[JamesSwift]

You still are pinning to exact versions of packages (and in fact are pinning to specific shas / content hashes of the packages) but you are doing so implicitly by going this "path of least resistance". The only thing that will change what _exact sha_ you are pointed at is an explicit update by someone. There is no risk of someone running install on one box and someone running it on another and them getting different _exact versions_ (or likewise, in CI it also will be the exact same). And in general thats what people are trying to achieve on teams by pinning to full semver. So thats what I mean by its generally not needed on nix.

However, to actually address your question it would be something like this (with ruby also added for comparison of whats possible depending on the language ecosystem you are using)

  ```devenv.yaml```

  inputs:
    nixpkgs:
      url: github:cachix/devenv-nixpkgs/rolling
    nixpkgs-node:
      url: github:nixos/nixpkgs?rev=9c0e2056b3c16190aafe67e4d29e530fc1f8c7c7
    nixpkgs-ruby:
      url: github:bobvanderlinden/nixpkgs-ruby
      inputs:
        nixpkgs:
          follows: nixpkgs

  ```devenv.nix```
  
  { inputs, pkgs, ... }
  
  {
    languages.javascript = {
      enable = true;
      package = inputs.nixpkgs-node.nodejs_24;
    };

    languages.ruby = {
      enable = true;
      versionfile = ./.ruby-version;
      // alternatively
      // version = "3.2.4";
    };
  }

[foldr]

Thanks for the code.

Pinning by version number isn’t a crutch; it’s more a question of mental models. Most people find it more intuitive to identify a world by describing the things in it than to fix the states of the things by identifying the world where they have those states. Nix sometimes feels as if someone read Kripke and missed the passage about possible worlds being a figure of speech.

In other words, many of us don’t think that version numbers are a crutch to be used in the absence of a totalizing hash of the entire state of the universe. We actually think that version numbers are, for many practical purposes, better than that.

[JamesSwift]

I dont mean to imply its a crutch. Im merely pointing out that even though people say they want to "pin to a specific version", there are usually two different motivations:

- one group of people who want guarantees that what they install on their machine matches on their colleague/CI/prod machines. For this group, the nix happy path basically serves all their needs really well. It can capture nearly any sort of software dependency they have regardless of ecosystem and it pins to actual content hashes in most cases. They only care that the high level versioning is met, because again its reproducibility that they care about

- one group of people want guarantees of specific software versions. Nix still handles this, but its "more work" because nix is by default a snapshot of the entire world, of which your target software is only a slice of. And most nix snapshots of the world are "what were the most recent versions of each software package at this point in time". So you generally need to compose together multiple snapshots of the world. It works fine, its just more work (and more disk space).

In practice, again, I think most teams are served better by being in group one, but dipping into group two when they either need an "unreleased" version for software they depend on or when they need an older/unsupported version of software they depend on (or a nix release breaks their workflow). Then nix shines by being able to mix and match between different snapshots in composable/reproducible ways.