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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-package
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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. Not for nil-safety and zero-value design without this library (→ See `samber/cc-skills-golang@golang-safety` skill), nor for native error wrapping with fmt.Errorf, errors.Is and errors.As (→ See `samber/cc-skills-golang@golang-error-handling` skill).
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. Not for nil-safety and zero-value design without this library (→ See `samber/cc-skills-golang@golang-safety` skill), nor for native error wrapping with fmt.Errorf, errors.Is and errors.As (→ See `samber/cc-skills-golang@golang-error-handling` skill).
name: golang-samber-mo
description: "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. Not for nil-safety and zero-value design without this library (→ See samber/cc-skills-golang@golang-safety skill), nor for native error wrapping with fmt.Errorf, errors.Is and errors.As (→ See samber/cc-skills-golang@golang-error-handling skill)."
user-invocable: true
license: MIT
compatibility: Designed for Claude Code, Codex or similar harness, and for projects using Golang.
metadata:
author: samber
version: "1.1.2"
openclaw:
emoji: "🎭"
homepage: https://github.com/samber/cc-skills-golang
requires:
bins:
- go
install: []
skill-library-version: "1.16.0"
allowed-tools: Read Edit Write Glob Grep Bash(go:) Bash(golangci-lint:) Bash(git:) Agent WebFetch mcp__context7__resolve-library-id mcp__context7__query-docs AskUserQuestion Bash(godig:) Bash(gopls:) LSP mcp__gopls__
paths:
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.
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 — refer to library documentation and code examples for more information:
samber/cc-skills-golang@golang-pkg-go-dev skill (godig), preferred over Context7 for Go package facts.samber/cc-skills-golang@golang-gopls skill (gopls).go get github.com/samber/mo
For an introduction to functional programming concepts and why monads are valuable in Go, see Monads Guide.
| 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 |
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.
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.
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.
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.
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.
type UserResponse struct {
Name string `json:"name"`
Nickname mo.Option[string] `json:"nickname"` // omits null gracefully
Bio mo.Option[string] `json:"bio"`
}
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)
// 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)
}
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"
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 existsTupleToResult at API boundaries — convert Go's (T, error) to Result[T] at the boundary, then chain with Map/FlatMap inside your domain logicResult[T] for errors, Either[L, R] for alternatives — Result is specialized for success/failure; Either is for two valid typesOption[string] distinguishes "absent" from "empty string"; use plain string when empty string is a valid valueresult.Map(...).FlatMap(...).OrElse(default) reads left-to-right; avoid nested if/else patterns when monadic chaining is cleaneroption.Pipe3(...) is more readable than nested function callsFor 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.
samber/cc-skills-golang@golang-samber-lo skill for functional collection transforms (Map, Filter, Reduce on slices) that compose with mo typessamber/cc-skills-golang@golang-error-handling skill for idiomatic Go error handling patternssamber/cc-skills-golang@golang-safety skill for nil-safety and defensive Go codingsamber/cc-skills-golang@golang-database skill for database access patternssamber/cc-skills-golang@golang-design-patterns skill for functional options and other Go patterns