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:

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

bash

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]

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
    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

golang-samber-mo

NPX Install

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