+---+feature: common-interface-package-sets+start-date: 2020-12-19+author: Frederik Rietdijk (@FRidh)+co-authors:+related-issues:+---++# Summary+[summary]: #summary++The Nixpkgs package set consists of a large amount of packages of which a+significant amount of grouped into sub package sets. This RFC recommends a+common interface for these package sets.++# Motivation+[motivation]: #motivation++The Nixpkgs package set consists of a large amount of packages of which a+significant amount of grouped into sub package sets. Sets exist for various+languages, frameworks and plugins.++They are typically grouped together for one of the following two reasons:+- clarity, e.g. because they're written in the same language or framework;+- necessity, e.g. for compatibility reasons.++Over time different methods for defining package sets were created. Currently+multiple methods are in use in Nixpkgs and pull requests are opened to modify+sub package sets interfaces from one kind to another. Not only is this confusing+for users but it also causes trouble; in some cases overriding of derivations+inside a set is not possible or cross-compilation is broken.++This RFC thus aims to unify the package sets to a common interface for the+following reasons:+- simplify usage of package sets and reduce confusion surrounding them;+- single approach for dealing with overrides;+- handle variants of a package set;+- ensure cross-compilation works.++Often one also wants to build an environment with an interpreter or main program+and some additional packages or plugins. This RFC will therefore also recommend+a function each package set should offer for doing so, when appliceable that is.++## Related issues++TODO: refer to these issues in correct place.++- Common override interface derivations https://github.com/NixOS/rfcs/pull/67+- Make PHP packages overrideable https://github.com/NixOS/nixpkgs/pull/107044+- Change in emacs interface https://github.com/NixOS/nixpkgs/pull/107152+- Package set for Octave https://github.com/NixOS/nixpkgs/issues/65398#issuecomment-743926570+- Python package set is not overrideable+  https://github.com/NixOS/nixpkgs/issues/44426+- Support `overrideScope'` in Python package set+  https://github.com/NixOS/nixpkgs/pull/105374+- Common `overrideArgs` for sub package sets+  https://github.com/NixOS/nixpkgs/pull/46842. May be resolved using+  `overrideAuto` in https://github.com/NixOS/rfcs/pull/67.++# Detailed design+[design]: #detailed-design++We will now look in more detail at what a common interface should offer.++## Attribute name of the package set: `fooPackages` versus `foo.pkgs`++Two different interfaces are common in Nixpkgs when referring to package sets:+- `fooPackages`+- `foo.pkgs`++TODO which one to pick? Consider also overriding of the interpreter or main program and how that should propagate. Consider also the function for generating variants, where you need to have a name under which your interpreter or main program is available in the subpackage set.+## Variants+Often multiple variants of a package set need to be created. E.g., in case of+emacs or Python there are different versions of the program and each of them+should have their own package set. For this reason it is important that one can+easily create a new variant++```nix+fooPackagesFor = foo: import ./foo-packages.nix { ... };+fooPackages_3_6 = fooPackagesFor foo_3_6;+```++## Set overriding+It should be possible to override packages in a sub package set and have the+other packages in the set take that override into account. To that end, a scope+is created++```nix+lib.makeScope pkgs.newScope (self: with self; {+  ...+}+```++that can be overridden using `overrideScope'`++```nix+fooPackages_3_6.overrideScope' overrides;+```++where `overrides` is of the form++```nix+(final: previous: { ... })+```++In case one uses overlays with Nixpkgs, one could now make the overrides+composible using++```nix+fooPackages_3_6 = fooPackages_3_6.overrideScope' overrides;+```++## Package overriding++Now that it is possible to override a set, a common interface to overriding the+packages inside the set is needed as well. This is treated in [RFC+67](https://github.com/NixOS/rfcs/pull/67).++## Cross-compilation++For cross-compilation it is important that `callPackage` in the sub package set+has access to the spliced versions of the sub package set. Until recently, there+were no spliced sub package sets in Nixpkgs, but support was added for Python+utilizing the `makeScopeWithSplicing` function. There is [room for+improvement](https://github.com/NixOS/nixpkgs/pull/105374).++An important aspect for making this work is that, when constructing the package+set, it needs to know its own top-level attribute.