{"action":"closed","number":567,"pull_request":{"url":"https://api.github.com/repos/rust-lang-nursery/futures-rs/pulls/567","id":138064906,"html_url":"https://github.com/rust-lang-nursery/futures-rs/pull/567","diff_url":"https://github.com/rust-lang-nursery/futures-rs/pull/567.diff","patch_url":"https://github.com/rust-lang-nursery/futures-rs/pull/567.patch","issue_url":"https://api.github.com/repos/rust-lang-nursery/futures-rs/issues/567","number":567,"state":"closed","locked":false,"title":"Add Void type and expect_no_error method","user":{"login":"srijs","id":2677165,"avatar_url":"https://avatars0.githubusercontent.com/u/2677165?v=4","gravatar_id":"","url":"https://api.github.com/users/srijs","html_url":"https://github.com/srijs","followers_url":"https://api.github.com/users/srijs/followers","following_url":"https://api.github.com/users/srijs/following{/other_user}","gists_url":"https://api.github.com/users/srijs/gists{/gist_id}","starred_url":"https://api.github.com/users/srijs/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/srijs/subscriptions","organizations_url":"https://api.github.com/users/srijs/orgs","repos_url":"https://api.github.com/users/srijs/repos","events_url":"https://api.github.com/users/srijs/events{/privacy}","received_events_url":"https://api.github.com/users/srijs/received_events","type":"User","site_admin":false},"body":"As discussed in #476.\r\n\r\nWent with the name `Void` for now, but as far as I can see we have the following choices:\r\n\r\n- Haskell: [`Void`](https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Void.html) (could be confusing for people coming from C-like languages?)\r\n- Elm: [`Never`](http://package.elm-lang.org/packages/elm-lang/core/5.1.1/Basics#Never) (works nicely for the error case IMO, smth can  \"never\" fail)\r\n- Scala: [`Nothing`](http://www.scala-lang.org/api/2.9.1/scala/Nothing.html) (subtype of every other type, referring to the bottom of the OO type hierarchy)\r\n- Type Theory: [`Bottom`](https://en.wikipedia.org/wiki/Bottom_type)\r\n\r\nI also added a combinator method `expect_no_error` that can be used to convert existing cases of `Future<Error=()>` into `Future<Error=Void>`. It might also make sense to add a function going the other way, e.g. `Future<Error=Void>` into `Future<Error=()>` in order to conform with existing APIs expecting `Error=()`. I'm also happy to remove this from the PR for now and open a separate one once we landed the `Void` type.\r\n\r\nLet me know what you 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