golang-samber-mo

Borrows from Rust and Scala the idea of a value that might be absent, or a result that might have failed, and gives Go a type for it — so the compiler makes you handle the empty case.

WHEN TO USE

SCOPE

USE FOR

Replace a nil pointer check with a type-safe Option marking a value present or absent.
Wrap Go's (value, error) pattern in a Result and chain transforms that skip on failure.
Model an outcome that can be one of two valid types, like cached versus fresh data.
Store a nullable database column or optional JSON field as a self-scanning Option.
Write straight-line code with mo.Do and let it turn a MustGet panic into a Result error.

NOT FOR

Nil-safety and zero-value design that does not use this library.
Native error wrapping with fmt.Errorf, errors.Is, and errors.As.

This is a saved copy of the skill's SKILL.md instructions. It may differ from the upstream file or the version you install.

In this document (10)

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:

This skill is not exhaustive — 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), preferred 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.

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

This skill ships inside the Samber Golang Skills plugin.

Install just this skill

Run this outside your AI tool, using Node.js 22.20.0 or newer. Skills CLI 1.7.2 skips skill selection; prompts cancel installation without an interactive terminal. If choosing Project when asked about installation scope, open your project's folder before running this.

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If asked, choose Symlink (Recommended) for shared files or Copy to all agents for separate copies; review the summary before confirming installation.
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Skills CLI 1.7.2 sends install data to skills.sh by default, including the source, skill names, target tools and file details; GitHub sources are reported only when GitHub confirms the repository is public, while other remote sources can also be reported. Confirmed-public GitHub sources also trigger a security-audit request. Before installing, set the environment variable DO_NOT_TRACK=1 or DISABLE_TELEMETRY=1 to disable both.

npx skills add samber/cc-skills-golang --skill "golang-samber-mo" --full-depth

After installing for Claude Code, open it in the installation project if you chose Project; Global makes the skill available across projects on this computer. Once Claude Code loads the skill, its description helps Claude decide when to use it.

If installation created a top-level skills folder that did not exist when your Claude Code session started, run /reload-skills in that session. Run it again after later changes in that folder.

Install the whole plugin

This route installs the Samber Golang Skills plugin package selected for your AI tool.

Add Samuel's marketplace, then install the plugin

Type both lines inside a Claude Code session, not at your shell prompt. The marketplace is a separate repository of Samuel Berthe's, samber/cc, which also lists his other plugins. The second line opens the details of cc-skills-golang, which carries this repository's 46 Go skills; pick one of the install options shown and Claude Code installs it.

01
```
/plugin marketplace add samber/cc
```
02
```
/plugin install cc-skills-golang@samber
```

Run it in your terminal. It copies the whole repository, with its 46 skills, into ~/.openclaw/skills/cc-skills-golang, OpenClaw's skills folder in your home folder. To install it for one OpenClaw workspace instead, Samuel gives ~/.openclaw/workspace/skills/cc-skills-golang as the folder.

git clone https://github.com/samber/cc-skills-golang.git ~/.openclaw/skills/cc-skills-golang

Run it in your terminal, not inside Gemini CLI; installing from GitHub needs git on your computer. It installs the repository, with its 46 skills, as a Gemini CLI extension. To update it later, run gemini extensions update cc-skills-golang.

gemini extensions install https://github.com/samber/cc-skills-golang

Run it in your terminal. It copies the whole repository, with its 46 skills, into ~/.cursor/skills/cc-skills-golang, Cursor's skills folder in your home folder.

git clone https://github.com/samber/cc-skills-golang.git  ~/.cursor/skills/cc-skills-golang

Type it in an interactive GitHub Copilot CLI session, not at your shell prompt. It installs the repository straight from GitHub as a plugin, with its 46 skills and no marketplace to add first.

/plugin install https://github.com/samber/cc-skills-golang

Run it in your terminal. It copies the whole repository, with its 46 skills, into ~/.agents/skills/cc-skills-golang in your home folder.

git clone https://github.com/samber/cc-skills-golang.git ~/.agents/skills/cc-skills-golang

Open plugin page ↗

What's inside

Files included with this skill.

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Same repository

golang-benchmark
Golang benchmarking, profiling, and performance measurement. Use when writing, running, or comparing Go benchmarks, profiling hot paths with pprof, interpreting CPU/memory/trace profiles, analyzing results with benchstat, setting up CI benchmark regression detection, or investigating production performance with Prometheus runtime metrics. Also use when the developer needs deep analysis on a specific performance indicator - this skill provides the measurement methodology, while samber/cc-skills-golang@golang-performance provides the optimization patterns.Read full description
golang-cli
Golang CLI application development. Use when building, modifying, or reviewing a Go CLI tool — especially for command structure, flag handling, configuration layering, version embedding, exit codes, I/O patterns, signal handling, shell completion, argument validation, and CLI unit testing. Also triggers when code uses cobra, viper, or urfave/cli. For cobra-specific APIs → See samber/cc-skills-golang@golang-spf13-cobra skill; for viper configuration layering → See samber/cc-skills-golang@golang-spf13-viper skill.Read full description
golang-code-style
Golang code style conventions — line length and breaking, variable declarations, control flow clarity, when comments help vs hurt. Use when writing or reviewing Go code, asking about style or clarity, or establishing project coding standards. Not for naming conventions (→ See samber/cc-skills-golang@golang-naming skill), linter configuration (→ See samber/cc-skills-golang@golang-lint skill), or doc comments (→ See samber/cc-skills-golang@golang-documentation skill).Read full description
golang-concurrency
Golang concurrency patterns. Use when writing or reviewing concurrent Go code involving goroutines, channels, select, locks, sync primitives, errgroup, singleflight, worker pools, or fan-out/fan-in pipelines. Also triggers when you detect goroutine leaks, race conditions, channel ownership issues, or need to choose between channels and mutexes.Read full description
golang-context
Idiomatic context.Context usage in Golang — propagation through API boundaries, cancellation, timeouts and deadlines, request-scoped values, context.WithoutCancel for background work outliving requests. Apply when designing context propagation across layers, debugging leaked or unexpired contexts, choosing between context.Background/TODO/WithoutCancel, or storing values in context. Not for code that merely accepts ctx as first parameter.Read full description
golang-continuous-integration
CI/CD pipeline configuration using GitHub Actions for Golang projects — testing, linting, SAST, security scanning, code coverage, Dependabot, Renovate, GoReleaser, code review automation, and release pipelines. Use when setting up or improving Go project CI, configuring GitHub Actions workflows, adding linters or security scanners, automating dependency updates, or adding quality gates.Read full description

samber/mo — Monads and Functional Abstractions for Go

Core Types at a Glance

Option[T] — Nullable Values Without nil

Result[T] — Error Handling as Values

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

Do Notation — Imperative Style with Monadic Safety

Pipeline Sub-Packages vs Direct Chaining

Common Patterns

JSON API responses with Option

Database nullable columns

Wrapping existing Go APIs

Uniform extraction with Fold

Best Practices

Cross-References

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Install just this skill

Install the whole plugin

Add Samuel's marketplace, then install the plugin

What's inside

More skills

golang-benchmark

golang-cli

golang-code-style

golang-concurrency

golang-context

golang-continuous-integration