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: Use ultrathink when designing multi-step Option/Result/Either pipelines. Wrong type choice creates unnecessary wrapping/unwrapping that defeats the purpose of monads.

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
• github.com/samber/mo

This skill is not exhaustive. Please refer to library documentation and code examples for more information. Context7 can help as a discoverability platform.

go get github.com/samber/mo

For an introduction to functional programming concepts and why monads are valuable in Go, see Monads Guide.

Core Types at a Glance

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.

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.

Result[T] — Error Handling as Values

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

// 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:

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.

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.

// 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.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.

Do Notation — Imperative Style with Monadic Safety

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

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)

result := mo.Do(func() int {
    val := mo.None[int]().MustGet()  // panics
    return val
})
// result is Err("no such element")

Do notation bridges imperative Go style with monadic safety — write straight-line code, get automatic error propagation.

Pipeline Sub-Packages vs Direct Chaining

samber/mo provides two ways to compose operations:

Direct methods (.Map, .FlatMap) — work when the output type equals the input type:

opt := mo.Some(42)
doubled := opt.Map(func(v int) (int, bool) {
    return v * 2, true
})  // Option[int]

Sub-package functions (option.Map, result.Map) — required when the output type differs from input:

import "github.com/samber/mo/option"

// int -> string type change: use sub-package Map
strOpt := option.Map(func(v int) string {
    return fmt.Sprintf("value: %d", v)
})(mo.Some(42))  // Option[string]

Pipe functions (option.Pipe3, result.Pipe3) — chain multiple type-changing transformations readably:

import "github.com/samber/mo/option"

result := option.Pipe3(
    mo.Some(42),
    option.Map(func(v int) string { return strconv.Itoa(v) }),
    option.Map(func(s string) []byte { return []byte(s) }),
    option.FlatMap(func(b []byte) mo.Option[string] {
        if len(b) > 0 { return mo.Some(string(b)) }
        return mo.None[string]()
    }),
)

Rule of thumb: Use direct methods for same-type transforms. Use sub-package functions + pipes when types change across steps.

For detailed pipeline API reference, see Pipelines Reference.

Common Patterns

JSON API responses with Option

type UserResponse struct {
    Name     string            `json:"name"`
    Nickname mo.Option[string] `json:"nickname"`  // omits null gracefully
    Bio      mo.Option[string] `json:"bio"`
}

Database nullable columns

type User struct {
    ID       int
    Email    string
    Phone    mo.Option[string]  // implements sql.Scanner + driver.Valuer
}

err := row.Scan(&u.ID, &u.Email, &u.Phone)

Wrapping existing Go APIs

// Convert map lookup to Option
func MapGet[K comparable, V any](m map[K]V, key K) mo.Option[V] {
    return mo.TupleToOption(m[key])  // m[key] returns (V, bool)
}

Uniform extraction with Fold

mo.Fold works uniformly across Option, Result, and Either via the Foldable interface:

str := mo.Fold[error, int, string](
    mo.Ok(42),  // works with Option, Result, or Either