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ADR-01: Unify config merging under a monoid

  • Status: Accepted
  • Date: 2025-12-12
  • Scope: config.ts, command-builder.ts

Context

mdflow's config precedence chain merges six layers, each overriding the last:

built-in ⊕ global ⊕ git-root ⊕ cwd ⊕ frontmatter ⊕ CLI flags

This is structurally a monoidal fold: {} is the identity, right-biased shallow merge is the associative binary operation, and reduce is the fold. But before this change, three separate code paths expressed that same operation differently:

  1. mergeConfigs() in config.ts: deep-clones base, then shallow-merges override per command key
  2. applyDefaults() in config.ts: spreads defaults first, then iterates frontmatter entries with assignment
  3. Inline spreads in command-builder.ts: { ...defaults, ...frontmatter } as AgentFrontmatter

The inline spreads don't deep-clone, so they can alias nested objects. applyDefaults uses iteration rather than spread, which behaves identically for flat objects but could diverge with getters or toJSON. There were ~18 tests across three files testing variations of the same merge logic, but no integration test verified the full precedence chain end-to-end.

Decision

Introduce concatFrontmatter(base, override) as the single canonical merge operation, with documented algebraic laws:

export function concatFrontmatter(
  base: AgentFrontmatter,
  override: AgentFrontmatter
): AgentFrontmatter {
  return { ...base, ...override };
}

Laws (verified by property-based tests):

  • Identity: concatFrontmatter({}, x) ≡ x and concatFrontmatter(x, {}) ≡ x
  • Right-bias: for any key k in both a and b, result[k] === b[k]
  • Associativity: concat(a, concat(b, c)) ≡ concat(concat(a, b), c)
  • No mutation: neither input is modified

All three code paths now delegate to this function:

  • applyDefaults becomes a thin wrapper: concatFrontmatter(defaults, frontmatter)
  • mergeConfigs uses it for per-command merging
  • buildCommand / buildCommandBase call it instead of inline spreads

Testing strategy

Property-based tests (fast-check, 200 randomized inputs per law) replace the case-by-case merge tests. A mutation proof (doc/mutation-proof.md) demonstrates equivalence: three deliberate bugs were introduced one at a time, and the property tests caught all three (3/3) while the case tests caught only one (1/3). The two bugs missed by case tests were phantom key injection (caught by identity law) and input mutation (caught by no-mutation law).

Consequences

  • One function to reason about for all config merging, instead of three
  • Property-based tests provide strictly stronger coverage with fewer test cases
  • applyDefaults is retained as a backward-compatible wrapper (thin delegation, no separate logic)
  • mergeConfigs still handles the GlobalConfig level (per-command dispatch), but its inner merge is now concatFrontmatter