Nim Code Organization

Nim Code Organization

Preamble

Use this skill when splitting Nim code into modules, helpers, and orchestration steps. Keep shared state explicit. Keep behavior in top-level procs. Keep exports narrow.

Rules

Orchestration state

• For multi-step orchestration, use an explicit state object.
• Prefer a plain object passed by var for orchestration state.
• Use ref object state only when identity, aliasing, or shared lifetime is part of the design.
• Pass shared mutable flow through proc parameters. Do not hide it in outer locals.

Helper placement

• Prefer top-level helper procs for orchestration logic.
• Keep reusable helpers out of the main driver proc.
• Use nested procs only for truly local logic or intentional closures.
• A nested proc may capture outer locals by default. If a nested proc must stay non-capturing, mark it {.nimcall.}.

Module shape

• Keep exports intentional. Export only the public surface.
• Keep orchestration internals, helper procs, and state details private unless another module must call them.
• Keep one module focused on one piece of state and the procs that operate on it.

Control flow

• Prefer explicit branching in procs over runtime dispatch for ordinary orchestration code.
• When behavior varies by kind, model the kind in data and branch with case before reaching for method.

Stdlib pattern

• Follow the stdlib parser-style shape when it fits: one state object, top-level open/next/close-style procs, and private helpers that mutate var State.

Workflow

1. Identify the shared mutable flow. If several steps touch the same evolving state, name it as an explicit object.
2. Choose the state representation. Start with a plain object and pass it by var.
3. Place helpers. Put orchestration helpers at module scope. Use nested procs only when the logic is local and short.
4. Define the public surface. Export only the entry points and public data types.
5. Choose the control flow shape. Use a small set of procs and explicit branching. Add runtime dispatch only if the design really needs it.

Minimal Pattern

type
  State = object
    pos: int
    ready: seq[bool]

proc markReady(state: var State; idx: int) =
  state.ready[idx] = true

proc flushReady(state: var State; ids: openArray[string]; out: var seq[string]) =
  while state.pos < ids.len and state.ready[state.pos]:
    out.add ids[state.pos]
    inc state.pos

Common Mistakes

References

• references/​orchestration_pattern.md — Explicit state object vs nested closure for a small orchestration flow
• references/​parser_state_pattern.md — Stdlib-style parser/​state object with top-level mutating procs

Changelog

• 2026-04-09: Initial verified skill.
• 2026-04-11: Added plain-object state, top-level orchestration, nested proc capture guidance.