(
T
) => (node: Node<$ElementType<T, "payload">>) => string;
(stringifiers: $Exact<$ObjMap<NodeTypes, TypedNodeToStringifier>>);
```
This expresses exactly (heh) the right type.
Test Plan:
Note that removing any of the elements of `NodeTypes` yields a Flow
error, due to the static assertion following the type definition.
wchargin-branch: node-types
SourceCred
The open-source community provides an enormous amount of value to the world. However, open-source contributors go largely unrewarded and unrecognized. SourceCred aims to help that situation, by building tools that enable quantitatively measuring the value that open-source contributors provide to individual projects, and to the community as a whole.
SourceCred will create a "Cred Graph", which is a graph that shows how the contributions that compose open-source projects are related to and derive value from each other. From this, we'll be able to assign "cred" to users based on how valuable their contributions are. Cred will be assigned based on a mixture of objective data (e.g. references between GitHub pull requests) and subjective feedback (e.g. projects' own judgments on how important different contributions were).
If you'd like to contribute, please follow along with our issues, as we are using issues to coordinate development and design decisions. We also have a slack.