Nim Error Handling

Nim Error Handling

Use this skill to decide where code should raise, catch, translate, or return structured failure data.

Rules

Choose the Failure Channel

• Raise exceptions for invalid input, semantic/​validation failure, and operational/​runtime failure (I/​O, OS, network).
• Return bool for probe-style queries where failure means “not found / condition not met” and no value payload is needed.
• Return Option[T] when the absent value is a value type with no natural sentinel and the caller benefits from composable operations like map or flatMap.
• For scanning/​parsing helpers, return consumed length as int (0 = no match) and write the parsed value via a var out-parameter.
• Use structured result objects only at the orchestrator/​batch boundary, where exceptions are converted into per-item outputs.
• Do not pass ad-hoc (ok, kind, message) step-result objects through straight-line internal flows.

Place Boundaries

• Internal step procs should raise (do not catch in the same layer).
• Catch only to recover, translate at a boundary, record failure, or clean up.
• Keep the success path straight-line between boundaries.
• Do not wrap each raising call in its own local try/​except.
• Validate at boundaries where data enters from outside the typed flow: files, network, user input, config text, FFI, and public API inputs.
• Add fallback logic only when the caller has a useful recovery path. If failure only leaves stale or unusable state, report the boundary error plainly.

Choose Exception Types

• Catch CatchableError as the recoverable catch-all. Do not catch bare Exception.
• Use specific existing exception types such as ValueError, IOError, and OSError when callers should distinguish them.
• Add a custom exception type only when callers need a narrower semantic type for distinct handling.
• Custom exception types must derive from CatchableError (recoverable) or Defect (programming bugs).
• Inherit from a more specific existing base like ValueError or IOError when the semantic fit is clear.
• Deriving directly from CatchableError or Defect is fine when no intermediate base matches.

Make Contracts Explicit

• Add explicit raises contracts on exported procs only when the exception surface is stable and narrow.
• Do not annotate every internal helper by default.
• Use .raises: [] for a proc that must not raise.
• Use .raises: [X] when one specific exception type is part of the contract.
• Treat raises as a compiler-checked contract, not documentation prose.

Translate and Inspect Errors

• Translate low-level errors only at module/​subsystem boundaries.
• Add local context and preserve the underlying reason in the new message (include the original exception message).
• If the handler only needs the message text, use getCurrentExceptionMsg().
• If the handler needs the exception object or fields, use except X as e or getCurrentException().

Cleanup

• Use try/​finally for cleanup.

Workflow

1. Decide whether failure is expected. If it is an expected miss, return bool, Option, or a parse-length value. Do not throw.
2. Mark the boundaries. Step procs raise. Parse helpers may catch once. Module boundaries may translate. Orchestrators may record per-item failure.
3. Pick the exception type. Start with an existing type. Add a subtype only if callers need distinct handling.
4. Write the contract. Add raises on exported procs when the exception surface is narrow and stable. Keep it accurate.
5. Verify the shape. Compile the code. Run the repo tests. If you wrote raises, ensure the compiler accepts the contract.

Minimal Pattern

import std/[options]

proc findConfig*(paths: seq[string]): Option[string] =
  for p in paths:
    if fileExists(p):
      return some(p)
  none(string)

proc loadConfig*(path: string): Config =
  if path.len == 0:
    raise newException(ValueError, "config path is empty")
  try:
    result = parseConfig(readFile(path))
  except IOError:
    raise newException(IOError, "cannot read config: " & getCurrentExceptionMsg())

Common Mistakes

References

• references/​batch_preview_boundary.md — Batch boundary that records per-item failures

Changelog

• 2026-04-09: Initial skill.
• 2026-04-11: Added raises contracts, exception base-class rules, expected-miss return channels.
• 2026-04-14: Refined failure channel guidance.
• 2026-04-17: Removed retry advice.
• 2026-05-08: Added boundary validation guidance.