zig-best-practices

# Zig Best Practices

## Type-First Development

Types define the contract before implementation. Follow this workflow:

1. **Define data structures** - structs, unions, and error sets first
2. **Define function signatures** - parameters, return types, and error unions
3. **Implement to satisfy types** - let the compiler guide completeness
4. **Validate at comptime** - catch invalid configurations during compilation

### Make Illegal States Unrepresentable

Use Zig's type system to prevent invalid states at compile time.

**Tagged unions for mutually exclusive states:**
```zig
// Good: only valid combinations possible
const RequestState = union(enum) {
    idle,
    loading,
    success: []const u8,
    failure: anyerror,
};

fn handleState(state: RequestState) void {
    switch (state) {
        .idle => {},
        .loading => showSpinner(),
        .success => |data| render(data),
        .failure => |err| showError(err),
    }
}

// Bad: allows invalid combinations
const RequestState = struct {
    loading: bool,
    data: ?[]const u8,
    err: ?anyerror,
};
```

**Explicit error sets for failure modes:**
```zig
// Good: documents exactly what can fail
const ParseError = error{
    InvalidSyntax,
    UnexpectedToken,
    EndOfInput,
};

fn parse(input: []const u8) ParseError!Ast {
    // implementation
}

// Bad: anyerror hides failure modes
fn parse(input: []const u8) anyerror!Ast {
    // implementation
}
```

**Distinct types for domain concepts:**
```zig
// Prevent mixing up IDs of different types
const UserId = enum(u64) { _ };
const OrderId = enum(u64) { _ };

fn getUser(id: UserId) !User {
    // Compiler prevents passing OrderId here
}

fn createUserId(raw: u64) UserId {
    return @enumFromInt(raw);
}
```

**Comptime validation for invariants:**
```zig
fn Buffer(comptime size: usize) type {
    if (size == 0) {
        @compileError("buffer size must be greater than 0");
    }
    if (size > 1024 * 1024) {
        @compileError("buffer size exceeds 1MB limit");
    }
    return struct {
        data: [size]u8 = undefined,
        len: usize = 0,
    };
}
```

**Non-exhaustive enums for extensibility:**
```zig
// External enum that may gain variants
const Status = enum(u8) {
    active = 1,
    inactive = 2,
    pending = 3,
    _,
};

fn processStatus(status: Status) !void {
    switch (status) {
        .active => {},
        .inactive => {},
        .pending => {},
        _ => return error.UnknownStatus,
    }
}
```

## Module Structure

Larger cohesive files are idiomatic in Zig. Keep related code together: tests alongside implementation, comptime generics at file scope, public/private controlled by `pub`. Split only when a file handles genuinely separate concerns. The standard library demonstrates this pattern with files like `std/mem.zig` containing 2000+ lines of cohesive memory operations.

## Instructions

- Return errors with context using error unions (`!T`); every function returns a value or an error. Explicit error sets document failure modes.
- Use `errdefer` for cleanup on error paths; use `defer` for unconditional cleanup. This prevents resource leaks without try-finally boilerplate.
- Handle all branches in `switch` statements; include an `else` clause that returns an error or uses `unreachable` for truly impossible cases.
- Pass allocators explicitly to functions requiring dynamic memory; prefer `std.testing.allocator` in tests for leak detection.
- Prefer `const` over `var`; prefer slices over raw pointers for bounds safety. Immutability signals intent and enables optimizations.
- Avoid `anytype`; prefer explicit `comptime T: type` parameters. Explicit types document intent and produce clearer error messages.
- Use `std.log.scoped` for namespaced logging; define a module-level `log` constant for consistent scope across the file.
- Add or update tests for new logic; use `std.testing.allocator` to catch memory leaks automatically.

## Examples

Explicit failure for unimplemented logic:
```zig
fn buildWidget(widget_type: []const u8) !Widget {
    return error.NotImplemented;
}
```

Propagate errors with try:
```zig
fn readConfig(path: []const u8) !Config {
    const file = try std.fs.cwd().openFile(path, .{});
    defer file.close();
    const contents = try file.readToEndAlloc(allocator, max_size);
    return parseConfig(contents);
}
```

Resource cleanup with errdefer:
```zig
fn createResource(allocator: std.mem.Allocator) !*Resource {
    const resource = try allocator.create(Resource);
    errdefer allocator.destroy(resource);

    resource.* = try initializeResource();
    return resource;
}
```

Exhaustive switch with explicit default:
```zig
fn processStatus(status: Status) ![]const u8 {
    return switch (status) {
        .active => "processing",
        .inactive => "skipped",
        _ => error.UnhandledStatus,
    };
}
```

Testing with memory leak detection:
```zig
const std = @import("std");

test "widget creation" {
    const allocator = std.testing.allocator;
    var list: std.ArrayListUnmanaged(u32) = .empty;
    defer list.deinit(allocator);

    try list.append(allocator, 42);
    try std.testing.expectEqual(1, list.items.len);
}
```

## Memory Management

- Pass allocators explicitly; never use global state for allocation. Functions declare their allocation needs in parameters.
- Use `defer` immediately after acquiring a resource. Place cleanup logic next to acquisition for clarity.
- Prefer arena allocators for temporary allocations; they free everything at once when the arena is destroyed.
- Use `std.testing.allocator` in tests; it reports leaks with stack traces showing allocation origins.

### Examples

Allocator as explicit parameter:
```zig
fn processData(allocator: std.mem.Allocator, input: []const u8) ![]u8 {
    const result = try allocator.alloc(u8, input.len * 2);
    errdefer allocator.free(result);

    // process input into result
    return result;
}
```

Arena allocator for batch operations:
```zig
fn processBatch(items: [