gen-scope

gen-scope is a demand-driven Higher-Order Attribute Grammar evaluator over algebraic scope graphs, implemented as a pure Nix library. It is gen’s sole evaluator: gen-scope is the one library in the roster permitted to evaluate at all, kept thin so a single incremental plane over it is even definable.

The problem it owns: a graph of nodes with attributes that depend on each other — some declared, some computed, some growing the tree as they resolve — needs one evaluation discipline everyone else can build on rather than reinvent. gen-scope is that discipline: nodes carry a distributed memoization cache, so attribute access is O(1) per lookup and nothing is evaluated twice.

Tip

“Sole evaluator” is a strong, load-bearing design rule enforced by a scanner that checks the rest of the roster for evaluation-driving constructs outside this library — and that scanner’s own documentation says it under-approximates the true property. A green run is a statement about the scanner, not a proof over every possible construct. Execution has the full picture.

It depends on gen-graph, gen-identity, gen-prelude, and gen-schema — gen-graph for the well-founded engine’s SCC partitioning, gen-schema for the identity mint that stages node creation. Two roster members build on gen-scope directly: gen-inspect and gen-link. The framework-tier libraries gen-assemble and gen-program take it as injected substrate rather than a declared input.

Take it alone

flake.nix

{
  inputs.gen-scope.url = "github:sini/gen-scope";
  outputs = { gen-scope, ... }:
    let engine = gen-scope.lib;
    in { /* ... */ };
}

# Or without flakes (all three dependencies auto-derived from the pinned flake.lock):
let engine = import ./gen-scope { };
in { /* ... */ }

Vogt, Swierstra & Kuiper — Higher Order Attribute Grammars (1989)The formalism gen-scope's demand-driven evaluator is built on.