golang-samber-mo

by samber

Monadic types for Golang using samber/mo — Option, Result, Either, Future, IO, Task, and State types for type-safe nullable values, error handling, and functional composition with pipeline sub-packages. Apply when using or adopting samber/mo, when the codebase imports github.com/samber/mo, or when considering functional programming patterns as a safety design for Golang. npx skills add https://github.com/samber/cc-skills-golang --skill golang-samber-mo Download ZIPGitHub2.4k 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. For Go package docs, versions, symbols, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill.

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

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.

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 := 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.

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
 func(v int) string { return fmt.Sprintf("got %d", v) },
 func(err error) string { return "failed" },
)
// "got 42"

Best Practices

• Prefer OrElse over MustGet — MustGet panics on absent/error values; use it only inside mo.Do blocks where panics are caught, or when you are certain the value exists

• Use TupleToResult at API boundaries — convert Go's (T, error) to Result[T] at the boundary, then chain with Map/FlatMap inside your domain logic

• Use Result[T] for errors, Either[L, R] for alternatives — Result is specialized for success/failure; Either is for two valid types

• Option for nullable fields, not zero values — Option[string] distinguishes "absent" from "empty string"; use plain string when empty string is a valid value

• Chain, don't nest — result.Map(...).FlatMap(...).OrElse(default) reads left-to-right; avoid nested if/else patterns when monadic chaining is cleaner

• Use sub-package pipes for multi-step type transformations — when 3+ steps each change the type, option.Pipe3(...) is more readable than nested function calls

For advanced types (Future, IO, Task, State), see Advanced Types Reference.

If you encounter a bug or unexpected behavior in samber/mo, open an issue at https://github.com/samber/mo/issues.

Cross-References

• -> See samber/cc-skills-golang@golang-samber-lo skill for functional collection transforms (Map, Filter, Reduce on slices) that compose with mo types

• -> See samber/cc-skills-golang@golang-error-handling skill for idiomatic Go error handling patterns

• -> See samber/cc-skills-golang@golang-safety skill for nil-safety and defensive Go coding

• -> See samber/cc-skills-golang@golang-database skill for database access patterns

• -> See samber/cc-skills-golang@golang-design-patterns skill for functional options and other Go patterns