[RFC 0058] Named Ellipses

rfcs/0057-named-ellipses.md

@@ -0,0 +1,35 @@
+---
+feature: Named Ellipses
+start-date: 2019-11-14
+author: deliciouslytyped
+co-authors: none
+shepherd-team: Eelco Dolstra, Michael Raskin, jD91mZM2
+shepherd-leader: Michael Raskin
+related-issues:
+  Named Ellipses - https://github.com/NixOS/nix/issues/2998
+---
+
+# Summary
+[summary]: #summary
+
+It should be possible to bind a name to ellipses in a function definition like `{ a, ...@extra }: null`, and `{ a, extra@... }: null`. This makes intuitive sense, and could remove the need for a lot of uses of `removeAttrs` that really just want to refer to the contents of ellipses.
+
+Nixpkgs often gets commits like https://github.com/NixOS/nixpkgs/commit/a50653295df5e2565b4a6a316923f9e939f1945b with code that would be cleaner without the need for extra `removeAttrs`.
+
+# Detailed design
+[design]: #detailed-design
+
+The intent of this syntax is for `{...@args}:` to behave as consistent with `{...} @ args:`
+ as practical. As both `args @ {…}` and `{…} @ args` forms happen in Nixpkgs, both would be supported for named ellipses, too. 
+
+ As `args @ {args}: ` is an error in Nix because of duplicate argument names, so should be `{extra, ...@extra}: `. As `(a @ { ... }: a) {a=1;}` returns `{a=1;}`, so should `({ a @ ... }: a) {a=1;}`. ```
+
+TODO: consider what other languages like Haskell do
+
+# Drawbacks
+[drawbacks]: #drawbacks
+This increases the amount of syntax Nix has, thus creating some maintenance cost both for Nix itself, and for tools intended to work with Nix syntax (from highlighting to hnix)
+
+# Alternatives
+[alternatives]: #alternatives
+Not doing this would not have any major impact besides not making Nix and nixpkgs nicer to use.