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rfcs/0110-inherit-as-list.md
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rfcs/0110-inherit-as-list.md
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---
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feature: inherit-as-list
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start-date: 2021-10-17
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author: Ryan Burns (@r-burns)
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co-authors: (find a buddy later to help out with the RFC)
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shepherd-team: @synthetica9, @infinisil, @kevincox, @bobvanderlinden
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shepherd-leader: @kevincox
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related-issues: (will contain links to implementation PRs)
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---
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# Summary
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[summary]: #summary
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This RFC proposes a new Nix syntax `<attrset>.[ <attrnames> ]`,
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which constructs a list from the values of an attrset.
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The goal is to provide a similarly-terse but more principled alternative
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to the often-used `with <attrset>; [ <attrnames> ]`.
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# Motivation
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[motivation]: #motivation
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It is currently cumbersome to create a list from the values of an attrset.
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If one has an attrset `attrs` and wishes to create a list containing some of
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its values, one could naively write:
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```nix
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[ attrs.a attrs.b attrs.c ]
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```
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To avoid typing `attrs` many times, one will typically use `with` instead:
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```nix
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with attrs; [ a b c ]
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```
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However, the `with` expression has many well-known drawbacks, such as
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unintuitive shadowing behavior [1][2], prevention of static scope checking [3][4],
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and reduced evaluation performance [3].
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* [1] https://github.com/NixOS/nix/issues/490
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* [2] https://github.com/NixOS/nix/issues/1361
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* [3] https://github.com/NixOS/nixpkgs/pull/101139
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* [4] https://nix.dev/recipes/best-practices#with-scopes
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Nonetheless, Nix expression authors are subtly guided toward the `with` form
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because it is (or at least appears) simpler than any existing alternatives.
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Some alternatives are suggested in
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https://nix.dev/recipes/best-practices#with-scopes, but as these
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are more verbose and complex than `with`, they are rarely used.
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The goal of this RFC is to provide a similarly-terse alternative which avoids
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these drawbacks.
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# Detailed design
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[design]: #detailed-design
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The proposed syntax is:
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```
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attrs.[ a b c ]
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```
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This expression is syntactic sugar for:
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```nix
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[ attrs.a attrs.b attrs.c ]
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```
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As the token `.` immediately preceding `[` is currently a syntax error,
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a Nix interpreter which supports this new language feature will be compatible
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with existing Nix code.
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This RFC is implemented here: https://github.com/nixos/nix/pull/5402
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For MVP functionality, only minor additions to `src/libexpr/parser.y` are
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needed. If accepted, the changes to the Nix interpreter can be backported
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to current releases if desired. Relevant language documentation and
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third-party parsers/linters would also need to be updated.
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# Examples and Interactions
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[examples-and-interactions]: #examples-and-interactions
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This would be useful for many languages and frameworks in Nixpkgs which
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extract packages from a package set argument.
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For example, `python3.withPackages (ps: ps.[ ... ])` will serve as a
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more fine-grained alternative to `python3.withPackages (ps: with ps; [ ... ])`.
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This would apply similarly to `vim.withPlugins`, `lua.withPackages`, etc.
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Certain list-typed `meta` fields could also make use of this feature, e.g.:
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```
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meta.licenses = lib.licenses.[ bsd3 mit ];
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meta.maintainers = lib.maintainers.[ johndoe janedoe ];
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```
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Note that only simple textual attrnames are allowed in the square brackets.
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For example, `pkgs.[ (openssl.overrideAttrs { patches = [ ... ]; }) ]`
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is currently a syntax error, as is `pkgs.[ "${some_expression}" ]`,
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`a.[ b.[ c d ] ]`, and `a.[ [ b c ] [ d e ] ]`.
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Future RFCs may add additional support for useful idioms such as
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`pkgs.[ python310 python310Packages.pytorch ]` on a case-by-case basis,
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but that is not planned for this RFC.
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For a comparison of other forms of syntax considered but not proposed
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in this RFC, refer to the Alternatives section.
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# Drawbacks
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[drawbacks]: #drawbacks
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* This will add complexity to the Nix grammar and any third-party tools which
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operate on Nix expressions.
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* Expressions reliant on the new syntax will be incompatible with
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Nix versions prior to the introduction of this feature.
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# Alternatives
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[alternatives]: #alternatives
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A number of alternatives have been considered, which can be roughly divided
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into syntactic (introducing new syntax which requires changes to the Nix language
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to parse) and non-syntactic.
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## Comparison of syntactic alternatives:
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Priority is given to syntax which would be "backwards compatible" with
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existing Nix code, meaning that any existing code would be evaluated the
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same under an interpreter supporting the new syntax. Conversely,
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evaluating new code under an old interpreter which does not support the
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new syntax would cause a syntax error.
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| Syntax | Notes | Cons |
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|---|---|---|
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| `inherit (attrs) [ a b c ]` | Initial draft | ["confusing that for attribute sets, the inherit keyword is on the inside but here it is on the outside" -jtojnar](https://github.com/NixOS/rfcs/pull/110#discussion_r730527443) |
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| `[ inherit (attrs) a b c; ]` | 2nd draft, [proposed by jtojnar](https://github.com/NixOS/rfcs/pull/110#discussion_r730527443) | ["not very happy with reusing inherit here. If I read inherit I read 'right, an attribute-set'." -Ma27](https://github.com/NixOS/rfcs/pull/110#issuecomment-947517675) <br /> ["There currently is no separator in lists, and this would add that." -Synthetica9](https://github.com/NixOS/rfcs/pull/110#issuecomment-959114390) |
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| `attrs.[a b c]` | 3rd (current) draft, [proposed by Synthetica9](https://github.com/NixOS/rfcs/pull/110#issuecomment-959114390) | Has ["ambiguities [...] which would have to be worked out" -Synthetica9](https://github.com/NixOS/rfcs/pull/110#issuecomment-971760508) <br /> ["what `[a b c]` means depends on if it is after `attrs.`" -kevincox](https://github.com/NixOS/rfcs/pull/110#discussion_r933500003) <br /> Needs care to limit scope of this rule, currently limited to single-identifier case (discussions [here](https://github.com/NixOS/rfcs/pull/110#discussion_r1001737515) and [here](https://github.com/NixOS/rfcs/pull/110#discussion_r1013099694)) |
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| `[ with attrs \| a b c ]` | [Proposed by nrdxp](https://github.com/NixOS/rfcs/pull/110#discussion_r933570815) | ["Looks more foreign", "may also cause confusion with other languages" -kevincox](https://github.com/NixOS/rfcs/pull/110#discussion_r934516433) <br /> ["introduces a small inconsistency" vs attribute-set `inherit` -rehno-lindeque](https://github.com/NixOS/rfcs/pull/110#discussion_r973033159) |
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| Exclusive `with`, `let with` | [Proposed by infinisil](https://github.com/NixOS/rfcs/pull/110#issuecomment-1319338335) and [later refined](https://github.com/NixOS/rfcs/pull/110#issuecomment-1319338335) | Requires some dynamic behavior to fully replace `with`, including nested usage [-oxalica](https://github.com/NixOS/rfcs/pull/110#issuecomment-1334537982), [-infinisil](https://github.com/NixOS/rfcs/pull/110#issuecomment-1334593541) <br /> May change semantics of existing Nix code |
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Some common threads here are the desire to introduce a syntax form which
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is simpler and more ergonomic than existing `with` or alternatives,
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naturally guiding users toward a "safer" form. We also desire consistency,
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reusing keywords or syntax patterns but only where it would be
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harmonious with the existing Nix language.
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## Comparison of non-syntactic alternatives:
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Other alternatives have been proposed where the motivation for `with`
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deprecation is acknowledged, but would be resolved without introducing
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new syntax to the Nix language.
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| Alternative | Notes | Cons |
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|---|---|---|
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| `builtins.attrValues { inherit (attrs) a b c; }` | Considered in initial draft | Verbose, cumbersome to compose, not order-preserving |
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| `select [ "a" "b" "c" ] attrs` | [Proposed by ocharles](https://github.com/NixOS/rfcs/pull/110#issuecomment-952704340) | ["highlights wrong (strings are not data but literal variable names)" -7c6f434c](https://github.com/NixOS/rfcs/pull/110#issuecomment-952817547) <br /> ["`with` is slightly more ergonomic", "the proposed change is arguably same [...] but without semantical gotchas" -7c6f434c](https://github.com/NixOS/rfcs/pull/110#issuecomment-952931398) |
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| Deprecation of list-types in NixOS modules and build inputs | [Proposed by infinisil](https://github.com/NixOS/rfcs/pull/110#issuecomment-959757180) | ["order of `buildInputs` is significant so unordered sets cannot be used" -jtojnar](https://github.com/NixOS/rfcs/pull/110#issuecomment-959799730) |
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# Unresolved questions
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[unresolved]: #unresolved-questions
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How would this feature be adopted, if accepted?
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# Future work
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[future]: #future-work
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Determine best practices regarding when this language construct should be used
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