golang-samber-mo

**Persona:** You are a Go engineer bringing functional programming safety to Go. You use monads to make impossible states unrepresentable — nil checks become type constraints, error handling becomes composable pipelines.

**Thinking mode:** Reason as thoroughly as possible when designing multi-step Option/Result/Either pipelines — wrong type choice creates unnecessary wrapping/unwrapping that defeats the purpose of monads. On Claude Code, use `ultrathink` to trigger extended thinking explicitly.

# samber/mo — Monads and Functional Abstractions for Go

Go 1.18+ library providing type-safe monadic types with zero dependencies. Inspired by Scala, Rust, and fp-ts.

**Official Resources:**

- [pkg.go.dev/github.com/samber/mo](https://pkg.go.dev/github.com/samber/mo)
- [github.com/samber/mo](https://github.com/samber/mo)

This skill is not exhaustive. Please refer to library documentation and code examples for more information. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See `samber/cc-skills-golang@golang-pkg-go-dev` skill (`godig`) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See `samber/cc-skills-golang@golang-gopls` skill (`gopls`). Context7 remains a fallback for docs not indexed on pkg.go.dev.

```bash
go get github.com/samber/mo
```

For an introduction to functional programming concepts and why monads are valuable in Go, see [Monads Guide](./references/monads-guide.md).

## Core Types at a Glance

| Type | Purpose | Think of it as... |
| --- | --- | --- |
| `Option[T]` | Value that may be absent | Rust's `Option`, Java's `Optional` |
| `Result[T]` | Operation that may fail | Rust's `Result<T, E>`, replaces `(T, error)` |
| `Either[L, R]` | Value of one of two types | Scala's `Either`, TypeScript discriminated union |
| `EitherX[L, R]` | Value of one of X types | Scala's `Either`, TypeScript discriminated union |
| `Future[T]` | Async value not yet available | JavaScript `Promise` |
| `IO[T]` | Lazy synchronous side effect | Haskell's `IO` |
| `Task[T]` | Lazy async computation | fp-ts `Task` |
| `State[S, A]` | Stateful computation | Haskell's `State` monad |

## Option[T] — Nullable Values Without nil

Represents a value that is either present (`Some`) or absent (`None`). Eliminates nil pointer risks at the type level.

```go
import "github.com/samber/mo"

name := mo.Some("Alice")          // Option[string] with value
empty := mo.None[string]()        // Option[string] without value
fromPtr := mo.PointerToOption(ptr) // nil pointer -> None

// Safe extraction
name.OrElse("Anonymous")  // "Alice"
empty.OrElse("Anonymous")  // "Anonymous"

// Transform if present, skip if absent
upper := name.Map(func(s string) (string, bool) {
    return strings.ToUpper(s), true
})
```

**Key methods:** `Some`, `None`, `Get`, `MustGet`, `OrElse`, `OrEmpty`, `Map`, `FlatMap`, `Match`, `ForEach`, `ToPointer`, `IsPresent`, `IsAbsent`.

Option implements `json.Marshaler/Unmarshaler`, `sql.Scanner`, `driver.Valuer` — use it directly in JSON structs and database models.

For full API reference, see [Option Reference](./references/option.md).

## Result[T] — Error Handling as Values

Represents success (`Ok`) or failure (`Err`). Equivalent to `Either[error, T]` but specialized for Go's error pattern.

```go
// Wrap Go's (value, error) pattern
result := mo.TupleToResult(os.ReadFile("config.yaml"))

// Same-type transform — errors short-circuit automatically
upper := mo.Ok("hello").Map(func(s string) (string, error) {
    return strings.ToUpper(s), nil
})
// Ok("HELLO")

// Extract with fallback
val := upper.OrElse("default")
```

**Go limitation:** Direct methods (`.Map`, `.FlatMap`) cannot change the type parameter — `Result[T].Map` returns `Result[T]`, not `Result[U]`. Go methods cannot introduce new type parameters. For type-changing transforms (e.g. `Result[[]byte]` to `Result[Config]`), use sub-package functions or `mo.Do`:

```go
import "github.com/samber/mo/result"

// Type-changing pipeline: []byte -> Config -> ValidConfig
parsed := result.Pipe2(
    mo.TupleToResult(os.ReadFile("config.yaml")),
    result.Map(func(data []byte) Config { return parseConfig(data) }),
    result.FlatMap(func(cfg Config) mo.Result[ValidConfig] { return validate(cfg) }),
)
```

**Key methods:** `Ok`, `Err`, `Errf`, `TupleToResult`, `Try`, `Get`, `MustGet`, `OrElse`, `Map`, `FlatMap`, `MapErr`, `Match`, `ForEach`, `ToEither`, `IsOk`, `IsError`.

For full API reference, see [Result Reference](./references/result.md).

## Either[L, R] — Discriminated Union of Two Types

Represents a value that is one of two possible types. Unlike Result, neither side implies success or failure — both are valid alternatives.

```go
// API that returns either cached data or fresh data
func fetchUser(id string) mo.Either[CachedUser, FreshUser] {
    if cached, ok := cache.Get(id); ok {
        return mo.Left[CachedUser, FreshUser](cached)
    }
    return mo.Right[CachedUser, FreshUser](db.Fetch(id))
}

// Pattern match
result := fetchUser("user-123")
result.Match(
    func(cached CachedUser) mo.Either[CachedUser, FreshUser] { /* use cached */ },
    func(fresh FreshUser) mo.Either[CachedUser, FreshUser] { /* use fresh */ },
)
```

**When to use Either vs Result:** Use `Result[T]` when one path is an error. Use `Either[L, R]` when both paths are valid alternatives (cached vs fresh, left vs right, strategy A vs B).

`Either3[T1, T2, T3]`, `Either4`, and `Either5` extend this to 3-5 type variants.

For full API reference, see [Either Reference](./references/either.md).

## Do Notation — Imperative Style with Monadic Safety

`mo.Do` wraps imperative code in a `Result`, catching panics from `MustGet()` calls:

```go
result := mo.Do(func() int {
    // MustGet panics on None/Err — Do catches it as Result error
    a := mo.Some(21).MustGet()
    b := mo.Ok(2).MustGet()
    return a * b  // 42
})
// result is Ok(42)

resul