Functions Over Classes: fn(args, deps)

Critical rules

• Business logic is fn(args, deps) — never service classes. args are per-call; deps are long-lived collaborators.
• Declare a per-function deps type. No god AllServiceDeps objects.
• Wire deps once at a composition root. Handlers call factories, not constructors mid-request.
• Inject only network/disk/clock collaborators. Import pure utilities directly.
• Use import type for infra interfaces; instances arrive via deps.
• Before writing examples or grouping choices, read references/examples.md and references/patterns.md.

Workflow

1. Identify business logic that should be a function (or extract it from a class).
2. Split per-call data into args and collaborators into a named XDeps type.
3. Before choosing individual vs grouped injection, read references/patterns.md.
4. Implement fn(args, deps). Keep classes only as thin framework wrappers or stateful resources.
5. Add a factory / composition-root wire-up so call sites never construct deps.
6. Test with mock<XDeps>() — no class instantiation. See references/examples.md.

Resources

• references/examples.md — WRONG vs CORRECT signatures, composition root, NestJS wrapper, tests. Read when implementing or reviewing.
• references/patterns.md — grouping, migration, when classes are OK, enforcement, rationalizations. Read before structuring modules.

Validation

• Every business function is fn(args, deps) with a per-function deps type
• No unused deps fields; no god deps bag
• Deps wired once at composition root
• Infra via import type only; instances via deps
• Pure utilities imported, not injected
• Remaining classes are wrappers, stateful resources, or builders
• Tests use mock<XDeps>() without instantiating a class

Constraints

• Framework-mandated classes, connection pools, caches, and fluent builders may stay as thin wrappers over pure functions.
• Related: result-types, validation-boundary, testing-strategy, writing-tests, observability, strict-typescript, pattern-enforcement.