rune-multi-lang - SKILL.md Agent Skill

name: rune-multi-lang description: Generate implementations and tests from a single RUNE spec in multiple programming languages. Use when a user needs the same function in several languages with identical behavior and error messages. license: MIT

How to Use

When given a RUNE spec and a list of target languages, generate an implementation + test file for each language, ensuring all implementations produce identical behavior.

Input: a RUNE spec + list of target languages. Output: one implementation file + one test file per language.

Language Adaptation

Any programming language is supported. When generating for a target language, adapt:

Naming convention — follow the language's standard (snake_case, camelCase, PascalCase, etc.)
Error handling — use the language's idiomatic mechanism (exceptions, error returns, Result types, pattern matching)
Testing framework — use the language's standard or most common framework
Type system — map types to native equivalents

Process

Step 1: Analyze the Spec

Read the RUNE spec and extract:

Function purpose (from INTENT)
Input/output contract (from SIGNATURE, even if language-specific)
All behavior rules (from BEHAVIOR)
All test cases (from TESTS)
Constraints and edge cases

Step 2: Adapt Signature Per Language

Convert the SIGNATURE to each target language's syntax and conventions.

From a spec with:

SIGNATURE: |
  def calculate_discount(price: float, percentage: int) -> float

Generate:

Language	Adapted Signature
Python	`def calculate_discount(price: float, percentage: int) -> float`
TypeScript	`function calculateDiscount(price: number, percentage: number): number`
Go	`func CalculateDiscount(price float64, percentage int) (float64, error)`
Rust	`fn calculate_discount(price: f64, percentage: i32) -> Result<f64, String>`
Java	`public static double calculateDiscount(double price, int percentage)`

Adaptation rules:

Follow each language's naming convention (snake_case, camelCase, PascalCase)
Map types to native equivalents (float -> f64, number, double)
Add error return channels where idiomatic (Go returns error, Rust returns Result)
Use the language's exception/error mechanism for BEHAVIOR error rules

Step 3: Adapt Error Handling

Each language handles errors differently. Adapt the BEHAVIOR error rules:

Spec says	Python	TypeScript	Go	Rust
`raise ValueError("msg")`	`raise ValueError("msg")`	`throw new Error("msg")`	`return 0, fmt.Errorf("msg")`	`return Err("msg".to_string())`

Preserve the exact error messages across all languages.

Step 4: Generate Implementations

For each language, generate the implementation following these rules:

Implement ALL BEHAVIOR rules in order
Use idiomatic patterns for the target language
Preserve the logic flow from the spec
Match error messages exactly
Handle all EDGE_CASES
Add a docstring/comment from INTENT

Step 5: Generate Tests

For each language, generate a complete test file:

Convert every TESTS entry to the language's test framework
Expand [...] into concrete fixtures per language
Use language-idiomatic assertion style
Group tests by category (happy path, boundary, error)

Step 6: Cross-Language Verification Table

After generating all implementations, produce a verification table:

## Cross-Language Verification

| Test Case | Python | TypeScript | Go | Rust |
|-----------|--------|------------|-----|------|
| `(100.0, 20) -> 80.0` | `calculate_discount(100.0, 20)` | `calculateDiscount(100.0, 20)` | `CalculateDiscount(100.0, 20)` | `calculate_discount(100.0, 20)` |
| `(-10.0, 20) -> error` | `raises ValueError` | `throws Error` | `returns error` | `returns Err` |
| ... | ... | ... | ... | ... |

All implementations must produce **identical outputs** for identical inputs.
Error messages must be **character-for-character identical** across languages.

Output Structure

Generate files organized by language:

<function_name>/
├── python/
│   ├── <function_name>.py
│   └── test_<function_name>.py
├── typescript/
│   ├── <function_name>.ts
│   └── <function_name>.test.ts
├── go/
│   ├── <function_name>.go
│   └── <function_name>_test.go
└── VERIFICATION.md          # cross-language verification table

Or if adding to an existing project, follow the project's directory structure.

Type Mapping Reference

Common mappings for popular languages. Any language is supported — adapt types to its native equivalents following the same pattern.

RUNE / Python	TypeScript	Go	Rust	Java
`str`	`string`	`string`	`String` / `&str`	`String`
`int`	`number`	`int` / `int64`	`i32` / `i64`	`int` / `long`
`float`	`number`	`float64`	`f64`	`double`
`bool`	`boolean`	`bool`	`bool`	`boolean`
`list[T]`	`Array<T>` / `T[]`	`[]T`	`Vec<T>`	`List<T>`
`dict[K, V]`	`Record<K, V>`	`map[K]V`	`HashMap<K, V>`	`Map<K, V>`
`tuple[A, B]`	`[A, B]`	`(A, B)` via struct/returns	`(A, B)`	custom class
`None` / `null`	`null` / `undefined`	`nil`	`None` (Option)	`null`
`Optional[T]`	`T \| null`	`*T`	`Option<T>`	`Optional<T>`

For languages not listed (C#, Ruby, Swift, Kotlin, Elixir, Haskell, PHP, etc.), map each type to its closest native equivalent.

Rules

All implementations must pass the same logical tests
Error messages must be identical across languages (same string)
Use idiomatic patterns per language — do not write "Python in Go"
If a RUNE feature cannot be directly expressed in a language (e.g., tuples in Java), use the closest idiomatic equivalent and document the adaptation
Always generate the cross-language verification table
If the spec has language: <specific>, ask the user which additional languages they want
If the spec has language: any, ask the user which languages to target (or default to Python + TypeScript)