Imported from mattstruble/skills (odin-design/SKILL.md). Install upstream with npx skills add mattstruble/skills --skill odin-design. Copyright stays with the author.

Odin Design Patterns

Odin is underrepresented in LLM training data. You will confidently produce wrong patterns from Go, Rust, C++, or Zig. This skill corrects those instincts and provides idiomatic Odin patterns.

Critical rule: Never guess Odin standard library APIs. If you are unsure whether a procedure exists or what its signature is, say so and direct the engineer to check https://pkg.odin-lang.org/. Fabricating procedures is the single most damaging mistake you can make.

What You Get Wrong (Corrections Table)

Before writing any Odin code, internalize these corrections:

Package & File Organization

Packages are for libraries. Not for organizing code within a program.

A package is a directory. All .odin files in a directory with the same package declaration form one compilation unit. No cyclic dependencies between packages are allowed.

Rules

1. One package per program is normal and fine. Games, CLI tools, servers — all work as a single package. This is equivalent to how C programs work, except external libraries get proper namespacing.

2. Create a sub-package only when the code is an independent library with no need to reference the parent. If your "renderer" sub-package needs to know about your "entity" type defined in the parent, it cannot be a separate package.

3. Split into files for logical grouping. All files in a package see each other — no imports needed. Moving code between files within a package requires zero refactoring (names stay the same).

4. Prefix procedure names with the concept they belong to:

   // entity.odin
   entity_create :: proc(...) -> Entity { ... }
   entity_update :: proc(e: ^Entity, dt: f32) { ... }
   entity_destroy :: proc(e: ^Entity) { ... }

   Frequently-used procedures (like add_entity, get_entity) can drop the prefix when the name is already unambiguous.

5. Use @(private="file") to limit access to file-level global state — prevents other files in the same package from touching globals they shouldn't. Don't overuse it for general encapsulation; Odin is not OOP.

6. Package names are for ABI linking, not for callers (callers choose their own import alias). Make them unique if publishing as a library; otherwise any reasonable name works.

Import Patterns

import "core:fmt"              // default name: fmt
import rl "vendor:raylib"      // alias: rl
import "../my_lib"             // relative path for local libraries

Import paths are relative to the importing file. Library collections use prefixes: "core:", "base:", "vendor:".

Naming Conventions

Allocator Patterns

Odin's allocator system is central to the language. Every allocation goes through an allocator, defaulting to context.allocator. Read references/allocators.md for worked examples and pitfalls.

Three-Tier Strategy

1. Temporary allocator (context.temp_allocator) — use for anything frame-scoped or short-lived. Cleared in bulk with free_all. No individual frees needed.

   actions := make([dynamic]Action, context.temp_allocator)
   append(&actions, action)
   // Cleared at end of frame via free_all(context.temp_allocator)

2. Custom arenas — for grouped allocations with shared lifetime. Use mem.Arena or virtual growing arenas. Beware: dynamic arrays in arenas leak old backing memory on grow (see references).

3. Default allocator — for long-lived or variable-sized allocations that outlive any single scope. Pair with tracking allocator during development.

Allocator Decision Tree

Key Rules

• new() and make() use context.allocator unless you pass an explicit allocator.
• Always defer delete(thing) or defer free(ptr) for default-allocator memory.
• Use context.temp_allocator by default; escalate to arenas or default only when lifetime requires it.
• For data loaded from files into long-lived structures, accept an allocator parameter or allocate from a dedicated arena owned by the parent structure.

Error Handling

No exceptions. Errors are values returned alongside results.

// Basic pattern: multiple return values
load_config :: proc(path: string) -> (Config, bool) {
    data, ok := os.read_entire_file(path)
    if !ok { return {}, false }
    defer delete(data)
    // parse...
    return config, true
}

// Modern pattern: or_return propagates errors up
load_config :: proc(path: string) -> (Config, Error) {
    data := os.read_entire_file(path) or_return
    defer delete(data)
    config := parse(data) or_return
    return config, nil
}

// or_else provides a default on failure
name := get_name() or_else "unnamed"

Error Type Design

• bool — sufficient for simple success/failure
• Named enum — when callers need to distinguish failure modes:

  Load_Error :: enum { None, File_Not_Found, Parse_Failed, Invalid_Header }

• Tagged union — when wrapping errors from different subsystems:

  Tilemap_Error :: union { os.Error, Parse_Error }

File I/O Idiom

Prefer os.read_entire_file over manual open/read/close. It reads the whole file and returns ([]byte, bool). Use defer delete(data) on the result if allocated from the default allocator, or pass context.temp_allocator if the data is only needed during parsing.

// Preferred: read entire file (no handle to manage)
data, ok := os.read_entire_file(path, context.temp_allocator)
if !ok { return {}, .File_Not_Found }
// data freed automatically on next free_all(context.temp_allocator)

// Only use os.open when you need streaming/partial reads:
f, err := os.open("large_file.bin")
if err != nil { return err }
defer os.close(f)

Cleanup with defer

// defer runs at scope exit in reverse declaration order
arena: mem.Arena
mem.arena_init(&arena, backing_buf)
defer free_all(mem.arena_allocator(&arena))  // resets arena; caller frees backing_buf

// Allocate into arena — all freed on free_all
context.allocator = mem.arena_allocator(&arena)
tiles := make([]Tile, count)

Worked Example: Loading Level Data with Arena

Tilemap :: struct {
    tiles:  []Tile,
    width:  int,
    height: int,
    arena:  mem.Arena,
}

tilemap_load :: proc(path: string) -> (Tilemap, Load_Error) {
    // Parse phase: temp allocator for scratch data
    raw_data, ok := os.read_entire_file(path, context.temp_allocator)
    if !ok { return {}, .File_Not_Found }

    width, height := parse_header(raw_data) or_return

    // Allocation phase: arena for level-lifetime data
    tm: Tilemap
    backing := make([]byte, width * height * size_of(Tile) + 1024)
    mem.arena_init(&tm.arena, backing)

    alloc := mem.arena_allocator(&tm.arena)
    tm.tiles = make([]Tile, width * height, alloc)
    tm.width = width
    tm.height = height

    parse_tiles(raw_data, tm.tiles) or_return
    return tm, nil
}

tilemap_destroy :: proc(tm: ^Tilemap) {
    backing := tm.arena.data  // capture before reset
    free_all(mem.arena_allocator(&tm.arena))
    delete(backing)           // free the backing buffer itself
}

Modern Features You Likely Don't Know About

These are real Odin features that may not exist in your training data:

Development Workflow

• odin check . — type-check without building (fast feedback loop)
• odin build . -vet — catches unused variables, imports, shadowing
• odin test . — runs procedures marked with @(test)
• Tracking allocator — enable in dev builds to catch memory leaks at runtime (see references/allocators.md)
• -debug — includes debug info and bounds checking
• -o:speed — optimization for release builds (disables bounds checks)

Declarations & Syntax Quick Reference

// Variables
x: int = 5
y := 10                    // type inference
z: int                     // zero-initialized (0)
w: int = ---               // uninitialized (rare, unsafe)

// Constants
MAX :: 100                 // untyped constant
PI : f64 : 3.14159        // typed constant

// Procedures
add :: proc(a, b: int) -> int { return a + b }

// Structs
Player :: struct { pos: [3]f32, health: int, name: string }

// Vector type alias (idiomatic — gets array programming + swizzle for free)
Vector3 :: [3]f32

// Enums
Direction :: enum { North, South, East, West }

// Unions (tagged)
Shape :: union { Circle, Rect }

// Distinct types (not aliases)
Meters :: distinct f64

All declarations use name : type = value or name :: value (constant). := is shorthand for inferred-type declaration.

Further Reading

• references/allocators.md — Arena pitfalls, tracking allocator setup, dynamic arrays in arenas
• references/data-oriented.md — #soa, array programming, bit_set, struct-of-arrays patterns, built-in matrix type
• references/context-system.md — Context propagation, custom context fields, thread-local behavior, using for subtype polymorphism
• references/procedures-and-polymorphism.md — Procedure groups, vtable pattern, and non-capturing closures
• references/profiling.md — Spall profiler, hot vs cold code, instrumentation attributes, optimization workflow
• references/sanitizer-integration.md — Read when integrating ASan with arena allocators, debugging stale-pointer bugs in reused memory, or adding sanitizer hooks to ownership transitions
• references/concurrency.md — Read when building concurrent Odin systems — state machines with tagged unions, cooperative scheduling via Effects, per-shard context setup, trap boundaries with sigsetjmp/siglongjmp, or lock-free primitives via core:sync