Add Evaluator.evaluate_constant to read a single constant fact - #16
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Constant facts (no inputs, no dependencies) previously could only be read by evaluating the whole filtered graph. `evaluate_constant` builds just the one Fact and returns its resolver, so callers can read a graph constant (e.g. a per-context tax_year or state_code) without paying for a full graph evaluation on the request path. Raises ArgumentError for an unknown name, a fact that has inputs or dependencies, or an ambiguous name across modules (disambiguate with module_filter:). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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July 21, 2026 00:44
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What
Adds
FactGraph::Evaluator.evaluate_constant(name, graph_class:, module_filter:)— reads a single constant fact's value (a fact with no inputs and no dependencies) without building or evaluating the rest of the graph.Why
Today the only way to read a constant is
Evaluator.evaluate(input, module_filter: [...]), which instantiates and runs every fact in the filtered module(s) — building Dry::Schema validators for facts you don't care about. Downstream apps (e.g. Gyraffe) have per-context constants likestate_code,max_age,tax_yeardefined asconstant(...)in the graph, but reading them on the request path meant a full filing-context evaluation. This gives a cheap, single-source read so those values don't have to be duplicated as literals in application models.evaluate_constantbuilds only the single matchingFactand returns itsresolver. It raisesArgumentErrorwhen:module_filter:to disambiguate.Tests
New
#evaluate_constantspecs inevaluator_spec.rbcover: reading a constant, not touchingprepare_fact_objects, unknown name, non-constant (has input), and cross-module ambiguity +module_filterdisambiguation. Full suite green (62 examples) and RuboCop clean.🤖 Generated with Claude Code