Fable Native Code

Your diff should read like the file's longtime owner wrote it. The file has a style — naming, error handling, comment density, formatting. Write in it. Your personal preferences are not improvements; they're an accent.

Purpose

Enforce codebase idioms and suppress foreign AI patterns. Eliminate defensive bloat, excessive comments, unnecessary try/catch blocks, unrequested logging, and conversational justification of coding style.

When to Use

• When writing or editing code in an existing repository.
• Before adding docstrings, comments, type annotations, or validations to a file that lacks them.
• When choosing naming conventions, error-handling patterns, or import structures.
• When reviewing a completed diff before finalizing the turn.

When NOT to Use

• When bootstrapping a brand new project from scratch with no established idioms (use fable-architecture or fable-execute).
• Refactoring architectural boundaries or decoupling modules (use fable-simplify).
• Performance benchmarking and algorithmic optimization (use fable-review).

Inputs

• source_diff: The proposed code modification.
• target_file: The existing source file, its surrounding conventions, and neighboring modules.

Expected Outputs

• idiomatic_code: Clean, minimal, idiom-matched code edits that blend seamlessly with surrounding style.
• clean_diff: A diff free of cosmetic churn, gratuitous comments, and defensive wrapping.

Procedure

1. Match the File: Follow naming, error-handling idiom, comment density, formatting, and import organization. If the file has one comment per 200 lines, your addition almost certainly has zero.
2. Comment Invariants Only: Comments state only constraints the code cannot show — invariants, gotchas, why-not-the-obvious-way. Never what the next line does, never narration of your change ("now correctly handles X"), never justification aimed at a reviewer.
3. Suppress Defensive Bloat: No try/catch around code that works, no validating invariants the types or callers already guarantee, no logging, config, or flags nobody asked for.
4. Don't Narrate Restraint: The urge to explain what you didn't add ("note: I deliberately left out the doc comment because…") is the same urge that adds it — displaced into the reply. Deliver the code; mention style only if the user asks.
5. Clean Exit: No debug prints, no commented-out code, no TODO crumbs you created.
6. Codebase Vocabulary: Names come from the codebase's existing vocabulary. Don't coin a second name for an existing concept.

Decision Rules

Contrast Example: Terse Go File

Task: add clampWindow to a terse Go file whose only comment in 200 lines marks a sorting invariant; callers guarantee max >= 0.

Foreign (AI habits, not the file's):

// clampWindow clamps start and end into the valid range [0, max] and
// normalizes inverted windows. Returns (start, start) when end < start.
func clampWindow(start, end, max int) (int, int) {
    if max < 0 {
        return 0, 0 // defensive: should never happen
    }
    ...
}

Native (what the file's owner would write):

func clampWindow(start, end, max int) (int, int) {
    if start < 0 {
        start = 0
    }
    if end > max {
        end = max
    }
    if end < start {
        end = start
    }
    return start, end
}

• If surrounding code uses early returns, use early returns.
• If surrounding code lets errors bubble up, bubble them up; do not swallow or wrap them in unrequested custom errors.
• Never add a docstring unless the enclosing interface or file strictly enforces docstrings on all exported symbols.

Tool Policy

• Read neighboring functions before editing to absorb file conventions.
• Use precise line replacement tools (replace_file_content) to prevent whitespace and indentation drift.
• Do not reformat untouched lines in the same file.

Evidence Requirements

• Provide the exact diff showing changes integrate natively with the file.
• Passing test suite and linter confirming zero stylistic or typing violations.

Failure Handling

• If linter flags a style mismatch, conform strictly to the project's linter rules.
• If an unneeded comment or debug print slipped into the diff, remove it immediately before completing the turn.

Completion Criteria

• Diff reads as if authored by the repository's primary maintainer.
• Zero defensive bloat, zero unrequested logging, zero artificial comments.
• Code compiles, passes existing style checks, and passes all unit tests.