gen-select

gen-select is a pure pattern matching library for Nix. Selectors are { __sel = tag; ... } attrsets, matched by matches against an ID-based accessor context.

The problem it owns: “does this node match this predicate” comes up everywhere a graph gets queried, and hand-rolling the predicate logic per caller means hand-rolling it wrong per caller eventually. gen-select gives you one small selector algebra — and, star, attrs, and the rest — and one matches function that evaluates any of them against a context.

It depends on exactly one thing: gen-algebra, for the conservative-equality regime a selector’s attrs matcher dispatches through — a tagged-sum comparison rather than a bespoke one, so gen-select doesn’t carry its own copy of a discipline gen-algebra already owns. One roster member builds on gen-select: gen-inspect, which matches against a materialized graph through it.

Take it alone

{
  inputs.gen-select.url = "github:sini/gen-select";
  outputs = { gen-select, ... }:
    let
      sel = gen-select.lib;
    in {
      # sel.matches, sel.star, sel.attrs, sel.and, ...
    };
}

Without flakes:

let
  sel = (import ./path/to/gen-select).lib;
in
sel.matches (sel.attrs { role = "backend"; }) "api" myContext
# => true if myContext.data "api" has role = "backend"